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Insulating/Designing Walkable Flat Roof Deck over Conditioned Space

Derek Lum | Posted in GBA Pro Help on

Hello,

I would appreciate your advice on a renovation to a 70 year old post war gable roofed 1 1/2 story in a Zone 6 cold climate high snow load climate (Rossland BC Canada) annual snowfall 145 inches. I am confused particularly on the feasibility of an unvented roof in this climate and how to convince my inspector that it is possible.

The long axis of the house is oriented east-west. There is a 6’x12′ bump-out with concrete foundation on the west gable end wall that I am currently reframing for a flat roof with a walkable deck surface, sloping downward to an outside drip edge. I have a total of only 9’4″ from the unfinished floor to the bottom of the door roughout on the attic level above. We can use 2×8 or 2×10 joists oriented North South. 2×10 would leave me only 10″ from the top of the joists to the door bottom. I have considered Dr. Lstiburek designs for drain/exterior foam/drain/ pavers with an outside drip edge to be ideal but my contractor feels there will not be enough room, and I worry about the “complexity” as nobody is familiar with these “inverted” designs here. Our original plan was an unvented assembly of Tufdek 60mm vinyl decking/2×10 with spray foam in the joists, but the inspector has countered with a demand for venting below the roof deck.

The rest of my house is in the midst of retrofit. Adjacent main floor exterior walls are Hardiboard/1x4rainscreengap/1.5″XPS/3/4″T&GShiplap/2x4blowncellulose/noVB. The gable end walls at the attic level walls are currently 2×4 walls with Roxul R14 batts with poly VB on interior, but I can easily remove the VB and add my foam 1.5″XPS wall system.

I have researched on this site my options. I believe an unvented roof is preferable. I believe an inverted roof with protected waterproof membrane makes more sense than an exterior vinyl deck that will eventually leak but no one is familiar with this design here. My main difficulties are with concepts of preventing ice damming in high snow load and how to convince my inspector I don’t need to vent my roof. Do I? I also worry about the roof/wall interfaces re leakage and condensation potential and how to connect my water/air/thermal barriers.

My current considerations are

1. Non vented assembly Tufdek/roof sheathing/2×8 or 2×10 cavity filled with minimum 6″ closed cell spray foam for R42. Will this address ice damming in my climate?

2. Nonvented Assembly with exterior XPS: Tufdek/4″XPS/Roof Sheath/2×8 with Roxul Batts in cavity

3. Inverted Roof Assembly as per Dr. Lstiburek Pavers/Drain/4″XPS R20/Drain/Roofing membrane/ Roof sheating/2×8 with ?roxulbatts. Challenges include limited space available above joists.

4. Vented: Tufdek/roof sheating/ventilation gap running North South, no perforation of my gable end wall/2×8 will 6″closed cell foam R42.

5. Vented Roof as per inspector: Tufdeck/Roofsheathing/Vented space from outside drip edge to my gable end walls, perforating it, venting vertically, and venting from the middle of my gable end wall.

I appreciate your help

Derek

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