Flat Roof Leak Repair

Flat Roof Leak Diagnosis

Today we'll be walking through a case study of a flat roof leak diagnosis and repair. Understanding the process that a qualified roofing contractor goes through can give you insights into your own roof and problems that may arise.

flat roof repair and coating on a Tucson house

There are two types of flat roofs. The ones that leak and the ones that are going leak. Additionally, many flat roof leaks are misdiagnosed and many times, unnecessary expensive repairs are performed.

The owner of this flat roof case study has had flat roof leaks since the day the building was purchased, which was roughly 10 years ago. The building owner employed many roofing contractors to make roof repairs which included the installation of new roof membranes, installation of new flashing, and the installation of new top reflective sealant coating. After all these repairs, the roof membrane still leaked. The building owner and his secretary would control the roof leaks with buckets. The last roofer decided to get an opinion from a building scientist (Marko Vovk) to help diagnose the root cause of these roof leaks.

Upon arrival and beginning inspections, the roofer cut the roof membrane and found trapped water. Water was running between roof membranes and leaking at two different room locations below. While on the roof, the maintenance man was adked to bring up a 5 gallon bucket full water. Several cups or water were thrown to an upper elevation brick wall. The water beaded off.

The maintenance man was asked who sealed the brick. He said, that 10 years ago, when they bought the building, the previous owner disclosed that the window at this location leaked and it was repaired. He wasn't sure about and brick sealing. He stated that since this repair, the window has not leaked.

Well it was quite obvious that somebody 10 years ago knew it was the brick that was leaking and not the window. This is why they sealed the brick with what appeared to be a water resistant coating.

While walking and inspecting the roof it was noticed that a higher elevation roof brick wall also existed at this area. Several cups of water were thrown onto this brick. This time, the water was absorbed or sucked into the brick. Brick and mortar joints are naturally absorbent; this is why you need weep holes in brick.

This brick upper roof wall did not have weep holes. This brick wall was once an exterior wall of an older building. The building roof that has the two flat roof leaks was an addition that was built over 20 years ago. The brick was getting saturated during long duration rains. This wall also faced the southern and western exposure which is more susceptible to weather. The water was getting sucked into Brick and mortar joints and running down that back side of the brick in the ¾ capillary space.

This newer addition building had roof joists that rested on pocket ledges that were cut into the brick 20 years ago. Instead of water running down into lower levels, it escaped at these cut pockets.

This was going to be an easy fix. Simply by sealing the exterior brick would fix this leak.

The owner was warned though, that when you seal brick, it no longer breaths and the potential of brick spalling may occur. I told the owner to control indoor humidity by running a dehumidifier during cold climates.

The reason for this is that if you have high indoor humidity it will travel to the exterior through a vehicle called vapor diffusion. If the brick is sealed on the exterior, it will act like an exterior vapor barrier. During the winter, when it is cold, it freezes, and brick spalling can occur. So controlling indoor humidity when you have sealed brick walls is very important.

The second leak diagnostic was also simple. Directly above this second leak was 12 year old HVAC unit. We cut into the roofing membrane at this location and encountered moisture. When a 5 gallon bucket of water was dumped into the HVAC unit fan area, it wasn't long before the water started dripping into the room below.

In the room below you could see a roof fasteners rusted and dripping water. The secretary said, the roof would leak for several days after rain storms. This leak existed for 12 years due the HVAC installation contractor not being a roofer.

Go hear to read about How To Patch A Roof Leak

This roof top unit had a roof duct penetration that was poorly sealed. The HVAC installing contractor created this leak 12 years ago. This was also an easy repair.

The HVAC unit needed to be lifted and roof membrane needed to be replaced.

Sometimes when looking for leaks you need to apply some building science knowledge, not just roofing knowledge. Sometimes, roof leaks are not roof membrane related as they were in this case.

DIY Flat Roof Repair


Today we'll be discussing making repairs to an old flat roof which, to be fair, is long overdue for a total replacement, but the owners haven't quite decided what to do with it yet. It's had a few minor repairs over the years including a temporary repair with an acrylic based paint on sealant, and in fact that's still waterproof and holding good, but what we'll be considering this time is a low-cost paint on repair that's within the grasp of most people, and it will seal the whole roof not just small areas.

The first place to start is with a clean dry roof and as you can see it's dry but not very clean. All we're going to do is thoroughly sweep the surface with a stiff bristle brush, and if any of the dirt or moss is really stuck, it may also involve scraping with a paint scraper as well. What we need is a dust free, dirt free contact with the roof surface so we can prime it up in a moment. You'll probably find the tiny mineral finish or mineral edging like this grips onto dirt really well so it's important to remember that any debris you leave on the face of your roof will into the bond we're looking to achieve. When you've removed as much as feasibly possible just sweep it up or into the guttering ready to removal later on.

This is a bitumen primer and all sorts of makes are available. The fastest and easiest way to apply the primer is with a standard 9 inch roller. And if you haven't got a dedicated roller extension you can make one. Simply sure a broom handle straight inside the roller and here I'm spinning a roller into place but you could also use gaffer tape if you really wanted. To just pour out some primer into a puddle about a mugful or two at a time don't go berserk this stuff goes a very long way. As you can imagine, it's not worth bothering to cut in neatly with a brush. I'm just doing the whole lot with a roller because I'll be stripping this roof off in the next year or two anyway. But if you want to do neat but full sheets best for the outside edges.

