Wednesday, October 18, 2017

Construction - The Last Major Dirt Work - Two Retaining Walls and More

It is late September in the lower Midwest and the first frost could be a month away. Even with global warming (which seems to have pushed back the deadline for planting grass and expecting it to germinate before cold weather), we might have at the most three weeks to finish the dirt work and plant seed.  Consequently, construction is now on hold until the dirt work is done.

"Dirt work" means several things:  constructing the
retaining walls that are essential for earth sheltering, finessing the exposed mouths of the drainage tiles, final contouring of the subsoil and redistributing the top soil that was removed for construction. Secondarily, a series of rain gardens need to be installed for controlling the run-off originating from our present residence next door and continuing down through, and augmented by, the hillside configuration of the new building site -- a total run of several hundred feet.  And some remaining sections of the insulation/watershed umbrella has to be installed in the process.

As for retaining walls, the plans call for two major "umbrella walls" plus two of smaller size that did not involve the umbrella.

Reminder:  To enlarge any photo for clearer detail, click on it.

The Umbrella Behind and Under the East Retaining Wall (one of two "umbrella walls")
As explained in a triad of prior posts, the umbrella comprises layers of sand, foam insulation board and 6 mil plastic sheeting plus a couple of layers of recycled
North elevation; notice transitions from 12' center wall and
8' walls on either ends; the dark line is the architect's take
on the final grade, including two retaining walls
carpeting.  (The design of the umbrella surrounding the house is based mostly on that described by Hiat in his must-read self-published treatise, Passive Annual Heat Storage.)  At the walls, the umbrella drops about 4' from the height directly behind the house to a lower level NE and NW from the house, which means that the umbrella runs behind and under the walls. 
The section of umbrella associated with the walls must be built first and the walls backfilled before the rest of the umbrella can be installed. 

Most of the north earth contact wall of the house is 12' high then it transitions abruptly to eight foot walls near each end (see drawing).  The most
Excavation complete, metal posts in
place and a sand bed for the
horizontal insulaiton
efficient way to 
maximize earth contact for the house while dealing with the transitions is with retaining walls.  Both the east and west umbrella walls have to be about 5' high next to the house and, by running them diagonally east and west, can be as low as a couple of feet 20' out from the house.  The excavations for the two walls were about half track loader work and about half hand work and took me the biggest part of two days. Then it was time to install the vertical and horizontal insulation and plastic sheeting that allows the umbrella to drop beneath the walls.

The horizontal insulation under the walls is merely a matter of making sure the excavation drains well, is smooth and
First layer of plastic lies under the horizontal insulation
and behind the vertical vertical insulation
covered with enough sand that the insulation can be nestled into it to handle better the load of the wall.  The vertical insulation is another matter.  It must be supported so that it is not damaged by laying

rocks against it on one side and backfilling against it on the other.  So I wired the insulation to metal fence posts that were shortened as necessary to stay below the
Horizontal insulation over two additional layers of
6 mil plastic sheeting
final height of the insulation.

I proceeded by setting two posts, one near the house and the other about 20' from the house. Their height matched the linear slope that the insulation will take from a high end next to the house to a low end next to the French (the French drain that will drain water from the top of the umbrella -- grist for another post). Intervening posts were driven along a mason line stretched between the first two posts (east wall) or, better yet,the edge of the horizontal insulation (west wall). I stood the vertical insulation on the horizontal insulation and stabilized it with clamps and props while I snapped a chalk line
Vertical insulation wired to posts

designating the sloping height of the insulation then used a wood-cutting blade in a reciprocating saw to cut the insulation to height.  I then set all of the insulation aside while I covered the soil on which the horizontal insulation will rest with a layer of sand and screeded it smooth for a solid foundation for the insulation and the rock wall above it.

A 6 mil plastic sheet was laid down over the sand such that there was 5- 6' of excess running in both directions for later shingling with the plastic sheeting of the umbrella above and below the wall.

After the horizontal insulation was returned to position and the vertical insulation was wired
to the posts and secured with more wire as needed between the posts, two equal-sized pieces of plastic sheeting were then draped over
Two more layers of 6 mil plastic sheeting covering the
the top of the vertical insulation to be spread out in two directions: (a) down the front side of the vertical insulation and over the top of the horizontal insulation plus plenty of excess running horizontally to be shingled over the plastic sheeting of the umbrella below the wall and (b) an equal amount of excess running horizontally off of the top of the vertical insulation eventually to be shingled under the plastic of the umbrella above the wall.

