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Membrane Roof Pitch


inspectorwill
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It's been over week since the last rain and this 3 year old membrane roof system still has 1" of standing water. This is a replacement roof system. Roofer was on site and told me the reason for going with the membrane roof was the original pitch was bad so they went with a material that is watertight and has a long warranty period. There are only 6 or so manufacturers of membrane roofing (TPO & PVC) and I read through the installation guidelines for several that I retrieved from the web. None provided information regarding roof pitch or whether standing water on the roof system will result in any sort of damage or void any warranties. Why bother installing the scuppers in the parapet wall if the water must get to a depth of 2-3" to drain? Keep it closed and it can double as a pool. I understood slope should be 1/4" per foot on all flat roof systems including membrane. Does anyone have any documentation showing that standing water for extending periods is detrimental or contradictory to general installation standards.

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As a general rule of thumb (and best practice), standing water should drain/dry within 48 hours after a rain.

There are several roof systems (some coal tar pitch systems, EPDM and other single-ply systems) that can withstand standing water indefinetly and remain watertight. There are even inverted sloped roof structures that are designed to hold water to help cool the building, although they are few and far between. It's usually fairly obvious if a roof structure was designed this way.

Large amounts of standing water on a roof that was clearly not designed as an inverted slope, can be a weight issue. Obviously I have no abilty to calculate roof load capacity, so on a roof that does not appear to have been designed to hold water but is holding substancile amounts, I would call it out for further evaluation by a structural engineer reguardless of the roof system or its condition.

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If you look long enough, you'll likely find what you need in writing somewhere.

I've been able to find requirements for flat roof systems (Mod Bit, EPDM, TPO & PVC) that indicate a minimum slope of 1/4:12 and/or no standing water after 48 hours. If you can determine the manufacturer (there are many brands, but few manufacturers), and it's not clear in the installation instructions, look for the ICC Evaluation Service report. I recently documented issues with a "Mule Hide" EPDM roof installation as part of an expert witness assignment. I obtained the ICC-ES report for the actual manufacturer (Carlisle). The design and installation section clearly states that their EPDM and TPO products must be installed with a minimum 1/4:12 slope.

Roofer was on site and told me the reason for going with the membrane roof was the original pitch was bad so they went with a material that is watertight and has a long warranty period.
Why the heck didn't they fix the slope with tapered insulation?
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If you look long enough, you'll likely find what you need in writing somewhere.

I've been able to find requirements for flat roof systems (Mod Bit, EPDM, TPO & PVC) that indicate a minimum slope of 1/4:12 and/or no standing water after 48 hours. If you can determine the manufacturer (there are many brands, but few manufacturers), and it's not clear in the installation instructions, look for the ICC Evaluation Service report. I recently documented issues with a "Mule Hide" EPDM roof installation as part of an expert witness assignment. I obtained the ICC-ES report for the actual manufacturer (Carlisle). The design and installation section clearly states that their EPDM and TPO products must be installed with a minimum 1/4:12 slope.

Roofer was on site and told me the reason for going with the membrane roof was the original pitch was bad so they went with a material that is watertight and has a long warranty period.
Why the heck didn't they fix the slope with tapered insulation?

$. A lower bid is easier to sell and most clients aren't savvy enough to know the difference.

Marc

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