What's a good way to seal a flat concrete roof?

Question

My guess is that a long time ago there was a fire in part of our house and what was the top storey was capped and the floor replaced with thick slabs of concrete. This was then sealed over with bitumen and left as a flat roof, with traditional pitched roofs on either side.

Now, however, it seems something is failing and the roof has started to leak. It's always pooled water on the top to half an inch or so, but the last lot of heavy rain caused a stain on the ceiling underneath. That was easily redecorated, and I've made sure the drain to the gutter is properly free flowing now, but I don't want it to start again.

What's a good way to seal this roof so that doesn't happen again? I've used flash-band to patch up a couple of obvious cracks in the bitumen but I'd rather have something more permanent.

Answer 1

If you don't have a problem with water pooling any more, the real answer is probably to have a professional roofer look closely at the roof both on the inside and outside. The problem with a lot of roof leaks is that where the water shows up in the house isn't where it comes in. Water has an annoying tendency to run along internal beams and such, then come into the house far away. You can go over any edges with new tar and hope you catch it but in my experience this approach ends up being like playing whack-a-mole, you patch and wait for it to rain, then patch more and wait more, etc.

So I would say if you can do it yourself, look closely at the underside of the roof as well as the outside. If you don't find anything, a professional roofer is probably in order.

Answer 2

Consider EPDM rubber roofing. It can withstand ponding water (the same material is frequently used to line ponds) and doesn't require expensive specialized tools to install. The materials are frequently warranted for 20 years, and if your roof isn't too large, you can probably get away with a single-sheet seamless installation.

I'm planning a DIY install of EPDM on the roof of my garage, a detached structure made entirely of poured concrete. The rubber comes in a roll and is adhered to the roof deck using a liquid adhesive. If the roof deck isn't clean -- mine has decaying coatings of tar/bitumen and some sort of elastomeric coating -- you first fasten down rigid insulation boards, and then adhere the rubber on top of that. (The insulation boards are made of polyisocyanurate, and sometimes called "iso boards"; a 1/2" thickness is common but 1" may be useful on particularly uneven surfaces.)

As Tester101 suggested, Thermoset Membrane roofing is probably a better solution, but not really a DIY option because it requires heat-sealing equipment. EPDM doesn't seem to require anything more specialized than a paint roller for applying the adhesive. There are several vendors who sell the materials online, although I'd also suggest checking with local roofing supply stores because the shipping charges can be high, especially if you're using certain chemical adhesives that require special handling.

Answer 3

You could look into installing Thermoset Membrane roof.

Answer 4

Consider a spray foam roof. This solves another problem you presumably have: how to insulate a floor-turned-into-a-roof. Spray foam at that level must be professionally applied. On top of the foam goes an elastomeric coating which keeps the foam protected from UV light, and granules which protect the coating. The DIY part may be preparing the roof, tearing off the old material, and designing the gutters.

Foam is a very good long term option. It's expensive to make minor patches. But unlike asphalt shingles there's no definite lifetime: keep up the coating and it can last your lifetime and beyond.

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Also consider having a structural engineer look at the slab: is concrete in that place really a good idea? Is that going to crash down someday?