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.

1 comment:

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