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Question DetailsAsked on 7/12/2013

Have a flat roof dormer on home with little pitch-- had water issues-- is a rubber roof best way to go?? 30x 15 area.

Long story short--- an improperly installed vent pipe to replace the original pipe years ago has caused water to seep in under the shingles and finally ruin the ceiling and one wall due to years of moisture building up and torential rains lately. Insurance will cover the interior repairs..... a contractor has given options to the roof after the dormer section is cleared of 2 layers and water damaged boards are replaced--- shingles, rolled shingles, rubber roof--- the dormer section has very little pitch--allows water, snow to stay on it.can tell the contractor thinks highly of rubber roof..... 30x15 or 16 area. Thoughts???? Ball park costs???

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You don't say what the slope is - I am assuming 1:12 or steeper, otherwise it would be considered a "flat" roof and would require a flat roof solution, which it obviously was not designed for in the first place.

I would not go with an exposed membrane roof - they are just too susceptible to wind and blown branch damage on sloping roofs and roofs without parapets, and fairly rapidly degrade due to sunlight if not protected with a paver or gravel covering. With a flat roof you have few choices, with a sloping roof you have more choices.

First, assuming the dormer is relatively small, I would have them put some slope on it - either add a peak roof, or put on a 4-sided sloped sheathing the same as you do on a chimney that is being blocked up and abandoned in place (though that could look funny), so you will not have standing water on it. This should also have an overhang (eaves) so you will not get water running down the side of the gable and possibly into the siding, then into the attic and under the adjacent lower roof sections.

Personally, if you are committed to a shingle roof, I would go with full-coverage ice and water shield underlayment on the entire roof, which is a waterproof asphaltic sheet material (not just water resistant, like usual underlayment) over the entire surface rather than just the usual 3-6 feet of ice dam protection at the eaves, then dark-colored smooth metal shingles over that. That way any backup under the snow or at ice damming that manages to get under the shingles will run down the ice and water shield to the gutters/eaves, rather than penetrate the sheathing. Additional cost for the ice and water shield about $0.50-0.60/SF over what it would have cost for just tarpaper underlayment. If you use ice and water shield rather than a breathable synthetic underlayment, it is CRITICAL that you have excellent attic ventilation, because any water than manages to get UNDER the shield will not be able to evaporate up through it as it can with felt or synthetic overlayment, so the only way it can go is into the sheathing, which then has to have good underside ventilation to evaporate any excess moisture in it. The metal shingles should shed snow better (even though it takes a 2:12 to 3:12 slope to prevent significant buildup), which would reduce water backup. For a low slope roof, I would recommend smooth surface shingles, which can be bought ith a paint scheme that makes them look rough surfaced. The smooth surfaced shingles will shed snow earlier than the textured surface ones, which I have seen hold over 1 foot of snow before avalanching. Dark color will enhance shedding of snow.

Now - for the ventilation - this is a problem area on low-slope roofs in snow areas, because snow buildup at the ridge can cause meltwater backup into the ridge vents. The only solution for this is to build a raised ridge - basically a full-width celestory that sticks up above the roof 3-6 inches to prevent water backup into it, with the ridge vent and ridge shingles on top of that. Sort of a full-width vent hood, which to make it long-lasting should be made of painted galvanized sheet metal over wood, though it could be wood with ice and water shield coverage, flashing, and shingle facing in addition to the ridge shingles. Would be noticeable, but not excessively so if crried full length of all ridges.

Another totally different solution, which would actually be expected to be more effective in solving the water intrusion and snow damming problem, would be raised (standing) seam metal roofing with raised ridge vent. This does introduce a major snow avalanche issue, however, which might require building a couple of ridged porch roofs to divert snow slides away from doorways, as would metal shingle roofing. I would EMPHATICALLY recommend AGAINST any "avalanche guards" or "snow tabs" - all they do is ensure ice damming and water backup, which is just inviting leakage, especially on your low slope. Metal roofs do cost 1-1/2 to 2 times as much as asphalt shingle, but if you are planning on living in the house for a lot more years, could well outlast you. However, the resale value will probably suffer, as most people do not like metal roofing, especially standing seam roofing. Your Realtor can advise you on how much metal or standing seam metal roofing is likely to affect your property value - obviously ift will be much more dramatic if yours is the only metal-roofed house in the neighborhood. However, to solve your problem, this is the solution I would go with, other considerations aside.

Answered 6 years ago by LCD

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