Flat roof leak diagnosis & repair (C) Daniel Friedman Flat Roof Leak Diagnosis & Repair
How to track down the source of leaks in a flat roof
How to apply roof sealants & coatings to stop roof leaks

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

Flat & Low Slope Roof leak diagnosis & repair:

This article describes the location of the leak source on a flat or low slope roof and the repair of a leaky flat roof by application of a roofing sealant.

This article series provides an extensive catalog of sources of leaks in all types of building roofs, this article describes procedures for finding and fixing all types of leaks in roofs, figuring out the actual spot where a roof leak is occurring, and methods for tracking down the source of water or wet spots on ceilings or in attics. 

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

How to diagnose and repair leaky flat & low slope roofs

Flat roof leak diagnosis & repair (C) Daniel Friedman The concrete roof described in this article is a nearly-flat roof sloping slightly (less than 1" in 12") to a roof drain in one corner.

This roof was constructed atop a home in San Miguel de Allende, in Guanajuato, Mexico in 2000. The flat roof leak diagnosis and sealant application to stop roof leaks described here were performed in 2015.

[Click to enlarge any image]

Flat and low slope concrete roofs on homes and other buildings are widely used in many areas of Mexico, the Southwestern U.S., Morocco, India, Australia and many other parts of the world where the roof surfaces are exposed to intense sunlight and often high temperatures.

Tracing roof leaks on a flat or low slope coated roof

On low slope membrane covered roofs water can travel a considerable distance horizontally before it appears inside the building. Roofers use a combination of visual inspection and moisture meters to scan membrane-coated roofs to find the wet areas below the membrane and thus the probable leak point - usually at a seam, mechanical damage, a roof penetration or at a roof drain.

But a leak in a coated concrete roof can mimic the hard-to-track leaks in a membrane type roofing too. That's because layers of roof coating on the concrete roof may look good from above but may not be adhered to the concrete surface. Poorly-adhered roof sealant can provide a hidden horizontal water creep pathway.

Reader Question: how can I find the source of a small leak in my flat roof?

31 August 2015 Dirk said:

I have a very special type of roof that has a leak. It was designed around 1940 as a passive air conditioned house in Florida. The roof is concrete slab (flat) with a deck and parapet walls so that the upper deck can be used as a sun deck. The entire deck was tarred and has j-bolts and standoffs then squares of concrete are mounted to the J-bolts and bolted down with flush nuts. The idea was that water would fill the area under the squares (which have a small crack between them).

The water would then evaporate through the concrete squares. The hotter the sun the more cooling the house got. This worked until about 10 years ago when the roof began to leak. (great idea but its useful life ended)

The client had the deck roofed over and AC installed. The problem is that there is a small leak somewhere. Annually, the living room ceiling needs repaired. I am wondering if you have any method of finding the actual leak?

The deck looks properly roofed. Without removing the roofing and lifting the concrete squares and re-hotmopping the deck I can figure no way to find and repair it. I am not even sure one could remove the special flush nuts!

I have thought of thermal cameras but unsure if they will show anything from the top side. remember that under the new roof is a "pool" in essence, so that the leak showing in the house will be where that "pool" leaks, not where the membrane leaks. Please advise?

Reply: steps in tracking down a flat or low slope roof leak


Tracking down the source of a leak in a flat roof is indeed challenging, as we discuss in the article above.

Roofers use a combination of

Keep in mind that both moisture meters and infra red imaging (the latter relies on temperature differences presuming wet areas are cooler) cannot accurately pinpoint a leak source if no water is present, that is if too much time has elapsed since the last rain or last leak.

I do not advocate tearing off a functional roof to patch a leak; I advocate very careful inspection by an experienced person (an inexperienced person won't have the eye to spot all useful cleak clues). Often we will identify several probable or most-likely leak points and apply a patch or sealant over those areas. Then we monitor the roof to see how successful we were.

Flat roof leak tracking & roof repair references:

Common leak points in concrete roofs

The most common points of leakage in these roofs are where the concrete has cracked, particularly if the crack is in an area of roof ponding or near a roof drain where the most water accumulated during wet weather.

Our flat roof photo above is typical of coated concrete roofs world-wide: poured concrete, typically reinforced with steel mesh or steel re-bar, has a gentle slope to a roof drain (not shown above). Imperfections in roof construction or less often due to sagging or structural movement can cause low areas that pond.

Areas of roof ponding show up as dark areas where dust and debris accumulate in the areas that stay wet longer on the roof surface. Ponding that has evaporated within 24 hours of a rainstorm is not considered an actionable roof defect. Roof ponding is illustrated and discussed at BUILT UP ROOFS.

Watch out: as you can see in our flat concrete roof photo above, there are no safety railings nor tall parapets on this roof. Pay attention to where you are walking or standing at every moment as it is easy to step backwards, slip, trip or fall to your death.

