Panel placement on roof of Class A & VHB Tape Mounting

[CarlCruzin]

Any recommendations of doing it differently, or does this appear to be an ideal layout given the factors involved?

Layout considerations:
1. Grouping panels that are next to roof protrusion (AC unit or vent) in series, since when the sun is not in their favor they will likely not perform.
2. Grouping panels that are not next to a protrusion in series so they can be fully utilized.

Edit: Added "VHB Tape Mounting" to title due to discussion.

 

[eXodus]

The physical layout should not only optimize solar layout - but more important fastening to the structure of the RV.

You need to figure out where the roof trusses are running to mount the panels secure. There are strong forces on the front and back of the RV trying to rip the panels of the roof.

Solar "Hump" for aerodynamics for Class A RV

Yup, hence why I said earlier the blockage ratio should be under 5% (and preferably under 3%). I would not treat these simulations as anything more than a bit of light comic relief. Do them for fun, but don't expect any actionable intelligence. This really is an area where many years of...

diysolarforum.com

I'm currently going down that rabbit hole with my Class A. - I'm going above the Vents. - installed low profile ventilation and cover them with the panels.

 

[12VoltInstalls]

Grouping panels that are not next to a protrusion in series so they can be fully utilized.

From the diagram it looks like the lower left has room for one more panel. I’m wondering if an advantage could be realized with three series strings 3S3P
It appears that this would result in at least two strings being unobstructed at any time potentially maximizing harvest watts per hour

 

[CarlCruzin]

From the diagram it looks like the lower left has room for one more panel. I’m wondering if an advantage could be realized with three series strings 3S3P
It appears that this would result in at least two strings being unobstructed at any time potentially maximizing harvest watts per hour

It isn't pictured, but that is where the ladder is so I left some extra space.

The physical layout should not only optimize solar layout - but more important fastening to the structure of the RV.

You need to figure out where the roof trusses are running to mount the panels secure. There are strong forces on the front and back of the RV trying to rip the panels of the roof.

Solar "Hump" for aerodynamics for Class A RV

Yup, hence why I said earlier the blockage ratio should be under 5% (and preferably under 3%). I would not treat these simulations as anything more than a bit of light comic relief. Do them for fun, but don't expect any actionable intelligence. This really is an area where many years of...

diysolarforum.com

I'm currently going down that rabbit hole with my Class A. - I'm going above the Vents. - installed low profile ventilation and cover them with the panels.

I've seen many different ways that people have secured solar panels, ranging from dangerously minimalistic to overkill.

I like the approach of this Youtuber with VHB tape and unistrut.

It seems that properly cleaned mating surfaces running the full length of unistrut will bond very well with it. Gravity also constantly reinforcing the bond. https://www.3m.com/3M/en_US/p/d/b40071691/
From 3m website:

• Fast and easy-to-use permanent bonding method provides high strength and long-term durability
• Virtually invisible fastening keeps surfaces smooth
• Can replace mechanical fasteners (rivets, welding, screws) or liquid adhesives
• Gray, 0.062 in (1.6 mm), multi-purpose adhesive and conformable acrylic foam core for good performance in a variety of applications
• Eliminate drilling, grinding, refinishing, screwing, welding and clean-up
• Creates a permanent seal against water, moisture and more
• Pressure sensitive adhesive bonds on contact to provide immediate handling strength
• Allows the use of thinner, lighter weight and dissimilar materials

 

[Browneye]

I've seen many different ways that people have secured solar panels, ranging from dangerously minimalistic to overkill.

I like the approach of this Youtuber with VHT tape and unistrut.

It seems that properly cleaned mating surfaces running the full length of unistrut will bond very well with it. Gravity also constantly reinforcing the bond. https://www.3m.com/3M/en_US/p/d/b40071691/
From 3m website:

• Fast and easy-to-use permanent bonding method provides high strength and long-term durability
• Virtually invisible fastening keeps surfaces smooth
• Can replace mechanical fasteners (rivets, welding, screws) or liquid adhesives
• Gray, 0.062 in (1.6 mm), multi-purpose adhesive and conformable acrylic foam core for good performance in a variety of applications
• Eliminate drilling, grinding, refinishing, screwing, welding and clean-up
• Creates a permanent seal against water, moisture and more
• Pressure sensitive adhesive bonds on contact to provide immediate handling strength
• Allows the use of thinner, lighter weight and dissimilar materials

I did too. Worked out really well. I did put three #12 screws into the roof on each channel to ensure good adhesion on the tape.
I've since added an additional 100W panel just for starter-battery charging, and used just the tape. Once that stuff sticks it's not coming off. And if you seal the edges with urethane caulk, it will never get water or dirt intrusion.

