Detailing wood framed parapets
I’m looking for anyone who has some parapet detailing experiences or opinions to share! Context is a wood framed 2-unit residential building with a flat roof with a TPO membrane, continuous rigid insulation and a parapet wall.
Typically, in a wood (or light-gauge steel) framed parapet situation, you seem to have two options to resist the wind loads on the parapet:
1. Diagonal kickers extending back from the parapet to the roof. It seems like these will cause a headache from a continuous insulation and roof membrane detailing standpoint.
2. Balloon frame the parapet wall. This causes a headache regarding moisture control. It seems that it requires that you use blocking and spray foam to form an air-barrier and vapor-retarder between conditioned space and the parapet like in figure 14 here: https://www.buildingscience.com/documents/insights/bsi-050-parapets-where-roofs-meet-walls
I think I’m leaning towards option 2, but I’m curious if anyone else has any experience with other options or approaches. Of if anyone has any experience with the details where the diagonal kickers, continuous insulation, and roof membrane all interact.
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Replies
Brendan,
The parapets I have built relied on the sheathing for their lateral support. The bottom plate is fastened through to the top chord of the trusses below. On the exterior side the sheathing is continuous with the wall below, and the interior side is sheathed so that the top and bottom plates are connected. Unless the parapet was fairly tall, I can't see why more lateral resistance would be necessary.
https://hammerandhand.com/best-practices/manual/6-roofs/6-3-parapet-wallls/
Hmmm. That was essentially our original detail, assuming that the sheathing would provide adequate support.
Our structural engineer came back with the balloon frame or kickers choice.
It's possible however that something got lost in the communication with the engineer and they didn't consider the support provided by the sheathing. Perhaps I will run it by them again.
Brendan, For what its worth, I have short parapets on three sides of the upper level roofs of my Santa Fe-ish style home. The wind here in the mountains can easily hit 50 and the parapets bending are not what I worry about. The membrane lifting is more of a concern despite heat welded seams and many washered screws. I don't know the installation details of TPO, but do be sure to ask about wind lift.
Portions of the parapets are constructed as ladder like frames where perpendicular to the truss alignment. Where parallel to the trusses, we framed balloon style adjacent to the truss which left a simple way to sheath both roof and wall. Like Malcolm describes, the sheathing on the "outside" of the walls is continuous, and inside nailed to the bottom of the ladder frame or blocking we added. The short height parapets sitting on the nail base deck, have shown no movement. Any movement would be revealed very quickly by the stucco skin over the outsulation attached to them. I am a bit terrified to now find out about membrane shrinkage potential pulling them over, but four years out, we are still good.
As for insulation details in conjunction with sheet insulation, I was faced with co-ordinating 8" nailbase on the roof decking and 6" of outsulation on the walls. The roof received the nailbase first before we applied the "perpendicular" parapets, so the only weak point in the insulation wrap occurs on the walls parallel to the trusses. The balloon framed parts of the 2x6 walls left pockets going up past the nailbase/roof deck which would have left a path to the very cold uninsulated roof side of the parapet.
Rather than try to wrap the parapet all the way around like Fig 5. in the referenced article, I chose to go with a version of Fig 14. By blocking between the vertical studs at the top height of the nail base deck, we created pockets to be filled with spray foam deep enough to block heat flow around the edges of the roof insulation. The exterior wall insulation had to go all the way to the top of the parapets, of course, in order to keep the exterior wall plane continuous. The wall caps, if full like in Fig 14., would be over 12" wide, but the pvc roof material we used is heat welded to a 2 1/2" L shaped drip edge nailed to 3/4" plywood parapet caps. The membrane covers the top of the parapet, the side and on down the roof plane. Looks fine from ground level and so far so good. I didn't put cants in and perhaps the roofers didn't either. It is hard to tell.
Using spray foam in the pockets was largely smooth sailing altough I did find two bays that required remedial filling of hidden air pockets. I might have been tempted to use a foam kit for what is a somewhat fussy detail, but the insulation contractor did pretty well. I did have them spray several other details where outsulation and roof member intersects made cut and cobble impractical. There has been no indication of heat loss along the parapets when it snows. I think I see more evidence of thermal issues from the 10" screws used to hold down the nailbase.
You haven't said what climate zone you are in so perhaps snow and ice build up isn't an issue for you. I opted to not create a fourth parapet with scupper exits for run-off and leave the entire width of the down slope roof edge open. I also left off gutters which don't play well with the ice build up we can get from sun driven melt off. Hope some of this info helps.