Flat roof leak diagnosis & repair (C) Daniel Friedman

Inside the building these roof leaks appear as stains or mold on concrete or on painted concrete or masonry ceilings and on the upper portions of walls at or near the ceiling-wall juncture. Tracing a low slope or flat roof leak to its source is important if you are to assure that the roof leak repair will be successful.

On coated concrete flat or low slope roofs the tracing of a leak to its probable source is often a bit easier because the horizontal travel distance will be less and the inside leak will appear close to a crack or close to a roof drain or point of overflow if the drains are blocked. See FLAT ROOF DRAINAGE SYSTEMS.

Water may also creep along underneath poorly-adhered roof coatings to leak some distance away from the point of water entry, as we will describe later in this article.

Occasionally water from the roof surface may follow a circuitous route through cracks in the concrete but you may see the crack on the roof surface if not also inside.

Prepare the Roof For Leak Repair by Cleaning, Scraping, Sweeping, Inspection & Repair

Flat roof leak diagnosis & repair (C) Daniel Friedman

What to bring onto the roof for surface preparation

The tools you'll want to bring onto the roof for surface preparation include at least a broom, dustpan, and one or more scrapers (shown above). I didn't bother to bring up the roof sealant nor any roof crack repair materials as the roof being repaired here is so small I wanted minimum clutter on the surface during cleaning and inspection.

Flat roof leak diagnosis & repair (C) Daniel Friedman Flat roof leak diagnosis & repair (C) Daniel Friedman

When I had finished prepping the roof I lugged up our impermeable roof coating: a Comex product called TOP.

Toxic material warnings on roof sealants & coatings (C) Daniel Friedman

Watch out: While this is a pretty nice material to brush out onto a roof the solvent vapors from this or any paint can be unsafe not only as a health hazard but it might make you light-headed enough to fall off of the roof - one more cause of deaths by roof falls. Wear a respirator whose canisters are rated for organic and inorganic vapors. The warnings above include:

Advertencia: No fume y no inginera alimentos durante su aplicacion.

Warning: Do not smoke and do not eat food during application.

A mere dust mask won't do a thing about solvent vapors.

Thorough roof surface preparation is key to a successful roof coating repair job

If you do a thorough job of preparing the concrete roof surface for coating with an impermeable sealant such as Comex® Top (illustrated below) you should find and remove all of the old, poorly-adhered roof coating to expose clean dry (let it dry if needed) concrete surface.

In our article series on painting buildings we emphasize the three key steps to a successful paint job: Prepare, Prepare, Prepare. If you do not adequately clean and prepare the paint surface the paint job will fail quickly.

Tips & tricks of the trade when preparing & scraping a previously-coated concrete roof surface

As when scraping painted siding for re-painting the question is always how much scraping, sanding, and preparing is really necessary. For the new coating (primer + one or two top coats) to adhere to the roof surface you must

Flat roof leak diagnosis & repair (C) Daniel Friedman Flat roof leak diagnosis & repair (C) Daniel Friedman

Take a look at my photo at above right: that green algae showed up where I removed some otherwise good-looking but old roof sealant coating. The fact that we see algae growing under the roof sealant coating confirms that water had been leaking under this area of sealant. The sealant had to be removed and this area dried and scraped again.

Flat roof leak diagnosis & repair (C) Daniel Friedman Flat roof leak diagnosis & repair (C) Daniel Friedman

[Click to enlarge any image]

Feathering or tapering the edges of thick but sound roof coating. This will help the new coating seal where it overlays the old roof coating edges and will reduce the chances of leaks at those locations. Without a perfect seal at the edges of thick roof coating water may enter under the thick old coating edge, lifting it and ultimately causing coating failure and roof leaks.

Flat roof leak diagnosis & repair (C) Daniel Friedman

Applying the Roof Sealant Paint or Coating

Watch out: Inspect the roof surface very carefully everywhere, on hands and knees, to be double sure you have not left any un-sound roof coating or loose areas that need more scraping and cleaning.

And do not start applying roof sealant coatings if you are not confident that your roof will have adequate drying time before any incoming rainy weather.

Watch out: sweep again before you start pouring out roof coating. You may be surprised how much more dirt and debris you'll collect.

Flat roof leak diagnosis & repair (C) Daniel Friedman Flat roof leak diagnosis & repair (C) Daniel Friedman

Watch out: plan your work sequence, starting at the area of the roof most distant from your ladder or exit point.

Above you can see we've poured out a dollop of our roof coating material and have begun to spread the roof sealant. Take extra care to seal the juncture of roof surface with any parapets on a concrete roof. I'm using a disposable long-handled brush that makes the application of roof sealant faster and more comfortable. If you try this with a little paint brush you'll be sorry.

I'm pulling a thick roll of roof sealant in front of the brush edge (above right). If I don't see that I've got that leading roll of sealant on the front edge of my brush I know I need to pick up more from the poured out coating. As the sealant is applied I stop and brush sealant into the joint of roof top and parapet wall, and I inspect for any bubbles, cracks, or pits that are forming voids in the coating. If necessary I brush back and forth a few strokes to be sure these are completely closed and sealed.

