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. 

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