The horizontal insulation under the wall loosely follows the plan for the rest of the umbrella behind the house, i.e., 4" thick for the first 8' out from the house, 3" thick for another 8', then 2" thick for the final 4' -- 20' in all.  The vertical insulation is 4" thick throughout with extruded polystyrene on the stone side and the weaker (but cheaper)
Generous layer of sand over the plastic to protect it from
injudicious stone placement; boards protect loose edges
 of the plastic from foot traffic during stone placement
expanded polystyrene on the backfill side.

Protecting the Vertical Insulation
The first retaining wall was erected a year and a half ago with the help of weekend volunteers using salvaged stones from a 19th century barn foundation. Similarly, we did the new retaining walls with weekend volunteers using more of the same stones . I either piled sand next to the wall or parked the track loader with sand nearby so that a layer of sand could be used between stones vertically and horizontally to help situate and stabilize them, considering their discordant sizes and shapes.  

Using sand as a filler meant that the space between the vertical insulation (covered with plastic sheeting) and the stones was also filled with sand.  But, without backfilling, the weight of the sand against the insulation or any pressure from the stones would distort the insulation -- a serious problem that partially collapsed the insulation and compromised the R-value of the wall built previously. Consequently, I fastened with 12" spikes a 2 x 6 over the horizontal insulation such that it abutted and stabilized the bottom of the vertical insulation.  I then was able to backfill the vertical insulation with sand, a shovel-full at a time, to a depth of a foot or so. The 2 x 6 was then removed and the spikes were reinserted to stabilize the insulation during wall-building.  The rest of the backfilling was done gradually in tantum with building the wall.

Building the Wall
I feel blessed to have had enough weekend volunteers, not only to build the wall we have
Stone wall well under way
been discussing here, but to do all of the other "stone work" that had to be done before I could finish the dirt work on on the critical east and south sides of the house.

Whereas the first retaining wall went up too fast for a high-quality outcome, the new wall took all of a half-day to build and with a satisfying result.  I had already stached nearby what I thought were enough stones and sand to build the wall. My role then was to give general instructions as to its size, height and the need to be careful not to damage the plastic sheeting or the stuccoed house wall.  The volunteers were free to place the stones as they saw fit and they did a great job.  Delegating paid off because I had under-estimated the number of stones and sand necessary and was kept busy with the track loader ferrying in more materials.

The Second Day
The second day was as much about hiding
Ugly French drain mouths
drain tiles as about building retaining walls.  The exposed mouths of the French drains and the north concrete wall foundation drains were so ugly as to beg camouflaging with something that would eventually enhance the native gardening that will surround them.  I had pinned down weed barrier fabric in the areas to be covered with stone and asked the crew to be
Stones added for aesthetics and to control erosion;
notice original retaining wall in the background 
creative in installing the stones.  Again the result exceeded expectations.

Then on to a couple of retaining walls. The first was next to the garage doors, which, by now, was hardly a challenge for the volunteers.  The second involved adding height to the first retaining wall at its house end.  Again, all I did was bring in the stones with the loader -- the crew did the rest un-micro-managed.

With the stones in place, I needed to take advantage of the relative dry fall to finish the
Raising the original wall next to the house
dirt work and plant seed -- the subject of the next post.

The Second Umbrella Wall
The retaining wall detailed above is the east-most umbrella wall.  The west-most umbrella wall was completed only to point of receiving stones.  With the weather window for the rest of the dirt work closing fast, the umbrella behind and to the west of the house became lower priority.  The wall will have to be finished in the ensuring months in conjunction with installation of the rest of the umbrella.

Sunday, September 17, 2017

Construction - The "Tall" Roof Revisited

The north-facing roof over the taller part of the house was begun a few months ago and covered with a 6 mil plastic until the time was ripe to complete it.  A prior post details its construction -- a cathedral roof using trusses -- and a recent post talks about temporizing it with sheet plastic.  Still another post discusses air and moisture barriers and uses the tall roof as a cathedral roof example.  

The design of the two main roofs, south-facing and north-facing, calls for cathedral roofs with "mini-attics".  The rationale for a mini-attic is detailed in the recent post, ventilated cathedral roof, showing construction of the south-facing roof from the truss phase to the temporization phase.  Now it is time to build out the mini-attic for the north roof and temporize the roof in a more substantial manner than with 6 mil plastic. 

