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CMUs over membrane roofing


mgbinspect
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Greetings brethren,

Workin' on a very large bay front property inspection.

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I've seen this roof setup a number of times before, and I've always been fine with it. But, I thought it might be prudent to poll the TIJ brain-trust to see if anyone has had bad experiences or is aware of any problems with this marriage. It's CMU patio stones over a membrane roof. I've always thought it was fine. Some of the stones rock a bit, which causes me to wonder if they can't cause some materials chaffing. Any thoughts or concerns?

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Thanks in advance for all insights.

Mike

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What membrane? Mod bit, granulated, what?

Definitely not selvedge (aggregate), Kurt. I could only see about 2" around the perimeter, but it appears to be what I've always thought of as a modified rubber membrane - the look/finish/feel of an automobile inner tube - heavier of course. Built in 2006.

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I don't see an issue with it, as long as there is no conditioned space beneath it.

Marc

It' the master bedroom balcony that is also the roof over the monstrous eat-in kitchen. As I said, in general, I'm fine with it. I mostly want to hear of any precautions or bad experiences with the setup.

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There's no such thing as modified rubber membrane. Is it EPDM? That looks like inner tube material, sort of.

EPDM can be ballasted, but I'm not sure about concrete pavers.

I think you're probably right - looking at photos of EPDM, that looks to be the animal. It has the look and feel of a rubber compound - thick material, of course with an apparent mesh reinforcement in it that slightly telegraphs through the material surface.

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The following is from the Building Envelope Design Guide produced by the National Institute of Building Sciences (NIBS) available here.

If crushed aggregate is specified, a stone-protection mat between the membrane and aggregate should be specified to avoid puncturing the membrane. A stone-protection mat is also recommended when smooth aggregate is used because some sharp fragments are often among the smooth aggregates. Also, aggregates sometimes fracture into very sharp pieces after they have been installed. It is also a conservative practice to specify a mat to protect against abrasion and puncture from fragments during paver installation. A somewhat thinner mat is normally sufficient for paver-ballasted jobs.

Marc

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The following is from the Building Envelope Design Guide produced by the National Institute of Building Sciences (NIBS) available here.

If crushed aggregate is specified, a stone-protection mat between the membrane and aggregate should be specified to avoid puncturing the membrane. A stone-protection mat is also recommended when smooth aggregate is used because some sharp fragments are often among the smooth aggregates. Also, aggregates sometimes fracture into very sharp pieces after they have been installed. It is also a conservative practice to specify a mat to protect against abrasion and puncture from fragments during paver installation. A somewhat thinner mat is normally sufficient for paver-ballasted jobs.

Marc

That was useful. I don't think there's any type of protective mat under the CMUs, which are about 12 x 12 x probably 2"

Kurt (or anyone else), any thoughts? I'll probably call for a roofer to bless the installation. I'm sure it's fine for now - no leaking and the material I can see looks great - like new. But the way some of the pavers rock suggest that maybe the roof sheathing should have been thicker or doubled.

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EPDM is synthetic rubber.

The top manufacturers (Firestone, Carlisle, etc.) require a protective barrier called paver mat, between concrete pavers and the roofing membrane. Carlisle has a rubber paver that doesn't need a mat. You should be able to easily find if the mat is present.

Thanks Bill, I believe the second photo shows the turned up edge of that mat. I think we're good to go then, except I still don't like the way units rock over the joists. I bet they should have doubled up the sheathing.

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Another concern is keeping the drain(s) clear. The drain has to be in the membrane below the level of the pavers, so it should be easily accessible. I've seen them covered so you have to lift pavers to find them, wrong.

John, No indication of a drain other than a slight slope. the balcony is probably about ten feet wide and fifteen feet long. I see no scupper or indication of an internal downspout system.

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