Flat Roofs

Flat Roofs Don’t Leak

Flat roofs have a bad reputation for leaks, but I don’t think flat roofs leak. My experience is that any leaks aren’t due to poor roofing or poor roof materials. The leaks are due to the flatness of the roof, which encourages use of the roof for HVAC equipment. 

Here is why that is a leak-producing situation, and what you can do about it. 
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Flat roofs don’t leak. 
What leaks are the holes you put in the roofing and poor roof terminations. 

Some Actual Cases
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First Case    A technical school had a 25 year old built-up tar and gravel roof. Maintenance was nonexistent. I could see the surface of the felts that made up the roof. The gravel and flood coat had completely eroded away. That’s not where the leaks were. In fact I have never seen a roof leak in the field of the roof. The leaks were at the exterior roof edge where the joints in the gravel stop had created cracks in the roof membrane due to expansion and contraction of the metal gravel stop. 

The school was re-roofed with edge conditions that met the roof manufacturer’s recommendations. 

Second Case   A seven year old school had leaks in a corridor and some offices on the first floor. Checking above the drop ceilings showed there were no penetrations through the roof above. A trip to the roof, however, made it obvious that all these leaks were along the back wall of the gym. Ahh, the flashing at the higher gym wall. Nope the flashings were intact and the counter-flashing was a copper design that I recognized from Graphic Standards; it looked perfect. After testing with a garden hose, the culprit was the mortar joint where the counter-flashing entered the brick gym wall. There was a hairline crack where the brick and copper met. The rainwater rolling down the masonry wall was flowing right into the hairline, back into the wall where it hit solid mortar, then went under the flashing and down the face of the wall and onto the ceiling. 

The counter-flashing was removed; the offending joint sealed; and a surface mounted counter-flashing installed with a mastic backing and a 45 degree ‘kick’ at the top to receive a non-setting sealant. 

Third Case   A manufacturing building had a leak in the middle of the floor. The culprit was pretty obvious. Directly above was a rooftop HVAC unit. The base flashings looked OK, but clearly there was a leak. After the flashings were redone...the leak was still there as though nothing had changed. We had the roofer check his flashings with a garden hose. No leaks. The rooftop unit was letting rainwater in through joints in its casing. No gaskets! 

Adding gaskets was extremely labor intensive for the HVAC unit manufacturer. 

​What You Can Do
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Intersections are the weak link. Pay special attention to where the work of two trades intersect. Mason and roofer. M/E contractors and roofer. Skylights and roofer. Siding / panels and roofer. 

Use big-name roofing manufacturers. Use manufacturers that include roof edges and flashings in their warranty and that publish recommended details for those conditions. Then make sure you get those details. 

Don’t put stuff on the roof. Whenever you can, put HVAC equipment inside or on the ground. It is more expensive, that’s why most buildings have the stuff on the roof. But it is better.

Space equipment so that the roofer can do a good job. This means being aware of what’s going to be on the roof. Then making sure every item has 30” clearance for the roofer to do his work. A diagrammatic roof plan is a good way to express the requirement. An accurate roof plan is even better. If you have consultants, make sure they know what you want.

Specify tall curbs. HVAC equipment needs base flashing around the curb that it sits on. Give the roofer a chance to do a good job. Specify 12” (min) high curbs. 

Design tall equipment platforms. When you have a large piece of equipment sitting on a raised platform, it will still be there when re-roofing is required. Raising it up makes roof maintenance possible. It may make the equipment more visible. You can solve that, but you can’t re-roof in 16” of headroom. And make sure the structural posts holding the frame are round. There are foolproof flashing details for ‘round’.  (Square is rarely square.)

Don’t use pitch pockets. Make the M/E contractors use pipe boxes instead. You should specify the pipe boxes and have the roofer install them. Here is a company that specializes in all types. Pitch pockets eventually leak, and quite soon if you have more than one item per pitch pocket. There are four items per condensing unit.  

Don’t attach piping and conduit to flashings and copings. Re-roofing is much more complicated, disruptive, and expensive when you let M/E contractors run items across the roof. There is always a way to run it below, but it’s cheaper to screw up your roof. 


​I am hoping you won't run into any of these situations like I did. Luckily, most of them were found re-roofing someone else's design.