Completion of the Mini-Attic
The mini-attic comprises two layers of sheathing separated by 2x4s on
The first layer of plywood sheathing was air-sealed with
flashing tape previously; the 2 x 4s on edge support the
second layer of OSB sheathing and provide for the over-
 hanging eaves and ventilation bays  
edge so as to create a ventilated space between the insulation and the roof cladding.  As rationalized in the post on the south-facing roof, I used plywood for the first layer of sheathing because it "breathes" better than OSB for drying purposes but used OSB for the second layer because it is cheaper and breathing was not an issue. We
sealed the cracks in the plywood layer, as well as its junction with the wall sheathing, with flashing tape to provide the definitive air and moisture barrier for the roof assembly (as opposed to relying solely on a barrier on the interior plane of the ceiling). The two-by-fours were fastened to the underlying trusses with construction screws to create the 3 1/2" tall ventilation bays, to provide the overhanging eaves and to support the OSB sheathing.

We installed a 1-by ridge board at the top of the roof in anticipation of the overhang for the second story clerestory windows extending southward from the ridge. Ventilation of the mini-attic will be by natural convection from eave vents at the lower end and an uninterrupted ridge vent at the upper end.  Accordingly, we left the sheathing 2" short of the ridge to provide an airway for the ridge vent.

Vapor Control
As discussed in the links in the first paragraph, the slope of the roof was less than originally planned by about 6" in 12', i.e., 12/2 instead of 12/2.5.  The slope is the minimum allowed by code for asphalt shingles and for standing seam metal roofing. The unfortunate decrease in slope was was due to building the first interior wall for 2 x 12 rafters instead of the 16" trusses that I eventually used and getting it a little too tall even then for 2 x 12s.  The low slope means that our moisture control strategy must use a moisture impermeable material that favors wet prevention over drying under the roof cladding (for details see the post on vapor barriers).  This strategy works because the mini-attic drys the OSB sheathing from below. Without it, any moisture that frustrated the drywall side of the roof assembly or found its way under the roof cladding would wet the sheathing and, over time, cause wood rot, mold growth and diminished R-value of the insulation.

Provisional Roof Cladding
For the other low-slope roofs, I plan to use 30# felt as the wet prevention material under standing seam metal roofing.  However, for the tall roof that is far above street level and faces away from the street, I am compromising in favor of asphalt shingles for the final roof in order to save money.  And it would be convenient to postpone installation of the shingles indefinitely for two reasons: to defer their cost and, since our dry months are August - October, to free up time to get on with the dirt work that needs to be done before it is too late for turf seeding.

A few years ago in anticipation for building the house, I bought on Craigslist 12 rolls of roofing.  It was cheap and I thought that it might somehow come in handy during construction.  As it turned out, I used it on the tall roof to double as the wet prevention layer instead of 30# felt and to serve as a provisional roof until such time shingles, or even a metal panels, are installed over it.

Ice and Water Shield
The code calls for a dedicated ice and water shield extending
Two courses of ice and water shield adhere directly to
the sheathing covering the drip edge at the eave
up-roof from the eave 3' beyond the plane of the interior wall.  For us with our wide overhang and thick wall, that spec meant two 3' courses. While shopping for the shield at the local home center, I lucked onto a roofing contractor who was buying it for himself. He recommended the cheaper of the two brands in stock as being the easiest to install and, indeed, it was a snap. (For installation details, go to the Tarco website.)  It is applied sticky side down directly to the sheathing.

Drip Edge
By covering the junction of the sheathing and the facia, the drip edge protects the junction from water penetration.
We installed abbreviated pieces of 15# felt at the rakes
so the drip edge could be installed over them; the felt
overlaps the ice and water shield down-roof; the rest of
the felt was installed just ahead of the roll roofing in order
to prevent wind damage to it before it could be covered
 As dictated by code, we fastened it under the ice and water shield at the eave and over the 15# felt under-layment at the rakes using roofing nails.

Imperfections in the Craigslist roofing were caused by
 improper storage but appearance was not critical since
 a definitive roof will cover the roll roofing eventually

Installation of the Roll Roofing
There are two common ways to install roll roofing. The simplest method is simply to fasten the overlap between courses with roofing nails but moisture penetration can occur around the exposed nails unless they are covered with roofing cement and horizontal rain might undermine the overlap and reach the sheathing. The better approach is to nail the top edge only and seal the overlap between courses with roofing cement such that the nails remain hidden and the lap is sealed.  Although it was much more time-consuming and costly (by the amount of four pails of cement), I opted for the leak-proof second method.
Only a couple of courses to go then its clean-up and 
on to a month's worth of dirt work

Due to the fact that the Craigslist rolls had been improperly stored laying down before I purchased them, they were imperfect and would not have passed muster if appearance had been important.  But, since the roll roofing will be covered with shingles eventually, protection was all that counted. Eighteen rolls total were required, ten of which were the Craigslist rolls.  We found the online installation instructions for roll roofing to be helpful but we deviated from them to the extent of using 15# felt as under-layment instead of fastening the roofing directly to the sheathing.

