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And yet another flat roof.


Richard Moore
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I had a 60 year old home today with a mostly flat roof (MLS described as torch-down). It had the deepest and worst alligatoring I have yet seen, all over the roof. Also had ponding stains in various areas. No question it needs repair/replacement, but I have a question.

The reflective coating looked to be maybe 5 years or less, although there were a couple of patched areas on top of that. As you may be able to tell from the photo, the cracks have appeared since the coating was applied. I thought these normally happened over time, but could our recent record heat wave have caused this deep cracking just in the last couple of weeks?

The reason I ask is that we haven't had any rain for almost a month now. There weren't any ceiling stains (no attic spaces) but I have to wonder if this whole roof isn't going to leak like a sieve if it doesn't get fixed before the next rain. (Rain showers are actually forcast to start tomorrow - we need it!)

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Unless the picture is very misleading, that looks like typical alligator cracking on the flood coat on top of a traditional built up roof. Admittedly, it looks like it was an overly generous flood coat. I used to see that all the time and the cracks rarely extended through the top layer of felt.

- Jim Katen, Oregon

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Here's a wider angle of the same area. The skylight is about 3'x5'. The alligator segments were larger and the cracks deeper (1/2"+ in places) than I've ever seen before.

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So, if the problem is too much asphalt, what's the solution...more?

The clients already have a couple of roofers lined up for evaluation and estimates.

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Here's a wider angle of the same area. The skylight is about 3'x5'. The alligator segments were larger and the cracks deeper (1/2"+ in places) than I've ever seen before.

So, if the problem is too much asphalt, what's the solution...more?

The clients already have a couple of roofers lined up for evaluation and estimates.

That's definitely a built up roof. If the flood coat was 1/2" deep, then the cracks will be 1/2" deep -- that's just what thick flood coats do. Look carefully into the cracks to see if the felt is damaged. I use a thin-bladed knife to gently feel for the severed edges of felt. 99% of the time, the crack doesn't extend below the felt. The internal faces of the crack are just a smooth fracture. If that's the case, there isn't any need for a solution.

- Jim Katen, Oregon

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Thanks Jim, but...

Look carefully into the cracks to see if the felt is damaged. I use a thin-bladed knife to gently feel for the severed edges of felt. 99% of the time, the crack doesn't extend below the felt.

That sounds good, but this was about 1500sf of roof with these areas everywhere. I would be up there for hours probing the thousands of cracks. On a flat roof, especially one with ponding issues in some areas, that 1% could cause major damage.

The internal faces of the crack are just a smooth fracture. If that's the case, there isn't any need for a solution.

I can see that the felt may be, even probably is, still intact but, with all due respect, I have a hard time with no solution needed. If I could see felt in the cracks then isn't that felt now subject to damage that the flood coat was supposed to prevent?

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Thanks Jim, but...

Look carefully into the cracks to see if the felt is damaged. I use a thin-bladed knife to gently feel for the severed edges of felt. 99% of the time, the crack doesn't extend below the felt.

That sounds good, but this was about 1500sf of roof with these areas everywhere. I would be up there for hours probing the thousands of cracks. On a flat roof, especially one with ponding issues in some areas, that 1% could cause major damage.

I'm not suggesting that you probe every crack. Just a few will tell you what you need to know. You'll either find torn felt down there or you won't. There won't be a lot of variation. By the time these cracks lead to leaks, they'll look different than they do in your picture; they'll be peeling up along with the top layer of felt below them.

The internal faces of the crack are just a smooth fracture. If that's the case, there isn't any need for a solution.

I can see that the felt may be, even probably is, still intact but, with all due respect, I have a hard time with no solution needed. If I could see felt in the cracks then isn't that felt now subject to damage that the flood coat was supposed to prevent?

It depends on a lot of factors. The roof in your picture might be toast, but not because of the alligatoring.

The thick flood coat wasn't put there to protect the felt, it was put there by lazy roofers who, when they were done with the roof, had a bunch of leftover asphalt in their applicator. To get rid of it, they just spread it around on the roof. After all, if a little floodcoat is good, a lot must be better, right?