For a bunch of panels like you're doing I would probably opt for aluminum cross rails instead of steel.
Toro had good pricing on aluminum strut rail as well.

Old RV - New PV's With Strut-Channel Mounting

I'm new here, starting a new thread on my 430W installation, so I can ask questions when they come up. This forum has been a wealth of information. I have also been scouring the 'net for, and learning about, LiFePo4 batteries as well. I'm not sure where I first saw strut channel mounting for...

diysolarforum.com

 

[12VoltInstalls]

seems that properly cleaned mating surfaces running the full length of unistrut will bond very well with it.

It does seem that way doesn’t it.

A safety retainer cable or securely bolting the unistrut is necessary. VHB is not good for 65mph imho. You could kill somebody.

 

[corn18]

No way the rubber membrane on my roof could hold onto much of anything. I wouldn't trust it. If I had a fiberglass or metal roof, VHB all the way. I can easily see my panels, unistrut and most of my rubber roof peeling off going down the road. The unistrut will still be attached to the rubber membrane as it flies through the air @ 75 mph.

 

[Browneye]

Rubber roof is a thru-bolt affair.

 

[corn18]

Rubber roof is a thru-bolt affair.

Agree. I got lucky and I can see exactly where the aluminium roof struts are (you can see them in that image). So I use unistrut to hit those attached with 2" stainless lag bolts.

 

[CarlCruzin]

(Disclaimer: I am attempting to provide constructive discussion, I hope it does not come off as argumentative)

I definitely prefer to lean on the side of overkill on matters of safety. I would like to sort out an effective way of adding a mechanical fastener to supplement. However, I believe this recommendation is for diy peace of mind as opposed to a mechanical necessity. I am just not convinced it is a true matter of life and death as some imply (when done properly in the manner that I am considering)

I am no aerodynamic engineer, but how much lift do you think a 1"x30" air passage can generate? Especially when setback from the front of the rv?

If I were to block the only gap under the front of the panel, wouldn't that eliminate any possibility of lift?

We have heard, or seen a couple videos of panels flying off an rv, but based off my observation there are plenty of people that venture into diy negligently. I would assert that almost certainly they failed to use the proper adhesive, and/or failed to clean the surface, and/or didn't use enough of a contact patch, and/or mounted the panels high enough to generate significant lift at highway speeds.

 

[corn18]

(Disclaimer: I am attempting to provide constructive discussion, I hope it does not come off as argumentative)

I definitely prefer to lean on the side of overkill on matters of safety. I would like to sort out an effective way of adding a mechanical fastener to supplement. However, I believe this recommendation is for diy peace of mind as opposed to a mechanical necessity. I am just not convinced it is a true matter of life and death as some imply (when done properly in the manner that I am considering)

I am no aerodynamic engineer, but how much lift do you think a 1"x30" air passage can generate? Especially when setback from the front of the rv?

If I were to block the only gap under the front of the panel, wouldn't that eliminate any possibility of lift?

We have heard, or seen a couple videos of panels flying off an rv, but based off my observation there are plenty of people that venture into diy negligently. I would assert that almost certainly they failed to use the proper adhesive, and/or failed to clean the surface, and/or didn't use enough of a contact patch, and/or mounted the panels high enough to generate significant lift at highway speeds.

You anywhere near Cincinnati? Stop on by and I will show you my rubber roof. It is not well attached to the substrate.

 

[jberger]

VHB Tape on a rubber (TPO, etc) style roof is a disaster waiting to happen with a panel. The roof is just not designed for it. You can tack down cables on a membrane roof, but nothing with uplift, it's either going to peel off or tear the membrane, both outcomes are going to ruin a trip.

VHB strength varies with heat too, so a hot panel on a hot roof in the desert is going to have more flex and stretch than you want. Mechanical fastening is the only safe option with larger panels.

 

[Browneye]

Here's how I calculated mounting strength on my motorhome. I'm no engineer, but I can do basic calculations based on test data.

Based on the wiki here on wind loading panel testing on a sloped roof, and the 3M testing stats on glazing in high-rise buildings, I estimated that at 90mph apparent, there could be up to 250psi of lift on a 26x50" solar panel 6" off the roof.

The 3M stat showed 18psi strength, what they use for rating glazing for wind loading.