My thanks to friend, "Pat the Plumber", who, though being a journeyman plumber, helped with the roof.  And to Keith, my step-son, who pitched in as well.  Thanks, guys.

Sunday, August 6, 2017

Construction - Temporary Protection

WARNING!  This post is surely the most boring one so far.  What makes it worth the effort, though, is that it could be a gift for any slow-paced DIY home-builder having to contend with the elements like we have to do here in the lower Midwest.

I have spent considerable time installing and maintaining temporary protection to keep the weather from degrading the bones and skin of the building before I could get the final roof and wall cladding installed. In the process, I have learned some things, good and bad, about temporary protection that might save others time and money. 

The pictures of the temporary covers start with a southeast view and rotate around the house counterclockwise to a southwest view.  Click on any pic to enlarge it.

Material Selection
Five materials for temporary protection come to mind.
Southeast view including east garage wall to the right
Six mil clear plastic sheeting is way too temporary. It allows UV rays to penetrate completely through its thickness so it starts coming apart in a matter of weeks during the hot summer. The consumer grade of tarpaulin degrades in the sun within a year or so.  First it starts leaking then begins coming apart in the wind. Housewrap such as Tyvec is often used for temporization but a web search suggests that UV damage undermines its water resistance under the cladding later -- and the amount of damage is directly proportional to the length of exposure. Therefore, it is only
Northeast view
material I have not tried. Six mil black plastic sheeting is a happier story. Only the surface is damaged by UV rays and at such a slow rate that it maintains its integrity for many months.  

My experience leads me to believe that, for wall sheathing, the best material for temporary protection is the wrap in which dimension lumber is shipped and stored. It repels water and is much tougher than my second choice, black plastic, making it ideal for use in windy conditions.  It is the only
North view
material discussed here that stays in place pretty well with staples. All the others rip loose easily, although tarps are a little better than plastic in this regard (at least when they are first installed). Since lumber wrap is usually white, it has the added advantage over black plastic of 
having a higher solar reflectance that keeps a building a little cooler in summer.  For this reason, I would make sure that it went onto the south and west walls if the
Northwest view
supply of lumber wrap was limited
. I would not use lumber wrap on the roof, however.  It typically has had a hard life and comes with many large and small holes that would have to be seriously patched for adequate protection.  Having said that, it would probably hold up to hail better than plastic. 

Sad Experience with 30# Felt Paper
For temporary roof protection, a sixth material, 30# felt, might seem to be a useful alternative, especially since it could be left eventually as underlayment for the final roof.
West view
 However, for whatever it is worth, my recent experience with it has been totally negative. Let me explain.

Back in the '70s, I used 30# felt for temporary protection of a large porch addition. The porch was protected by the felt for a couple of years while I waited to roof it in conjunction with roofing the garage. In retrospect, I think it held up as well as it did only because it was on the leeward side of the house and shaded most of the day.

In the most recent post, I made the case for using 30# felt as underlayment for standing seam metal roofing on our low-pitched roofs. So, based on my positive experience with the
Southwest view; notice that the white lumber wrap, as far
as it went, was used mostly on the hotter south walls
porch, I thought it made sense to make the leap to two layers of 30# felt, using the first layer as temporary protection until all of the roofs were ready for installation then using the second layer when the metal roof went on.  

We fastened the first layer of felt with roofing nails 18" or so apart and congratulated ourselves for getting the roof covered in time for the storm that night. Whoops! About a third of the felt was partially dislodged by the wind and had to be re-positioned and re-nailed as best we could considering that it now was badly wrinkled. As
30# felt before it was partially peeled off by high winds
for the second time despite the use of roofing cement
and extra nailing
we re-nailed it, we held the nails back from the edges to leave room for roofing cement in an attempt to seal down the edges. Unfortunately, by then the edges were so badly curled either by the storm or the hot sun or both and had enough memory to lift out of the cement before it had set. Or maybe roofing cement does not work well with felt.  In any case, we used another course of nails near the edges to augment the cement but by now the roof was becoming so irregular that I began to worry that a smooth profile of the metal roofing would be distorted.  To add to my frustration, another section of felt was dislodged with the next major storm and the hot sun caused the edges of the felt to lift in the areas that had been cemented but not been held down by a second course of nails..  

