I have a low slope 1:12 on the back of my house. The roof is Dibiten (if I recall the name correctly) It is torch bonded and has worked for 22 years. It has no granules. The deck is plywood. I need to have the Dibiten replaced. One roofer wants to use Bakor 1 ply, with a sticky back after stripping the existing roof. Is this product suitable and can it be removed in the future or does it adhere to well to the plywood? The Bakor website does not seem to have any application information for this product. Another contractor wants to put a one ply torch down with granules on the existing roof without stripping it. I am concerned with the additional weight as the plywood is rather thin.
Option 2 would work around here…
I am not familair with Bakor. I assume it is a type of single ply membrane. But there are lots, EPDM, PVC, TPO, etc…
If I were to strip the roof I would not install directly to the plywood. I would install a seperation, called a cover board, such as a high density fiberboard or preferrably a high density ISO. There are numerous reasons for this. If the plywood wasn’t properly installed, meaning that it wasn’t screwed instead of nailed, the nails will back out right through the plywood. The manufacturers have specifications for installing directly to plywood but you have to adhere to those specs. Also a typical grade C plywood wouldn’t be my first choice. At least B, but preferrably A. Installing new plywood would cost most than it’s worth so see my original comment about the HD fiber or ISO boards.
If R value is a concern I would instead install Poly Isocyanurate insulation board (assuming the ceiling is notinsulated in this area as I would not want to create a warm pocket. If the ceiling is insulated and isulation is still a concern, have some of the plywood removed and fiberglass installed, now is the time.
As for torching the new modified over the existing, I do not know how many layers you have now so can’t comment about weight nor code. What you should understand about modified bitumen is that it is somewhat like a built up roof in the fact that it is comprised of multiple layers. A typical modified bitumen roof may only be two layers, a base sheet and a cap sheet. Depending on base and cap this may only be a 10-15 year rated roof. If you were to install a base sheet and a smooth mid ply, like what you have now, and then install a granulated cap over that this would be rated as a 20 year roof (at time of original installation).
Have we ever torched a cap over a existing smooth modified? Yes we have, but never oer a 20 year old modified. i wouldn’t expect them to last 20 more years however. Maybe only 10-15 years max. I can’t see how the old roof lasted that long unless you were coating it. If you were coating it this may be a problem for torch adhesion. If you wern’t coating it and want to go over, I would apply a coating of asphalt primer first before torching just to soak into the weathered nooks and crannies and cracks of the existing modified bitumen membrane.
Personally if I were your roofer normally this is how I approach most residential jobs. 1) tear it off. You always get a better job when you tear it off. 2) If the flat roof is not visible and doesn’t tie in with the shingles, meaning the flat roof ends at a wall or something, then I would be recommending single ply like TPO, PVC or EPDM. 3) If the roof is visible and you don’t like the look of single ply I would be recommending modified bitumen with a granulated cap sheet. Normally I like to install self adhering on wood structures, if there is any vinyl or wood siding directly in my work area there is no way I will torch. Self adhering will cost slightly higher than torch however. 4) If the roof meets with the shingles I would be reocmmended a 3 ply self adhering modified bitumen roof system. Why 3 ply? Because this is rated at 20 years and it would be a bad idea to install a 15 year rated flat roof with a 30+ year rated shingle.
Here is a good read on my website which can explain more about various Flat Roofing Options
let’s talk about accessories because a roof is just not the membrane. You need to also factor in the accessories such as terminations. For example who is isntalling what at the edges of the roof? There shold always be some kind of metal termination around all edges. This is commonly a gravel stop or drip edge at the gables, a termination bar our counter flashing at walls, and of course gutter. There are a few threads on this forum that discuss flat roof gutters. If someone is not approaching the roof as a system, more than just the membrane, they are installing an incomplete roof.