Troubleshooting Roof Leaks on a Flat Roof

In August I purchased a 7 unit commercial building, which has a flat roof covering roughly 4,000 square feet.  I got a great deal on this property and one of the reasons was due to multiple roof leaks. For most people the first call is to a roofing company, who would likely want to replace the whole roof and charge around $8/ square foot to replace the roof, or around $32,000!  I really didn’t want to go this route.  Here is how I diagnosed and fixed all of these roof leaks.

Step 1: Walk the Roof!

I had a chance to speak to the maintenance man during our showing (he needed to show up with the keys to a couple doors the Realtor didn’t have) I asked about the roof leaks and he told me he had never been on the roof.   I went up to the roof and found 2 major red flags. ALL of the roof drains were clogged with dirt and debris.  Of course there were roof leaks, the water had no where to go! Any time it rained the water was stuck just sitting on the roof.

The next issue I found was that 1 of the 5 ventilation ducts didn’t have a cover on it!  This was an 18″ X 18″ open hole into the building!

Step 2: Immediate Actions For Fixing Leaks In Flat Roof

After viewing the property we closed on it 7 business days later.  My first action was to clear out the drains.  I took up a trash bag and scooped out all the mud and muck that was clogging the drains.  I then unbolted the drain covers and cleaned them out further, using a screwdriver and a rag.  This cost nothing.

I next addressed the large opening over the ventilation duct.  I couldn’t find a place to order an exact replacement, so I Macgyvered a fix.  The 3 bolts were rusted and I couldn’t remove the caps.  I used an angle grinder to cut the caps off, then I drilled holes through a heavy duty metal baking pan I had.  I set this in place and bent the bolts over it to ensure it couldn’t blow away.

Total cost: $0, 2 hours.

Step 3: Visit During Heavy Rains:

I walked through the building during a heavy rain and found that I still had roof leaks. The roof was leaking down into the bathroom in the front unit with enough water to push into the main area of the unit.  This is located at the corner of where the 1 story part of the building meets the 2 story part of the building.  There was a metal duct coming down into the bathroom for an exhaust fan, and I originally thought this was going to the chimney.

Step 4: Repairing The Chimney:

The chimney on the building didn’t have a cover at all.  It had a strong metal mesh cover over it to prevent birds and bats from coming in, but nothing to stop the rain. I looked into chimney covers and with this chimney being over sized that was not an option.  I ended up using a commercial baking pan with a railroad tie plate to hold it down and wired it into the mesh.

I revisited during rain and I still had leaks.

I then noticed a significant crack on each side of the chimney. This was surely letting in water. I used a silicone RTV caulk on this and assumed I had fixed the problem.  I returned during the next heavy rain….and there was no change.

I did some more walking around on the roof and realized I was in the wrong spot.  About 1 feet over from the chimney was a vent stack and this was the source of the leak.  This made a lot more sense, because why would the chimney terminate as just metal ductwork in the bathroom?  I was not feeling smart about this.  Anyways new area to investigate!

Total Cost: $15, 2 hours

Step 5: Patching the roof:

This duct cap is held on by 3 bolts and all of them had gaps that could let in water.  The top also was supposed to have a bolt and it didn’t. I caulked all of these positions.  But here is where I found the real issue.  Roughly 6 square feet of the roofing membrane had substantial wear.  Why? I have no idea.  But this area did not look like the rest of the roof.  The rest of the roof was smooth, and this area had a fine checkerboarded finish that water was sitting it.

Unfortunately I did not get a picture prior to putting on the waterproof sealant. This is after 1 coat of sealant. You can still see the checkerboard pattern.

I allowed the water to dry, cleaned the area out, and then came back with a special roof sealant for commercial roofs.  I used Liquid Rubber Waterproof Sealant.  I painted this onto the area and overlapped quite a bit.  I let it dry for about an hour, then did a second coat.  I came back a day later and did a third coat. I applied it with a small roller.

Total Cost: Roughly $80, 2 hours.

Step 6: Final clean:

I spent about 30 minutes clearing all the remaining debris on the roof.  There were rocks, pieces of the chimney, bird bones, and general muck on the roof.  Clearing all off this off prevents this debris from clogging the drains in the future.

Total cost: $0, 30 minutes

Additional Cautions:

I had other water spots on my hallway ceilings that I had attributed to roof leaks, but I was incorrect.  These leaks were caused by the condensate from the furnaces that are in the ceiling.  The connections were not tight, so all of the condensate wasn’t making it through the drain lines like it was supposed to.

Step 7: Final check:

I revisited during the next heavy rain and I had no leaks! We have since had 5 more heavy rains and no leaks! With no leaks I am now able to work on getting the front unit ready to rent out.  This unit doesn’t need a ton of work.  I need to paint the walls, replace about a dozen ceiling tiles, replace the bathroom exhaust fan, clean the windows and floor, and have one of the door locks adjusted.  This will likely cost somewhere between $1,000 to $2,000 and the unit should be able to rent for somewhere between $750 and $1,000 a month.

In total I spent under $200 and 10 hours to fix the roof leaks in this building!