Altogether, we wasted about 14 man-hours trying to make the felt work.  In the end, it was
6 mil plastic sheeting battened down over the felt;
the Masonite sheets reinstalled to protect the plastic
while the overhang for the second story is being built;
notice the black duct tape patches on the lumber wrap
just easier to cover it with black plastic and call it a day.  My plans now are to remove and discard the 30# felt, pull the nails, lay down a new single layer of 30# as the metal roof goes on but protect it from the sun until it can be covered by the roofing. Then the metal will lay flat and smooth for the best appearance.

Batten Boards Are Essential
The secret to success with any temporary covering is to secure it with batten boards. Otherwise the wind finds its way under the covering and whips it loose immediately (plastic) or eventually (tarps) and, occasionally (lumber wraps).  And the boards have to be screwed down.  The whipping action of the covering in the wind easily works nailed battens loose from 1/2" sheathing, especially on the roof. It is bad enough that the boards end up on the ground but, if on the roof, the protruding nails cut holes in the covering as they bounce along. (I used bright nail for the battens; maybe ring shank nails would have held but been harder to remove later.)

My batten boards were sawed from salvaged 1 x 6 tongue and groove flooring that would have otherwise gone unused.  One and five-eights inch drywall screws are long enough as fasteners.  I have noticed that, when the boards are removed, the holes in the plastic under the battens made by the screws are enlarged by wind action.  Chances are that, despite the overlying board, some moisture reaches the sheathing but not sufficient quantity to make a difference if the covering is not left in place indefinitely.

Batten boards at right angles to the flow of the water on the roof have to be installed at a slight angle to redirect the flow.  On the roof, plastic needs to have all its peripheral edges battened down.  On sidewalls, the top is sheltered enough by the eaves to omit horizontal battening but the bottom should be secured with angled boards that do not dam the water running down the wall.  The horizontal boards can be eliminated entirely for lumber wrap as it is tough enough to stay put at the top and bottom with just staples between the vertical battens.

The covering is best installed when there is no wind.  It can then be spread out and tacked down with only enough staples to hold it flat while the batten boards are screwed to place. And a covering installed on a calm day will definitely be tighter and smoother.

Patching the Coverings
Garden variety duct tape works well for patching holes in any of the coverings except felt paper.  Anywhere plastic is stapled there is the potential for leakage so, if the covering is going to stay in place for a long time, I recommend minimizing the number of staples and then covering them with tape, knowing full well that they will cause leakage eventually. Lumber wrap always comes with holes some of which require serious patching.  For large holes, I cut the right size piece of scrap and taped it to place; for smaller holes, a piece of tape by itself worked fine.

The slap stapler is a very handy tool for most stapling jobs.  However, it scuffs holes in plastic sheeting when the handle is not held parallel with the plastic.  Increasingly, I have resorted to the traditional squeeze stapler for plastic sheeting in order to be more precise with staple placement and minimize the number of holes that have to be patched.

Honeydew Time
Temporary protection buys time.  In due time, I will be either removing it or folding it back out of the way a small section at a time as needed for such tasks as installing the second layer of sheathing for the north-sloping roof (as described in the prior post for the south-facing roof), building the overhang for the second story windows and installing the metal roof and metal siding.

The temporary protection also eliminates urgency, allowing me to take a week or two off for several major "honeydew" projects that have gone wanting for several months.

Wednesday, July 5, 2017

Construction - Ventilated Cathedral Roof

The design of our cathedral roofs incorporates a "mini-attic" between two layers of sheathing.  The space between layers offers several advantages:  (a) primary air sealing at the sheathing level (secondary air sealing will be at the drywall level), (b) ventilation for a cooler roof in summer and to dry any moisture making its way into the ceiling/roof cavity and (c) a much more interesting interior space than would be the case with a typical horizontal 8' ceiling.

This post describes the construction of the south-facing cathedral roof over the single story portion of the house.  A prior post described the first cathedral ceiling/roof but left the story untold beyond the first layer of sheathing.  Here the entire procedure is described except for installation of the final metal cladding. Reminder:  click on any photo to enlarge it for detail.