Now let me explain at this point how much primer you want on the roof, and the answer is as little as possible to do the job. By all means be generous on your first pass, but on the second or third what you're looking for is the least amount of primer as possible. Once it's done its job by bonding the loose particles on the surface of the roof, that's it. Any puddles primer is not only a waste it's going to take longer for your roof to dry. The exact same applies to the mineral edges or drips. Once you have it primed roll it out to get rid of the excess less is more. With that done you just need to let it dry or flash off. In the summer this can be as little as 20 minutes but in the winter it can take a couple of hours. So go and have a tea break and come back when it's done.

Now this is a roll of glass fiber scrim and it's just like rendering or plastering scrim. And if you want to you can skip applying this stuff totally but I'm going to show you how it's done in case you want to. What I'm going to do now is place this onto the prime roof and roll it out. Just make sure that you get it nice and parallel now roll it to the outside edge of the roof and cut it just short of the drip edge.

Next roll back the other side all the way back to the halfway mark and when you get there pin it in position with something just to stop it springing back or blowing around. Fantastic now we're ready for the roof sealant and this is what I'll be using again there's many manufacturers but this is basically a solvent based bitumen roof sealer, and 25 liters of this should be enough to coat this roof twice.

Once you've popped the lid off you're going to need to give it a stir a good one the solids always settle to the bottom and the liquids to the top and obviously you want a consistent even coating. So with a flat sided stick nice and carefully pull up all the solids from the bottom and only when you're happy stop mixing.

Applying the roof coating needs nothing more elaborate than a decent soft bristled brush. A nice natural fiber like this works best not too stiff and not too soft. If you don't want to work directly from a 25 liter container consider putting a couple of inches off one side of your brush with a hacksaw. Not only does he make it slightly easier to work with but you can also get it in a standard builders bucket. Handy if you don't want to look a whole 25 litres up the roof you.

Back on the roof we can now start applying the roof sealant to the room and because we've already primed it it should stick like an absolute beauty. It's just a matter now of applying about two millimeters of sealer over the roof everywhere that the scrim will sit when it's rolled back out. That way when we apply more bitumen on top the scrim will effectively be sandwiched between the two layers.

Now using a scrim like this does make this type of repair slightly harder and messier than not using one, but there are two distinct benefits. Firstly it guarantees a minimum depth of coating of two millimeters, meaning no drama spots or missed areas. Secondly when dry it will add additional strength which is important if you have lots of cracks in your roof or it has a slightly soft feel to it. This as you can imagine helps to stop most cracks reappearing as the bitumen sealant dries out with age in the years to come.

When you have the first half done roll back the other half and start the process again. Now there are two disadvantages of using a scrim and here they are. Firstly it's a messier job and you have to keep pressing over the roof filling up any holes that reappear. This obviously takes a bit more time and you will use a bit more product. Secondly if your roof isn't flat or has nasty hollows or ridges in it the scrim can resist following these contours which will make it a lot more difficult otherwise known as a pain in the arse. Okay so here on the second run I'm going to do exactly the same process this time making sure that one scrim overlaps the first scrim by two to three inches. Then we're going to cover it up, just as we did before.

On the third one though I'm going to show you a slightly easier and lazier way to apply the scrim. Here I've just placed a couple of dabs of sealant onto the roof and I'm going to stick the scrim in it and roll it out into position. This time though I'm just pushing the bitumen straight through the scrim from the top surface alone whilst this isn't as good as the previous method it does still work. But like I said it's a lazy method not quite as good. Your roof your choice at the end of the day.

And of course there's the no scrim method. If you're looking for a repair that's a little easier and cheaper just apply the compound directly to the roof. Yes the depth of the sealent becomes harder to judge and it doesn't add the strength that we've talked, about but you can always have a second coat later on and to be honest I would do that as a matter of course anyway. Now all we're doing is covering the whole roof with a nice even coating trying to achieve the two millimeter depth that we require. Covering the whole roof and working back to the ladder or exit point.

With that done you should not have something that looks like this. On a summer's day it will be fully waterproof in about an hour, and re-coatable in five to twelve hours. What you want the surface to look like if something like this, nice and even with no pinholes. But sometimes especially if you've used scrim you might get some small pinholes like me near the edges where I was trying not to flip bitumen into the gutter or on the floor below.

A second coat of sealant here we'll pay absolute dividends and because the hard work has already been done recoating will be so fast it's almost embarrassing. Any small flecks of bitumen that find their way onto the floor can usually be dealt with by applying building sand generously to them and treading it in. Let it absorb the bitumen for as long as possible and then just sweep it off.

For a really nice finish and to protect your hard work for longer consider applying solar reflective coating. Again this stuff settles to the bottom and it will need a darn good stir until the solids flow freely. Then it's just a matter of cutting in around the edges with the two inch paint brush and applying the solar reflective coating with a cheap 9 inch roller just as we did with the primer.