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Mounting panels w/VHB on PVC roof/Forest River?

PlanetExcellent

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I have a year-old Forest River Surveyor Legend travel trailer with a PVC roof. The roof has a slight texture to it; it's not smooth and shiny like fiberglass. It's not uneven or "bumpy", just slightly textured almost like sandpaper.

I'm considering mounting 2 210w panels using unistrut rails secured with VHB 5962 tape, possibly with a few screws directly into the roof. I would run the VHB tape the entire 5.5 foot length of the unistrut. I've seen that AM Solar uses short lengths of VHB tape to mount panel brackets to metal/fiberglass roofs, with the addition of some screws on EPDM rubber roofs. I figure that if short lengths of VHB work on a perfectly smooth surface, long lengths with more holding power should work on a less-smooth surface. And if VHB + screws work on rubber roofs, they will work even better on a PVC roof.

Looking to hear from people who have used the VHB/unistrut method on a roof that is not as smooth as metal or fiberglass.

EDIT: I plan to clean the surface with alcohol, and I'm considering using the VHB primer / adhesion promoter that 3M recommends.
 
Using tape only on EPDM roof is pure insanity.

Agreed, which I guess is why AM Solar uses VHB + screws on rubber roofs. They claim they have never had a panel detach, and they've probably done as many installs as anyone.

So how do I determine where the aluminum roof crossmembers are? I've read that stud-finders don't work, and diagrams don't seem to be available from RV manufacturers. I suppose I could just thump on it and try to listen for a different sound.
 
Agreed, which I guess is why AM Solar uses VHB + screws on rubber roofs. They claim they have never had a panel detach, and they've probably done as many installs as anyone.

So how do I determine where the aluminum roof crossmembers are? I've read that stud-finders don't work, and diagrams don't seem to be available from RV manufacturers. I suppose I could just thump on it and try to listen for a different sound.

Many RV manufacturers will give that information out when requested.

Yep. Pushing/tapping/thumping on the roof surface may help you determine where the structural components are.
 
Agreed, which I guess is why AM Solar uses VHB + screws on rubber roofs. They claim they have never had a panel detach, and they've probably done as many installs as anyone.

So how do I determine where the aluminum roof crossmembers are? I've read that stud-finders don't work, and diagrams don't seem to be available from RV manufacturers. I suppose I could just thump on it and try to listen for a different sound.
The primary issue with rubber roofs is that the rubber isn't bonded to the substrate. It will lift right off. And if you press AM Solar they'll also tell you that they don't recommend mounting panels larger than about 100w on any mobile roof. I bought some of their little mounting brackets and they're sitting in a box in the garage after I decided to make my own. No way would I use those on anything but the smallest panels.

A stud finder will locate the trusses, I used one on my aluminum framed trailer. Just get one that will locate metal. It takes some experimenting to get the hang of it but it will at least get you in the ballpark. Drill a 1/8" (or smaller) test hole if you're unsure, easy to plug with Dicor lap sealant if you miss.

https://www.amazon.com/gp/product/B07NWMBB6N/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

But in most cases, especially with an older RV, you can see the truss lines on the surface of the rubber.

See the truss lines?

PXL_20220716_131047417.jpg
 
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Thanks, good ideas. To clarify, I don't have a rubber roof -- it's PVC plastic. Also since my trailer is less than a year old, there aren't any visible marks on the roof to go by. But I wasn't familiar with metal stud finders, I'll definitely get one.
 
In my install write-up I provided a couple of tips for finding the roof trusses:

 
Thank you, this is really helpful. I think I'll go with aluminum u-strut secured with VHB + screws into roof struts. Then secure the panels to the struts with 4 brackets on each side. Wire rope from panel to . . . something . . . as a safety tether. If it comes off at least it'll hopefully stay up there long enough for me to pull over.
 
Thanks all of you for the advice. I used the same aluminum u-strut channel to mount my 200-watt panels. The strut is 13/16 high, and the panels are mounted to it with Z-brackets, so the panel ends up being almost 2 inches above the roof surface for good ventilation and easy access to the cables.

My roof is 3/8 plywood with a thin PVC skin on top. The skin did not seem to be well-bonded everywhere so I mounted the struts with VHB tape and #14 screws. The screws took a good bite as they went in although I never felt anything solid like an aluminum cross member, even in places where it looked and felt like there was something underneath.
 
Thanks all of you for the advice. I used the same aluminum u-strut channel to mount my 200-watt panels. The strut is 13/16 high, and the panels are mounted to it with Z-brackets, so the panel ends up being almost 2 inches above the roof surface for good ventilation and easy access to the cables.

My roof is 3/8 plywood with a thin PVC skin on top. The skin did not seem to be well-bonded everywhere so I mounted the struts with VHB tape and #14 screws. The screws took a good bite as they went in although I never felt anything solid like an aluminum cross member, even in places where it looked and felt like there was something underneath.

Did you drill a pilot hole into the aluminum cross member? There should have been a noticeable pause after going through the plywood but before the drill bit penetrated the aluminum cross member.
 
Did you drill a pilot hole into the aluminum cross member? There should have been a noticeable pause after going through the plywood but before the drill bit penetrated the aluminum cross member.
I did on the first panel, but after 1-1/2 inches I never felt anything solid. I was scared to go deeper because I thought I would poke a hole in the interior ceiling.
 
I have a year-old Forest River Surveyor Legend travel trailer with a PVC roof. The roof has a slight texture to it; it's not smooth and shiny like fiberglass. It's not uneven or "bumpy", just slightly textured almost like sandpaper.

I'm considering mounting 2 210w panels using unistrut rails secured with VHB 5962 tape, possibly with a few screws directly into the roof. I would run the VHB tape the entire 5.5 foot length of the unistrut. I've seen that AM Solar uses short lengths of VHB tape to mount panel brackets to metal/fiberglass roofs, with the addition of some screws on EPDM rubber roofs. I figure that if short lengths of VHB work on a perfectly smooth surface, long lengths with more holding power should work on a less-smooth surface. And if VHB + screws work on rubber roofs, they will work even better on a PVC roof.

Looking to hear from people who have used the VHB/unistrut method on a roof that is not as smooth as metal or fiberglass.

EDIT: I plan to clean the surface with alcohol, and I'm considering using the VHB primer / adhesion promoter that 3M recommends.
I have a PVC roof too. It is a great and long lasting material but it’s glued down … it can only be as strong as the glue Bond to the roof ….time will gradually weaken the material and it will fail at some point.
If I ever put panels on my RV roof they will be attached mechanically and thru bolted ,sealed from water intrusion and with backing plates to spread the load ….. like a winch or hardware on a well built small to medium seagoing boat…
but then I’m usually a little bit over kill when anticipating loads on gear….
J.
 
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