Roof Trusses
The actual R-value of a roof or wall is not only about the R-value of the insulation but also the amount of thermal bridging through the structural members, especially through solid 2x lumber.  Because trusses minimize through-and-through structural members, they have less thermal bridging than solid rafters like 2 x 12s (manufactured I-beam rafters have even less).  The roof trusses that we used for the second story cathedral ceiling were 16" tall, i.e., they accommodated 16" of insulation that would provide R-50+ before thermal bridging.  

After the trusses were installed, I wondered how expensive it would have been to have increased the height of the trusses to 18" as a way of compensating for at least some of the thermal bridging.  When ordering the new set of trusses for the first story roof, I had them quoted both ways -- 16" and 18".  I was pleasantly surprised that the difference in price was less than $4 per truss so, naturally, I bought the taller trusses. Of course, the additional 2" will increase the cost of insulation by 10% but we will be using inexpensive rice hulls and, after buying a trailer-load, will have a surplus anyway.

Roof Slope Issues
The plans called for a roof pitch of 12:2.5 which would have more than satisfied the code
The left ends of the trusses were designed to rest equally
on both sets of double top plates; the right ends rest on

a double 2 x 6 ledge board; at a little more than 12:2
 barely meets code for standing seam metal roof
minimum of 12:2 for standing seam metal roofing.  However, the plans show 2 x 12s instead of 18" trusses and the truss manufacturer dropped the pitch by 3" inches for unexplained reasons. The two together meant that the slope was not as steep as planned. While it meets code, I am faced with the same issues that I discussed in detail in the most recent post that lead to the adoption of a wet prevention strategy under the metal roof as opposed to the more ideal drying strategy.  

Truss Installation
The trusses were supported on one end by the top of the exterior wall and on the other end by a ledger comprising two back-to-back 2 x 6s fastened to the second story wall with nails and construction screws.  After the layout lines were drawn on the ledger and top plate of the wall, installing the trusses becomes realistically a two-person operation.  One person lines one end of a truss with the layout line and with the outer edge of the top plate of the wall and the other person lines the other end with the layout line on the ledger.  We used both nails and construction screws to fasten the trusses.  The latter, angled up through the top plate and into the bottom chord of the truss, is a code-compliant alternative to steel rafter ties. In addition to nailing, the other end was secured with one set of construction screws up through the bottom of the ledger and another set near the top of the truss through blocking in the wall.


First layer of sheathing.  All
cracks are air-sealed with
 flashing tape
The function of the first layer of sheathing is twofold:  to provide easy air-sealing at the sheathing level and to serve as the floor for the "mini-attic".  The intentional gaps between sheets that would be hard to caulk or foam from below but are easily handled from above with flashing tape. Even the junction between the sidewall sheathing and the roof sheathing can easily be sealed with tape.  But before doing so at the eave side of the roof, we backed up the junction with 2 x 4 blocking internally.  We used 1/2" plywood for the first layer of sheathing because is "breathes" better than OSB should any moisture find its way into the ceiling cavity. 

Traditionally, a leaky area for air infiltration/exfiltration is the space between the double top plates which, if it is addressed at all, is typically caulked interiorly.  I as the nearby photo shows, I used tape on it, albeit duct tape instead of flashing tape because it is cheaper and fit better.  Duct tape is not nearly as sticky as flashing tape but will be held in place by the sheathing should the stickiness wane.  And, with my being the belt and suspender type, it will get caulked on the inside as well.
Duct tape is used to close the gap between the two top
plates before the sheathing goes on; the junction
between the wall and roof sheathing is not well supported
 until blocking is added between trusses for support from
the inside;  taping the junction had to be postponed until
it was supported

The next job was to add another layer of sheathing to create
The 2 x 4 supports for the second layer of sheathing
fastened directly on top of the trusses using construction
screws; notice the space at the left next to the second
story wall that will allow a free flow of air between all of
the bays; eventually roof vents will be installed over it to
allow hot air to escape
an air space for ventilation which I am calling a "mini-attic".  In order to create space and support the second layer of sheathing  we used construction screws to fasten 2 x 4s on edge over the trusses.  The first layer of sheathing was 16' wide, i.e., 16' from the second story wall to the eave. We held the 16' 2 x 4s away from the second story wall enough to accomplish two things (1) to extend past the lower story wall enough to serve as rafter tails for a 24" eave overhang and (2) to create space next to the second story wall that would be continuous from east to west such that air convected up the roof from eave vents could co-mingle with air from the other bays.  In this way only a four or five roof vents will suffice for 24 bays. 