Don't get me wrong. Alligatoring isn't a good thing and it isn't inevitable. It's usually the result of a less than perfect installation. Eventually, it might even cause the roof to leak, but by that time it won't look the way it does in your pictures. It will have started to peel. That is, each little "island" formed by the cracks will start to lift at its edges forming a dish shape and tearing away the top layer of felt below it. When that happens, water can enter between the plys and cause leaks.

At this point, the best thing for this roof is to have a reflective coating -- and it already does. I suppose that you could lightly pressure wash it and apply more reflective coating but I'm not sure it would help much.

None of that really matters at this point because the two roofers who will come look at it will recommend replacement anyway.

- Jim Katen, Oregon

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My comments are based on having looked at approx. 3500 "hot tar" flat roofs. I still see tar pot installations fromt time to time, but they're getting scarce.

Alligatoring isn't good, but it doesn't do anything bad. I see them where they are full of ice, and have been for a dozen winters. Nothing bad happens. Ever.

Probing the cracks might tell you something, but probably not.

Roofer consultants all zero in on it like it's going to make the roof explode, but nothing ever happens.

Years ago, I met a guy that was the lead foreman on replacing the roof on McCormick Place (at the time, the largest roof in the world), and he explained how all this stuff works in real time. It just means someone flooded too much tar on the roof.

Like Jim said, the roofers are going to recommend replacement anyway.

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Your observation that there are some areas of ponding is the significant point, not the alligatoring.

Ponding on flat roofs is death. Period. That alone would allow you to condemn the sucker.

Sure, I know that there are still a few coal tar pitch roofs intended to pond water (the water helped cool the building in hot weather), but in our world, ponding means failure in almost all cases.

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Your observation that there are some areas of ponding is the significant point, not the alligatoring.

Ponding on flat roofs is death. Period. That alone would allow you to condemn the sucker. . . .

Define ponding.

Personally, I'm not too bothered by 1/4" or 1/2" of water that covers a few square feet here & there.

- Jim Katen, Oregon

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An amount of water that is present >72 hours. Some folks say 36 or 48. A small skim of water that hangs around for a day or so isn't likely to do anything.

A couple square feet of water 1/4" deep, we call that a "bird bath".

About half the roofs I see are flat; it's the standard City condition. Water ponds on flat roofs are a surprisingly pernicious problem. Tiny pinholes in laps can leak with ponds. Ice in winter will stress out welds and seams.

There's a reason no mfg. of mod bit warrants their roofing material in ponding conditions.

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The ponding on this particular roof may or may not have met the 72-hour standard (I'd heard it as 48). Tough to tell as we have had no rain for many weeks and I was mostly going by stains and deteriorated coating. I doubt that it was deep enough to ever be a structural issue, but leaks...well, I just don't know.

As I said before, the deep cracking was all over this roof but the (suspected) ponding was more restricted. The below is a photo is of one area. So, for my info, how would you guys report this roof?

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Richard, in my experience when the flood coat gets that cracked its rate of expansion and contraction is VERY high and works at the underlying felt layers very hard---leading to failure and leaking. To me the roof you have pictured is very vulnerable to leaking and I would consider it in need of replacement. One could make a good case for having the roof scanned if you wanted to try and get some more life out of it. Once water gets into these multi-layer felt built up roofs all bets are off. Once water gets into it, you will have the elements working against it from the inside and the outside. Just my two cents.

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So, for my info, how would you guys report this roof?

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Well, it looks like the eave edge/gutter detail is wrong; typical wrap into an aluminum gutter.

Is that what they did? If it is, that makes it really easy to can.

Kinda hard to tell from just pics. So much of flat roof (or any roof) performance is about drainage.

Not disagreeing with Charlie because I don't know for sure what he's saying, but alligatoring doesn't mean expansion and contraction. It means someone flooded too much goop onto the roof.

Honestly, I've never seen alligatoring have any consequential effect on any flat roof. If there's problems, it's from some other combination of conditions. I used to get all freaked out about alligator'ed roofing, but after you see several thousand, you get the idea that it doesn't matter much.

Flat roofing, just like most other roofing, rarely goes out in the field (meaning the broad expanse of material); it goes at angles, vertices, flashing, eaves, parapets, and penetrations.

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Well, it looks like the eave edge/gutter detail is wrong; typical wrap into an aluminum gutter.

