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Directions on a Flat Roof Repair

Directions on a Flat Roof Repair

Directions on a Flat Roof Repair

My home has a flat roof, which requires complete removal and replacement due to "abundant" stachybotrys mold found on the underside of the plywood sheathing.  Root cause is thought to be insufficient venting of humid air from within the roof cavity, originating from leaks (fixed) and warm air leakage into the cavity, combined with poor intake at the mansard soffits and too few rooftop vents.   Original roof: tar and gravel applied in early seventies. Current roof was completely reconstructed in 1997.

Roof details: Mansard with parapet wall. Approx. 1000 square feet, gravel ballasted EDPM membrane, on fiberboard, on plywood sheathing, on 1x4 strapping, on top of 2x6 rafters filled with insulation, on top of and perpendicular to 2x8 joists filled with insulation over a kraft paper vapor/air barrier.

Venting Details: Eight 3x6 inch soffit vents on the underside of 2 sides of the mansard wall permitting air flow parallel to the "original" 2x8 joists. Roof vents: Eight 4-inch aluminum stacks.

Location: Near Toronto, Canada; environmentally similar to Buffalo with less snow, about 3 feet typical.

1. Is there a better replacement candidate than EDPM?  (somewhat sensitive to cost).
2. What's the best method of ventilation to overcome questionable air intake at the mansard soffits, which are 8 ft. under the cavity? I was thinking of replacing the plywood soffit with 100% perforated aluminum.  Is this sufficient?
3. Alternately, should I consider completely sealing the roof by completely filling the cavity with insulation and removing the intake and exhaust vents?
4. What's a recommended method of improving the air and vapor barrier when I will only have access from the top of the cavity?  I was thinking of using a product comprising plastic bubble warp with reflecting foil facing the warm side, over which I'd replace the insulation.
5. What thickness of plywood should I use to replace? Can I use OSB instead? If so, how thick?
6. I have 0% sloping to the scuppers.  What should it be? And how is it best achieved?

RE: Directions on a Flat Roof Repair

As a structural engineer - I have to skip your questions 1-4 as I can't help you there.

Question 5 - I would check with your local lumber yard or local residential builders as to the standard sheathing used in your area.  They would need to know what spacing your roof rafters are.  OSB is a viable alternative.

Question 6 - Most building codes anymore require 1/4 inch/ft of slope as a minimum.  You can achieve this with tapered insulation - but there's cost involved there.

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