Extensions of the lookouts (left) and rafters (right); notice
 the flashing tape at the junction of the rake wall and the
 roof but the absence of it on the eave side -- it had to be
 postponed until the junction was supported by blocking

We used OSB for the second layer of sheathing over the 2 x 4s for a couple of reasons -- in order to save cost and because, with the mini-attic below, trapping moisture will not be an issue.  If I had it to do over again, however, I would have installed 2 x 4 blocks in the gap between the long 2 x 4s and the second story wall.  I would have aligned them with the long 2 x 4s and made them short enough not to interfere with air movement but long enough not to split when the OSB was nailed to them.  Without them, the OSB was too springy as it rested on the ends of the long 2 x 4s then bridged the 21" gap before resting on the 2 x 4 next to the wall.  As it was, I had to use metal strapping to stiffen the junction between OSB sheets but could do nothing about supporting the sheet between edges.

Temporary Protection
It will be several months before all of the roofs are ready for installation of the metal roof so temporary protection for the sheathing was mandatory.  I used battened-
The mini-attic completed:  the subfacia and the second layer of
sheathing are installed and protected by 30# felt paper
down 6 mil plastic to protect the sheathing on the two roofs that were installed earlier -- the north facing roof over the second story and the west facing first story roof. After much waffling about what kind of underlayment to use under the eventual standing seam metal roofing, I settled on, not just one layer, but two layers of 30# felt.  The reasoning was that a second layer might add a measure of insurance for the lower than expected roof pitch.  My plan was
Masonite protects the felt while working on the wall
to use the first layer for temporary protection and lay down the second at the time the metal was installed.

As will be explained in the next post, using 30# felt as a temporary covering proved to be a bad idea.  Despite conscientious fastening with roofing nails, several areas peeled away with the first heavy wind. Augmenting with roofing cement was only marginally better -- several pieces even then lifted off as if the cement did not adhere well to the felt at least at high summer temperatures.  Also the felt that stayed put badly wrinkled but I am not sure why -- whether it was from getting wet or from thermal expansion or both.  Some of the wrinkles were so severe that I was afraid they might keep the metal roofing from laying flat as it should for good appearance. So I punted and added 6 mil plastic over the felt until it could be removed and replaced in conjunction with installing the metal roof.

I had a surplus of Craigslist Masonite so I screwed down a few sheets next to the second story wall so the felt will be protected while we build the overhang for the second story windows and install the windows.

Thursday, June 15, 2017

Construction - First Rake Walls; First and Only Conventional (Non-Cathedral) Roof

This post discusses three diverse topics.  First, it describes built-in-place wall trusses for the rake walls (as opposed to the pre-made trusses used for the eight foot walls).  Second, it explains how our low pitched roofs evolved.  And, third, it shares a bad design for a roof assembly that necessarily extends the conversation on vapor barriers that was started in the first of two posts on air and vapor barriers and a conversation that will undoubtedly come up often in future posts until the building envelope is fully completed inside and out.  When I set out to build a house, I understood the importance of air sealing but lacked an appreciation for the importance of moisture control and how controversial and misunderstood it is for contractors and permitting authorities.

Rake Walls
We stick-built the east rake wall for the second story on top of the first story pre-made wall
The east rake wall on top of the first story wall trusses
trusses that separate the living quarters from the garage. The west second story rake wall rested on top of the beam over part of the master bedroom. Two-by-sixes were used for both walls instead of 2 x 4s only because I had exhausted my supply of salvaged 2 x 4s but had plenty of salvaged 2 x 6s left.  In order to provide a 15" wall cavity for insulation like the rest of the stick-built exterior walls, the rake walls were essentially "double walls" patterned after the pre-made wall trusses, even to the extent of turning the 2 x 6s 90 degrees like the 2 x 4s in the trusses and stabilizing them with gussets like those in the trusses.  

The west rake wall on top of the LVL beam

Sheathing the rake walls had to be postponed until the shed roofs extending from them had been completed. So we covered them with battened-down sheet plastic in order to protect them, especially their plywood gussets, from the elements.

A Word About Temporary Coverings
The blue tarp covering the roof in the second photo had enough UV deterioration from a prior use that did not protect the sheathing and had to be replaced with battened-down 6 mil plastic sheeting. Black plastic resists deterioration longer than garden variety tarps and much longer than clear plastic.  The reason for its longer life is that UV rays cause minor damage only to the surface rather than penetrating through and damaging the full thickness of the material as with clear plastic.  Staples alone do a pretty good job of securing tarps in the wind while plastic easily tears loose from staples.  It takes screw-retained batten boards to hold it. Nailing the boards to 1/2" thick sheathing, particularly with bright nails rather than serrated nails, does not work either.  In a stiff wind, the boards end up on the ground or, worse yet, flailing around on the roof with nails protruding, tearing holes in the plastic.

