Finding Cause of Leak on a Flat Roof

Finding the source of a leaf goes from tricky to trickier when air conditioners and other units provide countless opportunities for water to get inside. Learn a simple way to deduce the source of a roof leak, as well as…

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LESLIE: Now we’ve got Pam in Illinois on the line dealing with a flat roof. Tell us what’s going on at your money pit.

PAM: We are having a problem finding a leak on the roof. We have a roof that’s flat but it has a slope to it. We have our air-conditioning units that sit up there, along with the roofing vents. We’ve used that white vinyl to seal it; it’s a rolled roofing, you know. We’ve sealed it with a white vinyl.

Around the air-conditioning units and the vents, we’ve used the black mastic tape but we can’t seem to get – to seal them. So do you have any suggestions? Something that would work?

TOM: So, do you know where the leak is?

PAM: Well, we’re thinking around the vent or the air-conditioning unit.

TOM: Have you tried to take a garden hose up there and strategically sort of flood that suspected area to see if you can cause the leak to happen?

PAM: No, we haven’t.

TOM: So that might be a good next step. Start low on the roof, because it’s sloped, and flood that area with the garden hose for 15 or 20 minutes. And then if nothing happens, move it up a few feet and a few feet and a few feet to see if you can narrow down the exact area where the leak is happening.

You’ve got a difficult situation, Pam, because first of all, rolled roofing is the weakest roofing material out there when it comes to low-sloped roofs. Secondly, you’ve got an air-conditioning compressor on the roof, probably sitting on 4x4s or something of that nature. So when the – where the air conditioner sits, as it goes on it vibrates. And so that vibration breaks down that roofing material, makes it really difficult for you to get something that’s leak-free.

If the roof was built in a different way – and by that, I mean if the rolled roofing was stripped off right down to the sheathing and the roof was built with a rubberized roof or something of that nature; and there are special types of support mechanisms for air conditioners that have flashing built into them, and guidelines for installing an air conditioner on a roof - then you wouldn’t be having this issue.

So there’s no sort – there’s no easy way to kind of make this go away. All I can really suggest is that you strategically try to find out where this leak is and then focus your tarring-application efforts right around those spaces. Unfortunately, it’s not going to be easy to find it but once you do, hopefully you can identify the weak link. And then, as a matter of preventative maintenance, you can get up there and then reseal it every once in a while.

PAM: OK. Well, you’ve certainly helped us and given us a lot of food for thought.