Watch out: pay attention to where you are standing and walking on the roof at all times. On a commercial roof job in the U.S. OSHA will require safety barriers that are typically eschewed by small painting contractors on residential roof projects. It is particularly easy to step backwards off of a roof or to lose your balance, step into wet paint, and slip to your death. Don't do that.

Flat roof leak diagnosis & repair (C) Daniel Friedman

When my first roof sealant work area has been coated I step back (Watch out: don't step back off of the roof or you die) to inspect the work again for voids or defects. It's important to make these checks while you can still reach with your long-handled brush into the work area to apply more coating if necessary. You don't want to ever walk into the wet paint - that's another way to slip and fall off of the roof besides looking like an idiot with paint on your shoes.

Watch out: in hot sun this is a good point to remember to stop and take a drink of water. It's easy to suffer from sunstroke when working on a hot roof and in turn easy to then fall off of the roof. Be sure to consume any drinks or snacks before you've opened the container of roof sealant (as we warned above).

If you think I'm being an old maid take a look at construction accident statistics: falls from roofs are among the most common causes of injury or death in construction. Falls from roofs cause between one tenth and one third of all construction fatalities! (U.S. CDC 2000 and U.K. HSE 2014).

Preparing to Leave the Newly-Coated Flat Roof

Flat roof leak diagnosis & repair (C) Daniel Friedman Flat roof leak diagnosis & repair (C) Daniel Friedman

Above you can see that I've painted myself over to my roof exit point. This area also happens to be near the roof drain and best of all it happens to be directly over the areas of roof leakage shown in our interior photo at the start of this article. I finished spreading roof sealant with great care, working from the top of my access ladder to complete the job.

At above right you can see the roof drain opening. I took extra care with sealant at the abutment of the roof surface and roof parapets in this area because this was over the area of roof leaks.

Flat roof leak diagnosis & repair (C) Daniel Friedman

At above left our first roof sealant coating has been completed. You note that I did not paint all of the parapet wall - something to to correct when the roof is dry or when applying the second roof coating layer.

Below you'll see that I store the roof coating paint brush in a heavy plastic bag so that it won't harden before we need it for the second roof coating layer. Please don't leave that long paint brush brush handle on the floor where I'll step on it, slip and break a leg, or the roof's second coating could be delayed for quite some time.

Store the roof paint brush in a plastic bag with a bit of water for re-use in the second coat (C) Daniel Friedman

Applying the Second Coat of Flat Roof Sealant

When the flat roof sealant coating had dried for several days we were ready to apply the second coat recommended by Comex, the roof coating manufacturer. But it rained. We waited for an additional three days of dry weather, then inspected the roof surface and found that it was dry and hard enough to walk on even in the areas of thick coating.

Second coating of Comex brand roof sealant paint on a flat concrete roof in San Miguel de Allende, Mexico (C) Daniel Friedman

Comex advises that the second or top coat of roof sealant be applied in a direction orthogonal to the original coating - that is, opposite or across the original coating brush-on direction.

If you've used a long-handled coarse roof paint brush you'll see why: the brushed coating will often show areas of raised ridges in the original coating direction. Brushing across those ridges for the second roof sealant application will improve the quality and smoothness of the top coat.

Inspect the first roof sealant coat for thin areas, pinpoint holes or voids, or for visible cracks that telegraphed through the coating. Be sure that these leak-suspect areas are given a sound, smooth, uniform coating of sealant. Also pay careful attention to a final seal at any junctures of roof surface and parapet walls.

Flat roof sealant coating second coat detail (C) Daniel Friedman Flat roof sealant coating second coat detail (C) Daniel Friedman

In our photo at above left you'll see that close inspection of the first roof sealant coating showed that I had left small voids in and around the course of a crack in the parapet wall. The second coat is an opportunity to seal that area (red arrows) and thus to reduce the chances of roof leaks at that point. You'll also see an area (in the blue oval) where the roof sealant coating was a bit thin.

At above right is the same roof area after my second coat of roof sealant paint.

[Click to enlarge any image]

It's worth taking the time to be a bit careful and compulsive now about roof sealant coating rather than to have to do the job over again or spend time looking for that little hole or crack you neglected to repair and seal the first time.


Continue reading at LEAKY ROOF DIAGNOSIS & REPAIR or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.



Or see these

Key Articles About Low Slope & Flat Roofs

Suggested citation for this web page

FLAT ROOF LEAK REPAIR at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


Or use the SEARCH BOX found below to Ask a Question or Search InspectApedia


Frequently Asked Questions (FAQs)

Click to Show or Hide FAQs

Ask a Question or Search InspectApedia

Questions & Answers diagnosing & repairing leaky roofs

Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.

Search the InspectApedia website

HTML Comment Box is loading comments...

Technical Reviewers & References

Click to Show or Hide Citations & References

Publisher's Google+ Page by Daniel Friedman