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. I used aluminum drip edge but, if the final roof were likely to be steel, I would have used steel drip edge in order to minimize the possibility of galvanic deterioration of the aluminum in the presence of moisture.  In fact, I probably should have used aluminum nails instead of steel roofing nails for the same reason.

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 lying 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.


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As a do-it-selfer-in-training, I welcome your comments.