The way I calculated my mounting struts was 1.5" x 60 x 2 struts x 18psi = 3240 pounds of strength. If the potential is 250lbs of lift from a headwind at freeway speeds, I was shooting for ten-times strength redundancy in the mounting. Due to the slotted holes I estimated that I could not get the full strength of the strut mounting surface, so even with half that, it's still six and a half times. I was confident that I hit that. Plus I mounted my panels 1.5" off the roof - the height of one strut crossing the other. Yes, low-profile superstrut from home depot - they're about $20 for a 10' strut. Another cool option is the ability to mount angle propping mounts to the struts if you want to slope your panels. I decided I wasn't going to want to climb up there every time and fiddle with them. Flat was going to be good enough. Solar production is surprisingly good.

As mentioned, I also put 3 #12 self-tapping screws in each strut - the fiberglass roof is very thin, and it's applied over very thin luan plywood, but that large of a screw gripped quite well, and ensured I got good contact pressure on the tape on my struts. Then sealed over the screws with sikaflex urethane caulk. If nothing else it ensures the fiberglass is held to the plywood substrate. Under that is just expanded foam insulation. But it's plenty rigid to walk on, although I don't like traffic up there such as for event viewing and such.

I don't think you could pull my panels off the roof without tearing half the roof off. I have over a thousand miles of freeway traveling with them in headwinds up to 40mph. I no longer worry about them, but will keep a close eye on the sealant and mounting area.

I also applied 1" wide eternabond tape to the inside of the struts to seal the slotted holes that exposed the VHB tape underneath.

This is for a fiberglass or metal roof. For a rubber sheeted roof you do need some way to mechanically fasten your mounts.

 

[Browneye]

BTW, the strut-nuts are the schiznit for fastening to the struts. I painted mine for rust resistance as they are just cad-plated.
1/4-20 is plenty strong, I used SS bolts and fender washers, and locktited them in.

And you have the bonus of being able to remove them should you need to replace a panel - and they're size adjustable. Could pretty much fit any old solar panel on there. The struts on the roof are permanent.

 

[CarlCruzin]

Here's how I calculated mounting strength on my motorhome. I'm no engineer, but I can do basic calculations based on test data.

Based on the wiki here on wind loading panel testing on a sloped roof, and the 3M testing stats on glazing in high-rise buildings, I estimated that at 90mph apparent, there could be up to 250psi of lift on a 26x50" solar panel 6" off the roof.

The 3M stat showed 18psi strength, what they use for rating glazing for wind loading.

The way I calculated my mounting struts was 1.5" x 60 x 2 struts x 18psi = 3240 pounds of strength. If the potential is 250lbs of lift from a headwind at freeway speeds, I was shooting for ten-times strength redundancy in the mounting. Due to the slotted holes I estimated that I could not get the full strength of the strut mounting surface, so even with half that, it's still six and a half times. I was confident that I hit that. Plus I mounted my panels 1.5" of the roof - the height of one strut crossing the other.

As mentioned, I also put 3 #12 self-tapping screws in each strut - the fiberglass roof is very thin, and it's applied over very thin luan plywood, but that large of a screw gripped quite well, and ensured I got good contact pressure on the tape on my struts. Then sealed over the screws with sikaflex urethane caulk. If nothing else it ensures the fiberglass is held to the plywood substrate. Under that is just expanded foam insulation. But it's plenty rigid to walk on, although I don't like traffic up there such as for event viewing and such.

I don't think you could pull my panels off the roof without tearing half the roof off. I have over a thousand miles of freeway traveling with them in headwinds up to 40mph. I no longer worry about them, but will keep a close eye on the sealant and mounting area.

I also applied 1" wide eternabond tape to the inside of the struts to seal the slotted holes that exposed the VHB tape underneath.

This is for a fiberglass or metal roof. For a rubber sheeted roof you do need some way to mechanically fasten your mounts.

Thanks for the details of your project Browneye, I appreciate it. That sounds like a good objective approach to calculating load.

I plan to also block the gap under the panel, which I figure will prevent the possibility of lift in any scenario.

The roof on my rv appears to be fiberglass. I attached the build diagram supplied by the manufacturer. Although, I don't know that I can rely on their measurements to find the aluminum trusses.

What method is typically used to locate the trusses? (aside from them noticeably protruding)

VHB Tape on a rubber (TPO, etc) style roof is a disaster waiting to happen with a panel. The roof is just not designed for it. You can tack down cables on a membrane roof, but nothing with uplift, it's either going to peel off or tear the membrane, both outcomes are going to ruin a trip.

