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Bubbling / Blistering of MB


Mark P
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What does the brain recommend about blistering / bubbling of MB? I've read where some recommend slicing them open and re-sealing. I've also read that it is not a problem (assuming there is no active water intrusion) and leave them alone until the roof is replaced.

I understand it can be caused for a number of reasons: moisture is getting underneath or it was applied over a wet spot.

This is a commercial property. I found no evidence of leaks.

Most of the bubbles were the size of the one in the 2nd picture. The one in the 3rd pic was the biggest by far.

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Water will expand approximately 700 times it's volume when it goes from liquid to vapor. It doesn't take much water to create a couple of those blisters.

I'd be much more concerned with the parapet, and not just the crappy seams. What's going on under the membrane that's (incorrectly) wrapped up over the coping? In most cases, it's parapetitis.

Also, what's the roof structure and sheathing, is there a closed/sealed attic "plenum", what insulation.....(?)....

Reason I ask......I've been finding some mightily screwed up roof structure that's got mod bit on top, drywall on the bottom, and insulation stuffed between the two.

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Here is a picture of the other side of the parapet. It has coping stone on top.

What do you recommend be done with the parapet? Remove the MB? Would you explain furter please what is wrong with it.

I don't know what is under the MB. The rooms below have 15 foor ceilings with acoustic tile. There was no attic or access to see what was between the MB and ceiling.

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That rectangular area below the scupper looks like a repair. A failure in that area can be expected when such a large amouint of water is dumped on a level roof. It should be piped away to the ground or rainwater drainage system.

Also, there's no gravel on that rectangle. Asphalt degrades rapidly when exposed to the UV content of sunlight.

Given that it's commercial and a big one, I'd likely punt the entire roof to a Registered Roofing Consultant.

Marc

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What do you recommend be done with the parapet? Remove the MB? Would you explain furter please what is wrong with it.

Moisture migrates up through the structure by stack effect. It hits the underside of the roof sheathing/roof membrane and condenses like crazy. It either soaks into the roof sheathing and rots it, or it moves out to the perimeter.

Wet moves to dry, hot moves to cold. It's cold and dry out at the perimeter, so the moisture migrates over there, where the parapet stack effect draws it up and between the membrane and the masonry. Massive condensation occurs, the moisture soaks into the brick, and the result is interior wythe brick that turns to powder.

Cut the membrane back down to about 12" above the roof line, mechanically fasten it to the masonry, then put a counterflashing over the termination bar.

That is, of course, if there's anything under that membrane to mechanically attach to; usually, it's mush, and the parapets need rebuilding, or at minimum, the inner wythe needs rebuilding.

Roofers do this because it's the cheap easy way for them to do it, and they can honestly say the roof won't leak. But, it destroys the parapet from the inside out.

All the problems with stack effect in old buildings are exacerbated and increased at parapets. I first started noticing this about 18 years ago when all the improperly installed roofs started damaging masonry.

Joe L. talks about it ins some of his Insight articles, so this is established building science, not conjecture.

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Here is a picture of the other side of the parapet. It has coping stone on top.

What do you recommend be done with the parapet? Remove the MB? Would you explain furter please what is wrong with it.

I don't know what is under the MB. The rooms below have 15 foor ceilings with acoustic tile. There was no attic or access to see what was between the MB and ceiling.

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Well, if you're gonna bring it up and over the parapet, the coping goes OVER the membrane. But as Kurt always mentions, it's better to terminate well below the top of the wall with a reglet or termination bar flashing method.

Where the vertical wall meets the roof, a cant strip should be installed (without a cant, the membrane always tears and cracks at this joint). The wall flashing cap sheet then has to extend 6" beyond the cant strip onto the roof surface. The seams are a mess too.

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