No, no gutters. Just a drain hole to a downspout here and there, and not many of them. The metal you are seeing is at top of a slight rise at the edges...maybe an inch or so.

This home also had a "fun" foundation under a 30'x15' added wing. At least 30' of the perimeter "footing" (if you can call it that) wasn't in contact with the soil.

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An amount of water that is present >72 hours. Some folks say 36 or 48. A small skim of water that hangs around for a day or so isn't likely to do anything.

A couple square feet of water 1/4" deep, we call that a "bird bath".

Yes. I agree. Small puddles just aren't "ponding." The thing is, that NRCA definition is out there and, in my neck of the woods, if I pour a cup of water onto a small depression on a flat roof in Novemeber, it'll still be there in May. According to the NRCA, that cup of water that hasn't evaporated is "ponding."

- Jim Katen, Oregon

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As I said before, the deep cracking was all over this roof but the (suspected) ponding was more restricted. The below is a photo is of one area. So, for my info, how would you guys report this roof?

Sorry, I just can't tell anything from the picture. If the ponding were restricted to the very dark areas, then I wouldn't be too concerned.

As for the rest, I'd be looking really hard at the perimeters and all of the flashings.

- Jim Katen, Oregon

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Richard, in my experience when the flood coat gets that cracked its rate of expansion and contraction is VERY high and works at the underlying felt layers very hard---leading to failure and leaking. To me the roof you have pictured is very vulnerable to leaking and I would consider it in need of replacement. One could make a good case for having the roof scanned if you wanted to try and get some more life out of it. Once water gets into these multi-layer felt built up roofs all bets are off. Once water gets into it, you will have the elements working against it from the inside and the outside. Just my two cents.

Gotta disagree. As Kurt said (I think) those cracks aren't expanding and contracting -- and they never were. They're just contracting. That's what thick flood coats of asphalt do over time, every one. If you look at them carefully, they're not letting water into the built up assembly at all. It's strictly a surface phenomenon. You can have a bazillion cracks that go all the way through the flood coat right up to the surface of the felt and it won't be a problem.

Now here's where I disagree with Kurt, although it might just be that our different climates yield different results. As that cracked flood coat ages it will continue to shrink, but its bottom edge will be held in place, mostly, by the felt that it's adhered to. As a result, the top surface continues to shrink and it lifts up the corners of each little island of asphalt. When that happens, the felt tears. *That's* when the problems begin. A good reflective coating can postpone that shrinkage for years.

My best advice: when evaluating a built up roof, don't focus too hard on the cracks -- they're a red herring -- focus on the flashings and the slope.

Also, in the better built-up roofing jobs that I see, they use a granule-coated cap sheet. It makes for a really nice, very long-lating job. Is that common elsewhere?

- Jim Katen, Oregon

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Jim you disagree with me in the first paragraph and eloquently get at what I am talking about in the second:)

Well except for the part about the flood coat expanding and contracting and the part about the roof in the picture being vulnerable to leaking and the part about it being in need of replacement.

Other than that, yes, I agree with you.

- Jim Katen, Oregon

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I agree with the assessement; when the roof is toasted, the little islands of asphalt start to lift. In my woods, by the time it gets to the lifting island thing, the whole mess is as toasted as Ozzie Osborne, and the little islands are the least of the concerns.

"Little drainholes running to a downspout"......(?)......there's no sump or metal clamping? It's just a hole running to an interior drain? That, by itself, would make me can the thing.

It's always about the flashing, and if one looks over all the NRCA flashing details, one quickly learns that no one does it right. Never. Condemning a flat roof is really, really easy, because they're always wrong someplace.

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Richard, your second post is the answer.

You inspected, the roof, either by your recommendation on on their own, your clients are having it inspected, hopefully by a certified roofing contractor, for an evaluation.

Maybe you could attend, with the roofer, and report back.

Have a great day

Tim

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  • 3 years later...

Flat roofs are more often found on commercial buildings than on residential homes. However, in areas where the climate is arid, a flat roof can provide the homeowner with a relatively inexpensive option for a gable roof or hip roof. While flat roofs save money in construction costs, they often make up the difference in the price of maintenance. Thanks.

Regards,

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