Flat & Low Slope Roof leak diagnosis & repair FAQs.

This article series 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.

Q&A on how to repair leaky flat & low slope roofs

These questions and answers about how to find the source of leaks in low slope roofs and how to seal or fix low slope or flat roof leaks were posted originally at FLAT ROOF LEAK REPAIR - be sure to also read through those suggestions.

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On 2018-11-28 by (mod) - roof leak continues after repair but there was no more rain

Becky

If a roof leaks AFTER a repair was AND if there has been no rain since the repair I figure

1. there is a pool of water in the roof structure left over from the leak - you need to inspect the roof cavity to find and drain that water as well as to remove and replace wet or moldy insulation or drywall ceilings.

or

2. It's not a roof leak it's another leak source such as a water pipe, hot water heating pipe, or air conditioner condensate drain line leak.

On 2018-11-28 by Becky

I have fix leak 3 times n roof still leaking it hasn't rain n days still leaking

On 2018-09-26 by (mod) - using roof repair tape such as Ultra Bond from QuickRoof

Thanks for the added details - which point out the shortcoming of even guessing at roof leak diagnosis by e-text.

It is possible that a cement could be incompatible with the mod-bit material - and would be instructive to find the specs on the adhesive and to send a query to the mod-bitumen manufacturer. Some solvents in adhesives and mastics (including trowel-on or paint-on sealants and coatings) are not compatible with modified bitumen while others are specifically-recommended.

I like seam tape and have just been using one of those products on a flat roof myself, but again you have to be sure the tape is rated for your specific product.

Even among mod bit the tape may not work if your mod-bit is granule-coated.

For example check with Ultrabond about their seam tape that they describe as

"Repairs EPDM, TPO, metal, modified asphalt, vinyl, fiberglass, aged and rigid PVC, OSB, rigid extruded polystyrene, wood and tile"

I don't see modified bitumen in that list but then maybe that's what they meant by "modified asphalt"?

And even then they don't distinguish (on the advertisement) between smooth and granule-coated modified bitumen roofing.

In sum, check the compatibility of any products you add to the roof

See details at EPDM ROOF SEAM REPAIR TAPE

On 2018-09-26 by Jim

Hi,

I put this roof on myself. I briefly gave some thought to doing a torch-down roof but, wisely decided to use cold-process adhesives. Today I went over the entire roof, looking for cracks in the mod-bit. I found a couple of small areas but nothing like the area immediately up stream of the leaks.

As mentioned the cracks conform perfectly to the seam seal flashing cement I applied about a year or more ago. When I put down the mod-bit sheets I used Karnak flashing cement on the first 6in of the sheet's edges and then used roofing cement in the sheet's center.

I wonder if that contributed to the cracking? Whatever the cause, I've now covered the cracks with Karnak flashing cement.

Your earlier comment that the leak I'm dealing with is---despite my seam-sealing efforts---most likely coming from water getting under a lap-seam and not from the mod-bit roofing sheets themselves makes than suspecting that the mod-bit has failed (aside from the cracks, it looks solid as can be).

We have more rain coming later in the week so I'll know if I've found the problem.

But, if I still have a leak or even less of one, I wonder if I could just go ahead with covering the lap-seams with UV-resistant reflective seam tape. This is admittedly a brute-force solution, but I can buy the tape in 12 in width's and that stuff will stick to anything, especially mod-bit

My initial intent was to use the tape sealed seams in combination with the flashing cement as a way to create a fully sealed roofing membrane.

If you think that will work, I won't care very much if the tape covers a leaking seam.

I'm a firm believer that perfection can be the enemy of good so, if this will fix the problem, I won't care if the tape is covering an other leaking seam.
Again, thanks so much for your help.

On 2018-09-25 by (mod) - I wouldn't assume that all of your seams are perfectly sealed even though you think they are.

Thank you, Jim, that's helpful.

Do you think it's possible that this was a torch applied roof? If so overheating is a common source of problems in the areas of the same's including the points of overlap.

One clue to check his email to run out at the same if there's a lot of run out the same was overheated. Sometimes I've seen the run out covered up by seam tape when the roofer was both trying to fix a problem and also cover up the overheat.

On 2018-09-25 by Jim

Hi, Dan. "I wouldn't assume that all of your seams are perfectly sealed even though you think they are.

A leak in a seam is far more likely than a leak through undamaged membrane material." Here is what I found while looking for evidence of water filled blisters

. I was looking at an area upstream of the leak and found that the mod-bit roofing (TAMCO SBS cold process) had actually cracked all along the edges of the seam-sealed laps. I used Karnak adhesives throughout, including #19 Ultra flashing (which is compatible with mod-bit roofing) cement to seal the laps---both during the initial install and later along the outside lap edge.

The cracking is very apparent and follows exactly along the edges of the two year old flashing cement used to seal the outside lap seams. I found identical cracks in other parts of the mod-bit roofing---which correspond to the areas where my leaks are concentrated.

Further investigation showed that this specific upstream area was the only section of my roof which show this kind of cracking. I have sealed these cracks, again with Karnak cement.

Careful examination of the roof doesn't show any other evidence that the TAMCO roofing material is degraded in any way. The cracking I found is decidedly odd since it doesn't appear elsewhere on my rather large roof.

As I said, I've used the same adhesives from the same manufacturer throughout. I understand that the leaks may or may not become coming from these cracks so I'm going pay additional attention to the these initially sealed the lap seams.

My initial plan was to use the flashing-cement as a first step which would be followed by UV protected reflective seam-tape for a hopefully permanent fix. I don't what to go to this final step, however, until I've solved my leak problem. Again, thank you again for your most helpful advice.