VHB strength varies with heat too, so a hot panel on a hot roof in the desert is going to have more flex and stretch than you want. Mechanical fastening is the only safe option with larger panels.

That is a good point. Temperature is something I had not yet considered. The only data I could find from 3M's website shows
Temperature Resistance: Short Term (minutes, hours): 250°F (121°C) Long Term (days, weeks): 200°F (93°C)

 

[Browneye]

Well that's pretty cool, you got more than I did.

Forest river says they use edpm rubber roof covering: https://www.forestriverinc.com/brochures/0000/0000RoofDicorfaq.pdf
Climb up on there and it's easy to tell - it's either shiny fiberglass gelcoat, or it's a rubber surface. I've had both. Below is a pic of our 17 year old fiberglass roof. I treat it twice a year with 303 Aerospace protectant - I buy that stuff by the gallon. Everything gets sun-bleached here.

My prep was a scotch-brite pad with acetone, and then burnish with isopropyl alcohol and a clean rag. The test is, does your finger squeak when you pull it across the surface.

I don't know how you could tell where the roof framing is if you can't detect it visually. I have heard of people tearing out the headliner to find them.
You could also judge to the best of your ability based on the location of roof vents and their associated framing, and shoot for there. The 'aluminum tubes' look like they're pretty wide, and you would want to put screws into them.
I'm not sure I would relish drilling a bunch of holes in my roof. I wasn't going to put any screws in mine, then thought the better of it.

Another option is some new cross members fastened into the shoulder of the roof, or the sides, or where they meet. But yeah, you need to locate those cross beams.

 

[corn18]

Well that's pretty cool, you got more than I did.

Forest river says they use edpm rubber roof covering: https://www.forestriverinc.com/brochures/0000/0000RoofDicorfaq.pdf
Climb up on there and it's easy to tell - it's either shiny fiberglass gelcoat, or it's a rubber surface. I've had both. Below is a pic of our 17 year old fiberglass roof. I treat it twice a year with 303 Aerospace protectant.

My prep was a scotch-brite pad with acetone, and then burnish with isopropyl alcohol and a clean rag. The test is, does your finger squeak when you pull it across the surface.

I don't know how you could tell where the roof framing is if you can't detect it visually. I have heard of people tearing out the headliner to find them.
You could also judge to the best of your ability based on the location of roof vents and their associated framing, and shoot for there. The 'aluminum tubes' look like they're pretty wide, and you would want to put screws into them.
I'm not sure I would relish drilling a bunch of holes in my roof. I wasn't going to put any screws in mine, then thought the better of it.

Another option is some new cross members fastened into the shoulder of the roof, or the sides, or where they meet. But yeah, you need to locate those cross beams.

If I had your roof, I would not hesitate to use VHB. On that type of roof with the proper prep, I think your RV will fly before those panels come off.

 

[Rocketman]

On my fiberglass roof - I used four 2”x 6” alum angle - four on the roof and four on the panels - bolted together.

I cleaned the fiberglass roof well. Then I got the 3M VHB primer stick - primed and put the VHB tape on the bracket - two 1” x 6” pieces. Then used Dicor to seal around the brackets. One the very front panels I added one screw into the roof below (with its dicor).

I looked at the commercial brackets that were 2”x2” with VHB and I figured my 2”x6” would be 3x stronger. 3 years(and rolling) and no problems.

Good luck!

 

[12VoltInstalls]

put the VHB tape on the bracket - two 1” x 6” pieces. Then used Dicor to seal around the brackets.

I’d use polyurethane adhesive caulking to bond the rails to the roof and bolt it. One product, very strong, permanently waterproof. If access for a nut and fender washer from the backside is a problem, 2” flush white plastic hole plugs are an esthetically sound means of covering the hole in the ceiling you made for getting into the ceiling space. You locate the plug holes with a feeler bit through the hole in the bracket on the roof.

commercial brackets that were 2”x2” with VHB and I figured my 2”x6” would be 3x stronger

It is but I still wouldn’t trust that driving on roads with other vehicles behind me.

Electricians, security system installers, audio people, and plumbers fish things into impossible places without destructive techniques; and no lives are at stake. Through-bolting things that could kill somebody if let loose at 60mph should be no less possible.

 

[deeuubee]

Pardon my ignorance, but can't you find the cross beams with a stud finder just like in a house?
I'm gathering up stuff for a solar install on my RV, so I'm following along.
I'm in the tape/screw overkill camp.