Threshold crack width for "damage" to a TPO roof membrane?

[jeffandmike]

The attached document is a study on the durability of TPO roofing.

Page 15 of the document describes how “hailstones larger than 3/4 inch in diameter were among those that impacted” one of the roof membranes (the one in Texas). This roof was sampled at some of the impact locations and examined under a scanning electron microscope. They reported that “Micro cracks were observed in some of the images but the cracks were reported by laboratory personnel to be very minute, ranging from 2 to 4 micrometers”. It goes on to say that “It appears that the membrane was undamaged by the hail impacts”, even with this level of cracking.

This implies that the cracks must reach some threshold before they can be considered to be "damage" to the membrane. It is not sufficient to have just any cracks, of any width. This doesn’t seem to be much different than looking at a reinforced concrete wall and determining that cracks that aren’t visible with the naked eye are not considered “damage”, for example.

What then should the threshold width be, for cracking in a TPO roof membrane? The threshold would likely be something larger than 0.004 mm (the upper limit in the attached study) and possibly something smaller than 0.05 mm to 0.1 mm (the generally accepted maximum crack width in water-retaining structures, according to ACI). It would seem that if the cracks didn't affect the water-tightness of the membrane, and didn't decrease its life expectancy, they couldn't be considered as "damage".

Your input is appreciated.

 

[EngineeringEric]

I would say that any visible crack or crack that penetrates the multi-layer roofing membrane is a failure. Anything larger than the capilary action width will result in failure of membrane. Cracks at the .004 micron cannot be seen and may or may not really be able to allow the infiltration of water... I haven't ever really looked too into it. But i would not compare TPO/PVC/EPDM to concrete.

 

[jeffandmike]

Yeah... I think the issue I had is that if you look at a document like UL 2218A (Outline of Investigation for Impact Resistance of Roofing Systems), for example, the "acceptance criteria" reports that "The roofing system exposed surface shall show no evidence of tearing, fracturing, cracking, splitting, rupture, crazing or other evidence of opening". Again, this is referring to the "exposed surface"... not the underside.

However, if you then look later on in the document, it reports that "Observations and measurements of each impact location" shall include "Under layers/components examined for any tearing, fracturing, cracking, splitting, rupture, crazing, or other evidence of damage to the roofing system". The question becomes if this isn't part of the "acceptance criteria", what is the significance of it?

By 2218A, it appears as though you could have fracturing/cracking of any width in only the "exposed surface", and that would be considered a failure (regardless of whether or not the membrane is still water-tight, and whether or its life expectancy had been affected). However, it also appears as though you could have fracturing/cracking of any width in only the "under layers/components", and this wouldn't be considered a failure.

To respond to one of your comments... I wasn't trying to "compare TPO/PVC/EPDM to concrete". I was just proposing a possible upper limit on the acceptable crack width, to get the discussion going. I was really hoping that someone would have research to support a better alternative upper limit on the crack width.

W/r/t "any visible crack or crack that penetrates the multi-layer roofing membrane is a failure"... visible by any means, or only with the naked eye?

 

[EngineeringEric]

I didn't mean anything by the ACI comparison, I figured you were using it as a comparison since they state things clearly (the novelty of that!)

Yeah... I think 2218A is a document that is used for looking at the roof in place. I cannot see the underside layers so if the exposed surface is broken then it is failed. If the underside is broken, this would only be known by cutting out samples and well that defeats the purpose and cannot be done at every impact (how many hail stones will hit a roof? )

I have done one investigation with a roofing manufacturer for certain areas and it was done with naked eye and a seam tool. One area he broke out a magnifying glass but this was not done at every location.

Sadly I cannot help much since it sounds like you may dealing with more of a specialty claim/investigation than i have done. I do think that by any means is a bit excessive so i tend to think that if i cannot see it with a naked eye or magnifying glass then it isn't too big.

 

[JAE]

I didn't realize that TPO roofing was a structural system.

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[jeffandmike]

Fair enough. I submitted a "Report" requesting that this thread be moved to the Forensic Engineering forum. That is the closest thing I could find to the topic of the thread. Thanks.