Which adhesive caulk for solar panel mounts??

On metal and EPDM roofs people use either Dicor self-leveling lap sealant or Geocel Advanced RV EPDM roof sealant. Cant's say about fiberglass. I tried both, and like Geocel better, cures faster and no shrinking. Though have to be careful when it's very hot - might skin too fast. Dicor is also better to use when it's not too hot.

Other sealants are mostly inferior for this application, from what I hear from people.

Both metal and EPDM membranes require screws in addition to sealant, you can't rely on sealant for fastening. Again, fiberglass is different.

I used VHB tape, screws and Dicor. Don't expect the screws to hold in thin fiberglass. What's below the fiberglass?

I believe that "non structural FG" - all those roofs except for Scamp, Casita, etc - have nothing but block foam underneath. Such things should better be checked by prying the trim off, inside the vent hatch.

Could have 1/8" ply backing, but this is still nothing. On surfaces thinner than 3/8" I prefer well nuts. Tapping screws won't hold.

In that case he should use appropriate support brackets that will allow screws to be secured into the roof beams.

Under the very thin dimpled fiberglass is luan .20". The RV manufacturer recommended hollow wall anchors that flare out when tightened. Using Stickyfeet and 1 or 2 anchors per mount. then covering with the adhesive caulk. Adding extra mounts and using the anchors because we encounter high wind gusts in our travels, sometimes head on. Might be overkill, but it's for peace of mind.

3M 4000UV it's a marine adhesive and caulk. It's made for bedding cleats and such on boats above and below water line. It's UV resistant and has a tensile strength of 500psi. They also make a 5200UV but that's overkill.

CA Traveler wrote:
I used VHB tape, screws and Dicor. Don't expect the screws to hold in thin fiberglass. What's below the fiberglass?

got 5 panels held down like this

VHB tape and cover the edges/screws with sealant of choice. I like the Geocel products.

kaz442 wrote:
3M 4000UV it's a marine adhesive and caulk. It's made for bedding cleats and such on boats above and below water line. It's UV resistant and has a tensile strength of 500psi. They also make a 5200UV but that's overkill.

Good choice. I used 3M 4200 and it's also holding up well.

These 3M products are very adhesive, as well as good sealants. 3M 5200 is extremely strong.

AFAIK the other products, Dicor, Geocell, are less adhesive but also good sealants.

3M 4200 is a sealant, it cures somewhat soft. Adhesion, particularly to FG, is a different matter, not everything adheres well to cured polyester resin but this is not crucial when sealant is used to seal rather than to hold.

The RV manufacturer recommended hollow wall anchors that flare out when tightened.

Sounds like well nut.

Hitting the rafters is a good idea too, when practical.