Anyone ever loose lay slip sheets under a ballasted PV system? I am being asked to approve this but I don't think it is a good plan. I have seen loose sheets work their way out from under dura-bloc conduit supports and end up in the parking lot. Anyone do this regularly?
Loose lay slip sheets
- Thread starter coop3339
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Typically it depends on what the roofing material is. On some membrane roofs they are required in order to keep the warranty. You should check with the roofing manufacture. DuraBloc is compatible with some manufactures and slip sheets are not required. Others require the slip sheet. Make the call as you do not want to void the roof warranty. Depending on the size of the roof you could be talking major mega bucks to replace.
- Location
- Massachusetts
I am curious what code section could be cited for not accepting it.:huh:
ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Consulting Electrical Engineer - Photovoltaic Systems
It's not a code thing, it's at the discretion of whomever is responsible for enforcing the warranty.
I have run into this numerous times with different roofing companies/vendors/suppliers. Some allowed loose laid slip sheets, some required they be torched down. Some required that the slip sheets be the exact material (theirs) as the existing roof, some didn't care as long as it was an equivalent material. Some required that their rep inspect the roof before and after installation, some didn't, and some of those who required it charged a fee for the inspections.
Make the call. Failing to follow their directions could result in the revocation of the roof warranty, and you don't want that. Your customer would be very unhappy.
- Location
- Massachusetts
I don't think the OP is the roofing manufacturer based on their profile and they stated:
I am curious what basis the OP would have to not approve it.
I am very familiar with roofing warranties and oddities, been working on commercial roofs well before PV was a thing.
The first PV job I did here we needed custom pieces from Firestone the roofing manufacturer.
I don't think the OP is the roofing manufacturer based on their profile and they stated:
I am curious what basis the OP would have to not approve it.
I am very familiar with roofing warranties and oddities, been working on commercial roofs well before PV was a thing.
The first PV job I did here we needed custom pieces from Firestone the roofing manufacturer.
Hi iwire,
My understanding is that the slip sheet is meant to protect the roof membrane from abrasion caused by the array shifting from thermal expansion. If the slip sheet it is not adhered to the roof, it is likely that it will slide against the membrane and cause abrasion. The roof is TPO and the slip sheet is TPO. The solar racking has rubber pads. I would think that rubber to TPO has a higher coefficient of friction than TPO to TPO, making it more likely that the TPO to TPO joint will slide instead. Also the ballasting weight is determined by wind tunnel test data. This test was probibly done without slip sheets in place. If the TPO to TPO joint has a lower coefficient of friction then the array would require more ballasting to stay in place. The roof warranty only suggests that the slip sheets be adhered but i would bet they will not warranty the roof if a leak was caused by a missing or sliding slip sheet that was not adhered. Last but not least, I think the loose slip sheets are going to end up shifting all over the place over time and look like hell.
Carultch
Senior Member
- Location
- Massachusetts
The wind tunnel testing is done to determine the incident wind forces on the array. They may not even have your particular roof membrane in the test assembly. Knowing the traction coefficients in question, is needed to extrapolate how to keep the array in place, given the incident loads.
Also, wind load on a nearly flat array is primarily vertical. Seismic loads are more important in determining lateral support.
- Location
- Massachusetts
What code section would you cite to 'not approve it'?
I am not an AHJ and I am not referring to any electrical code but to respond to your question, it would be 110.12. Installing electrical equipment on something that is likley to shift or become dialoged and blow off a roof is surely not installing in a neat and workman like manor.
My question is mostly for quality control. I was hoping for other installers to chime in on their experiences with loose slip sheets. This was offered as a value engineering option and I'm not sure that it is a good idea for the reasons stated above.
Carultch
Senior Member
- Location
- Massachusetts
Where roof protection pads really belong, is adhered or attached to the racking system itself. The problem is that the roofing manufacturers often require a super-specific match, and no racking manufacturer will inventory every possible membrane sample.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
So for best performance you want to guarantee that the protection pad will slip relative to the roof membrane instead of that slipping happening between racking and the protection pad?
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