Solar panel mounting to ~1/4" motorhome decking with EDPM roof.

[brainbone]

I'm getting ready to mount some panels on a motorhome roof that seems to have only 1/4" plywood decking, with a small radius. Will be mounting them lengthwise down the center of the roof. 3 x ~30" x ~50" panels, mounting between A/C and vents, so not exactly a continuous line of panels.

Trying to keep weight and profile down as much as possible, I was planning on using 2 x 48" 13/16" unistrut along the length of the each panel, with Z brackets mounting the panel to the unistrut, spreading pull on the brackets over the unistrut, and hoping to overlap a roof truss or two so I wouldn't have to rely on just the thin decking. Unfortunately, it looks like there are spans larger than 48" on the roof where there are no trusses spanning side to side.

I'm entertaining the idea of just relying on butyl tape and 8 or so screws into the 1/4" decking over the length of the unistruts to hold, though I'm thinking it's probably a bad idea.

Anyone run into this? Should I just get 10' unistrut in order to span some trusses and deal with the extra weight, or is screwing to the decking alone enough?

 

[mmmmmdonuts]

I think a picture would help. Also recommended using the standard strut especially if you're trying to use washer or tire the solar panels down. You have to slide the whole length of the channel on the smaller stuff where you can angle it in on the taller strut.

 

[scrubjaysnest]

I used VHB tape, 3/4 inch SS number 12 screws, and adhesive sealant. I made z brackets from 1 5/8 inch aluminum angle 2 inches long. 4 brackets per panel. Then sealed brackets and screws with Dicor self leveling. Been there since 2017. Also made it so I could tilt the panels when parked.

 

[wme]

MOST RV with EPDM roofs have the trusses attached to the top of the walls which are load bearing. Putting extension on the ends of the uni-struts, matching the radius of the roof, will be the strongest method. You may have to jiggle you layout to march reality.

 

[brainbone]

I think a picture would help.

Not to scale, or pretty, but the attached should give a rough idea of the issue. Where I had originally planned on putting 4' (48") unistrut doesn't line up with the motorhome's steel trusses, and instead are on 1/4" plywood.

To hit trusses, I guess I'd need to run longer unistrut pretty much from the first strut to the last, though wondering how necessary it is.

I used VHB tape, 3/4 inch SS number 12 screws, and adhesive sealant. I made z brackets from 1 5/8 inch aluminum angle 2 inches long. 4 brackets per panel. Then sealed brackets and screws with Dicor self leveling. Been there since 2017. Also made it so I could tilt the panels when parked.

Only screwed into thin decking?

MOST RV with EPDM roofs have the trusses attached to the top of the walls which are load bearing. Putting extension on the ends of the uni-struts, matching the radius of the roof, will be the strongest method. You may have to jiggle you layout to march reality.

See attached sketch. I was expecting trusses every couple feet or so, but this motorhome does it a little differently from what I anticipated. I'd prefer not to move the panel locations.

 

[HRTKD]

If you're trying to keep the weight down then Z brackets are the way to go. They weigh a lot less than strut. I've done PV installs on RV roofs both ways. Strut gives you a lot of flexibility and a very sturdy mount. But it takes more time and weighs more. Z brackets may not be as sturdy as you're going to get only one screw for each mount in a roof truss. Pretty quick install, but if you ever need to replace the panel the Z bracket may need to be removed from the roof, especially if using a different sized panel.

If I was a professional installer, I would use Z brackets simply because they take less time to install.

There are different types of strut available. Full height and low profile. Steel and aluminum. I used a low profile aluminum strut that doesn't have a standard channel. I'll never do that again. Standard channel is critical.

 

[brainbone]

If you're trying to keep the weight down then Z brackets are the way to go.

The problem is the thin decking. I'm concerned just 4 z-brackets screwed into 1/4" plywood isn't going to be strong enough since none of the screws would hit a truss, hence the unistrut. Problem is, with shorter sections of unistrut I'd still only be screwed into the thin decking, though spread over more surface area.

 

[HRTKD]

Here is another option. I ran short sections of strut perpendicular to the length of the trailer, right at each truss with three screws in each section. The sections are short because of the roof arch. Any longer and they couldn't be attached without putting stress on the strut and the roof.

 

[MisterSandals]

The problem is the thin decking.

How about this:



I'm not sure how critical your weight restrictions are but personally my first choice would be (has always been) VHB with safety tethers.
Next i'd consider full length strut, aluminum if weight that critical and cost not a problem.

 

[brainbone]

How about this:

View attachment 303357

That's too... obvious... (Now I need to question my own sanity a bit.)

Other than the serviceability issue (z-brackets directly to roof make it difficult to remove a panel without dealing with adhesive, etc., though ideally I wouldn't have to do that once installed) I'd need to add some spacers to the side-mounted z-brackets to deal with the radius/arc of the roof, though not difficult to solve at all.

Here is another option. I ran short sections of strut perpendicular to the length of the trailer, right at each truss with three screws in each section.

That, in combination with MisterSandals' suggestion, may be the way.

Thanks for the input!

 

[Backyard]

Is the roof thick enough to use molly bolts to hold your brackets?

 

[brainbone]

Is the roof thick enough to use molly bolts to hold your brackets?

Yeah, I thought about short molly/jack-nuts or rivnuts for thicker stock, though wasn't sure how well they'd work in 1/4 plywood.

 

[Zwy]

I used a stud finder to locate the roof truss and screwed into those. 1/4" plywood won't hold long term. Ever see solar panels flying on the interstate? I have and I see panels on the side of the road.

I used non perforated aluminum unistrut. Lighter and no corrosion plus panels can be mounted directly to the strut without any galvanic reaction. Under the unistrut where the fasteners went, I used double sided Eternabond. This seals the hole from water intrusion. Once screwed down, I used Dicor around the unistrut. If I did it again, I'd find a different product than Dicor. It gets that dirty look and has to be replaced over time.

The trusses always seem to never line with the correct spots on the panels plus there is an air gap under the panels with unistrut. I purchased the unistrut from Grainger online and picked it up from the store 2 hours away when I had a doctor appointment and was driving thru.

 

[x98myers7]

yes, as above is all great advice... use aluminum strut. vhb tape + screws. make sure you use a stud finder and get the screws into the trusses. seal it up well. there's a bunch of videos on youtube on it. I would also consider adding a wind screen for the front edge of the roof to direct air over top of the panels when travelling.

 

[occuplus]

I used vhb tape on roof. 2 aliminum rails. Each 280cm to 8 cm. 2 winter n one summer. There s no problem. Camper s CTP panel 370cm n PV r 375watt each.
I

 

[Zwy]

I used vhb tape on roof. 2 aliminum rails. Each 280cm to 8 cm. 2 winter n one summer. There s no problem. Camper s CTP panel 370cm n PV r 375watt each.
I View attachment 303924

That looks to be an aluminum skin on the roof and not TPO. It all depends on the roof material.

 

[occuplus]

That looks to be an aluminum skin on the roof and not TPO. It all depends on the roof material.

Its CTP. 30mm xps foam covered by 2mm laminated wood on each side.

 

[Zwy]

Its CTP. 30mm xps foam covered by 2mm laminated wood on each side.

Doesn't appear to be coal tarred pitch. The other CTP is Transparent CTP sheets. It doesn't appear to be transparent.

It could be the fiberglass type sheeting commonly used on sidewalls. That is glued to 1/4" luan plywood.