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built up roofs


John Dirks Jr
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Consider old built up roofs on city rowhomes. Let's say 60, 70, even 80 years old.

Over time they get added to, layer after layer. At some point, enough is enough.

How does one tell when its time to tear it all off? Do reputable roofers ever cut into them to measure the thickness and calculate loads?

I know theres lots of roofers out there looking for the quick buck of the overlay. I assume that many times they add more layers when they shouldn't.

Do any of you focus on this potential problem and how do you go about it?

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If it's 60+ years old, and it's never been torn off, it should be torn off. You've probably got 6-7 layers of material, meaning major tonnage on the old structure. Get rid of it. Weight loads are not the only consideration, though.

Overlays cannot get the parapet wall flashing correct. In almost every case, the parapets have parapet-itis, which is massive condensation under the sidewall flashing that's demolished the parapet inner wythe. If the inner wythe is demolished, you can't mechanically fasten the new membrane to it.

Yes, competent roofers will drill a core sample to see what's going on. That happens about twice a year around here. And, it's not really necessary in most cases; you can see what's going on in every other detail.

I will grudgingly concede that sometimes an overlay is OK for some very specific conditions, but as a general rule, it's a dogmeat way to go about installing a new roof.

I focus on it continually, simply by looking at the roof, and deciding what I would do if it was my roof.

Pay close attention to the detail @ the rear eave. Folding the membrane down into the gutter is wrong; you should have a flanged gutter, and the membrane should go under the flange and then be stripped with material over the flange. That way, it can't leak in ice dam conditions. Can't.

Make sure the gutter is replaced with a new 8" galvanized or keynar coated sheet metal, primed to accept the new membrane. Don't accept some crap extruded seamless aluminum gutter for flat roof applications.

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