Conventional Roof
The living space between the LVL beam and the west concrete wall as seen in the second photo needed to be covered by a shed roof attached to the rake wall and resting on the concrete wall.  I wavered between the cathedral type trusses we used for the second story roof or more typical trusses.  I made the mistake of choosing the latter.
OSB blocking attached to the top chord of the trusses; they
extend from the outside wall to  a height on the chord that
 will allow 18" of  insulation without any of it blocking the
ventilation pathway between the attic and the eaves  

The cathedral approach would be easier to insulate (by a technique I have planned for the other cathedral ceilings and will be detailing in future posts). The conventional attic created by the low roof pitch is so confining that crawling around in it to blow in the insulation won't be fun. Extra work was required for installing blocking between the rafters to hold the future insulation at bay and maintain a patent airway to the eaves for proper ventilation.  And, to make matters worse, I was unaware that I needed specifically to request that the truss company make the height of the two end trusses 3 1/2" shorter to allow for stick-built lookouts to support the fly (facia) rafters. As a result, I will have to "Jerry-rig" (pun intended) something to make the lookouts strong enough to support a two-foot overhang.

Why Such Low-Pitched Roofs?
The building code specifies that the window area be 8% of inhabitable space.  All of our windows but one are confined to the south facade so, in order to meet code, the clerestory windows on the second story had to be much larger than is typical for clerestories.  The height of the second story wall was increased by 30" over a standard 8' wall in order to create enough space between the bottom of the windows and the shed roof to allow for the pitch of the roof.  The 30" figure was purely arbitrary as I was trying to strike a balance between making the wall inordinately tall and providing for adequate roof pitch.

The plans called for a 2.5 - 12 roof pitch for all of the roofs but envisioned 2 x 12s as rafters. The trusses that I chose instead of 2 x 12s were 16" (north slope) and 18" (south slope) deep to give more room for insulation. The added height caused both roofs to drop below the target roof pitch.  Knowing what I do now, I would have raised the second story wall by at least 6" and lowered the exterior walls by at least 6" to give a steeper roof pitch.  Even then, since there were no windows involved, I could have pitched the west-facing conventional roof higher but, for aesthetic reasons wanted to hold it to slightly below the height of the south-facing shed roof.

Foil-backed Sheathing
I had already purchased and installed OSB sheathing with foil backing over the conventional trusses by the time I had finished the research for the recent post on barriers. The research convinced me that OSB without foil or plywood would have been a better choices.

(In order to make more sense of the next couple of paragraphs, I would recommend recent post on barriers.)

The reason why foil-backed OSB was not a wise choice is that it is a vapor impermeable on the foil side, meaning that any moisture that breaches the roof cladding and the fabric or felt paper under it will have to dry towards the attic side of the sheathing.  And it has to be assumed that some moisture will find its way under the metal roofing, especially at our low roof pitches.  As discussed at length in the most recent post on barriers, a vapor permeable barrier such as Slopeshield could be used to shield the OSB from moisture penetration but allow any moisture that does breach it to return back out through it for drying, but it is not recommended for roof slopes as low as ours.

In a recent post on barriers. I quoted Listiburek's as follows:

"Avoid using vapor barriers where vapor retarders will suffice; avoid vapor retarders where vapor permeable materials will work; thereby "encouraging drying mechanisms over wetting prevention mechanisms."  (Italics and underlines are mine)

Since our low pitchness for standing seam metal roofs increases the potential for moisture penetration, I am stuck with going against his recommendation and using a wetting prevention approach for all of the roofs, relying on the interior surfaces of the sheathing to be vapor permeable enough for drying .  Therefore, I intentionally scuffed up the (expensive!) foil so as to make the OSB at least somewhat vapor permeable interiorly without totally compromising the radiation reflectance of the foil.

Temporary Protection  
The black plastic temporarily protecting the rake wall and the roof was removed in conjunction with sheathing the rake wall above the new roof. It was reapplied over the wall sheathing but not the roof sheathing -- 30# felt was applied instead.  As will be fleshed out in subsequent posts, 30# felt was or will be used on all of the roofs as interim protection until the metal cladding is available.  Then a second layer of 30# felt will be applied in conjunction with installing the cladding.