On 2018-09-24 by (mod) - don't assume you know where the leak is

Thank you for the interest in question. Thank you also for the nice notes about our website. I'm always very pleased when readers find it useful.

About the roof leak that you're discussing, it's a common mistake and diagnosing any problem to start with a set of assumptions that's too strong or in which we have too much confidence. I wouldn't assume that all of your seams are perfectly sealed even though you think they are.

A leak in a seam is far more likely than a leak through undamaged membrane material.

But you might do is dry infrared or thermal scanning or moisture meter scanning to try to track and map the leak path. Leaks and low slope and flat roofs can be devilish to find because water can travel horizontally for considerable distance.

Of course the other area that's most suspicious are any penetrations in the roof and the flashing around those. It's also possible that the material that you used as a seam sealer was not compatible with the modified bitumen that you put down. I'm not stating that as a fact because I don't know exactly what you used. But it's something to consider and to review.

On 2018-09-24 by Jim

First of all, thank you for maintaining such an informative site. The information here in invaluable.

I have a low-slope roof with a mysterious leak. My original roof was BUR. I treated it for several years with coatings. Two years ago I decided to install mod-bit roofing over the original BUR roofing which, although old and abraded, was smooth and had no penetrations.

I decided it was safe to use it as a base for the new mod-bit install. A few weeks ago, anticipating fall rains, I sealed the new roof's lap-seams (all of 'em) with good quality flashing cement reinforced with mesh. Now, with the torrential rains (I'm in Texas) my new roof has an unexpected leak I'm trying to diagnose.

With very heavy rain, it appears that water is entering and flowing to a low spot where the leak appears. I found some evidence of lifting and some blisters which proved to be dry. I also found a water filled blister which I remained these and stopped a bedroom leak.

Owing to the construction of the roof, the new mod-bit install constitutes a full coverage membrane---water shouldn't be coming in at corners, etc.

Here's my question:

My goal was to create a new roof with a 100% sealed membrane. I'm confident my seam-sealing effort was successful so this leaves a question about the roofing material itself. I intend looking for water-filled blisters to fix which may be the source of my remaining leak but this raises another question.

I thought that mod-bit was supposed to be highly resistant to leaks, yet my install appears to have some I didn't expect to have

. I realize that an obvious solution would be to remove the existing mod-bit and start over, something I want to avoid if I can. If I find the source of this leak can I count on the mod-bit to be strong enough to have a permanent repair?

I was 74 last summer and I really don't want to have to do this again in a few years. What I want is to create a long lasting fix that I can trust. To this end I'd like to know what you think of coating the mod-bit roofing with 100 % silicone coating?

An alternative to a paint-on covering is to put down fiberglass webbing cemented by asphalt emulsion. Is it possible for me to create a completely sealed roof using one of these methods? Thanks in advance for your help.

On 2018-06-29by (mod) - what would cause leaking at the seam where they meet?

Improper / inadequate flashing of the flat roof covering up under the shingles, sometimes combined with wind-blown rain or depending on where you live, melting snow-cover.

Or of course a leak higher on the roof that finds its way in at the change in slope.

Or ice damming at the point of change in slope or just below it.

On 2018-06-28 by Fred

We have a flat roof over out family room and an asphalt shingles on the rest of the house. what would cause leaking at the seam where they meat?

On 2018-02-22 by (mod) - is there a warranty that it wont leak?

Margaret

Roof job warranties mighr be offered by the roofing company, commonly in the roofing contract.

Commonly the roofing matetial manufacturer warrants the life of the material but not the workmanship of the job.

So a place to start is with an indepindent expert's opinion of the causes of your roof leaks.

Without knowing anything about your building, location, roof material, and installation, I can't speculate further.

On 2018-02-22 by Margaret Mara

If an entire roof is done is there a warranty that it wont leak?

How long does this type of repair work? Also when it had dried will it crack if walked on.

I see the references to this roof work in warmer areas. How does it work in places where the climate swings well below zero to a season where it could go above 100 degrees? Please reply

On 2017-03-01 16:18:03.175072 by (mod) -

Nicole said:
Flat roof repair becomes the problem when it gets ponding water for long time.

On 2015-08-31 by (mod) - very special type of roof that has a leak. I

Dirk please see the article FLAT ROOF LEAK REPAIR where we now include your question and a detailed answer.

On 2015-08-31 by Dirk

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?

...

Continue reading at FLAT ROOF LEAK REPAIR or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Key Articles About Low Slope & Flat Roofs

• BUILT UP ROOFS
• CONCRETE ROOFING
• DEFECTS LIST - ROOF LOW SLOPE
• EPDM, RUBBER, PVC ROOFING
• EPDM ROOF COATING REPAIRS
• EPDM ROOF LEAK REPAIRS
• EPDM ROOF SEAM REPAIR TAPE
• FLAT ROOF DRAINAGE SYSTEMS
• FLAT ROOF LEAK REPAIR
• FLAT ROOF MOISTURE & CONDENSATION
• LOW SLOPE ROOFING
• LOW SLOPE ROOF CONVERSION
• MEMBRANE & SINGLE PLY ROOFS
• MODIFIED BITUMEN ROOFING
• MODIFIED BITUMEN ROOF DEFECTS & ROOF LIFE
• ROLL ROOFING, ASPHALT & SBS
• ROOF LEAK SOURCE DIAGNOSIS

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INDEX to RELATED ARTICLES: ARTICLE INDEX to BUILDING ROOFING

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