Lag bolts to fasten Uni-Strut to RV roof

[inspectorrgadget]

I am building a roof-mount solar array consisting of six 100 watt panels, which will be secured to the roof by a pair of aluminum uni-strut rails.

This is the sort of strut I am using. It is 14 gauge, and can hold 90 pounds over its 10-foot length, unsupported:

https://www.grainger.com/product/GRAINGER-APPROVED-Strut-Channel-Slotted-Aluminum-45YV89

Assuming a worst-case scenario of going 75 mph into a 15 mph headwind, the whole array could face a 90 mph wind load. (Don't worry, I usually drive 55 mph or less). How many lag bolts should be used to secure that uni-strut to the roof is my question.

My RV has aluminum rafters which support a rubber-covered plywood roof. The rubber roof is so old and crusty I can't get duct tape to adhere to it (forget about using VHB tape). So I am going to secure the uni-strut with lag screws into the metal rafters. I will use lots of caulk to protects against water intrusion. The fewer roof penetrations I have, the better. The more lag screws I use, the stronger the roof mount will tend to be. Determining the correct number of lag screws is something I can find no guidance for. Setting lag scew each 16 inches seems like overkill.

I used a studfinder (as well as an infra-red scope on a cool morning) to determine where the RV's rafters are. In general, the rafters tend to follow a pattern of about 16 inches apart (just like residential construction).

The pair of aluminum uni-strut rails have a total length of 15 feet. Actually, they are 10 feet long and 5 feet long, and will have to be lag-screwed down where the short and long struts meet together.

Herewith, I am diagraming the bare minimum of lag screws, as represented by e!clamation points:
0 ft----------10ft-----15ft
!-------------!--------!
Lag----------Lag----Lag

How many lag screws should be in between the Zero and 10 foot mark? How many lag screws between the 10 foot and the 15 foot mark?

If anyone has any guidance as to how many lag screws should secure a rail holding 6 solar panels over a length of 15 feet, please let me know.

 

[MrThisIsME]

VHB tape to hold the channel down is a lawsuit waiting to happen. You are only bonding to to rubber which you pray is actually glued down to the plywood. Ask anyone who has replaced a rubber roof on an RV, and how easy it comes up, and they will tell you what an insane idea it is. The guy who made that video on Youtube should take it down.

Are you sure the rafters are aluminum? Is this a Class A?

For wood rafters I usually use 1.5 inch galvanized lag bolts to each rafter and double washers to spread the load (one washer larger than the other, fender washers will not fit in the channel). AL rafters you can go at least 1.25" screw length and self tapping. There is no reason not to hit each rafter - (every 16 inches) as in the more the better. Most RV roofs are built on a jig, so the rafter spacing seems to be better than a home.

Once you get the rafter layout, you can predrill the channel and use an inch or so of butyl tape on the backside of the channel. Drill just a tiny hole to guide the self tapper. Once mounted cover each bolt head with Dicor self leveling completely.

FYI you can buy the same channel at Lowes or Home Depot for cheaper and no shipping.

What I like to use is 14ga solid aluminium channel. It will feel a little light on the ground but once you mount it to the roof it will feel secure. Solid meaning it has no holes on the back side, so it ensures better engagement with the rafters. You can pick it up at Zoro shipped for $42 each 10' length.

I have mounted a few 500w 54 pound panels with the AL rail no problem. Plus the AL saves a few pounds and will never rust. Gold plated steel channel will rust no matter what they tell you (it was not meant to be mounted to a roof).

 

[HRTKD]

^^^ What he said.

Use a self tapping screw that is sized for the job. It doesn't have to be huge. I used #10 hex head 1.5" self tapping screws to go into wood trusses on the roof of my RV. But I used Z brackets, not strut. (Next time I'll use strut) I spend a lot of time camping and driving in Wyoming. The winds there can be brutal. My panels are still solidly attached to the RV.

As stated above, the aluminum channel is a better - but more expensive - option. When I was looking into that, it seemed like the aluminum channel was all low profile. That may not provide enough elevation off of the roof for air movement under the panels while stationary.

 

[inspectorrgadget]

This RV not a Class A. It is a 32 foot Keystone Montana 5th wheel, with an all-aluminum superstructure built atop a steel support frame. I am sure the rafters are aluminum, because I removed some interior ceiling vents and saw for certain that they are aluminum.

MrThisIsMe: You think a fastener @ each 16 inches is the best?

HRTKD: I am addressing the low too-low issue with plastic spacers. I wanted to raise up the rail for more air under the solar panels. The plastic spacers are cut out of a synthetic decking board. Aside from more air under the panels, the spacers also elevate the array so it will clear (go over) a rooftop plumbing vent. I should have 2.25 inches of spacers on the right edge, and 1.5 inches near the center of the roof. This counteracts roof's slight curvatures, which run both laterally and longitudinally.

Plastic spacers and stainless steel lag bolts should be corrosion-resistant. If I miss-drill and don't hit the rafter, then the plywood likely has enough holding strength do the job.

Another effect I'm worried about is chassis flex. Raising the 5th wheel on and off the landing legs shows how much flexing the frame undergoes. How much more stretching-and-shrinking-and-warping does that roof undergo when the RV is bouncing down the highway? I'm not sure, but it's my belief that the spacers will allow more "wobble" and allow this flexing to be better accommodated.

 

[MrThisIsME]

MrThisIsMe: You think a fastener @ each 16 inches is the best?

I tend to do larger panels in the 450 to 500w range so like to hit every rafter. It is quick and easy as your are already there. If concerned about damaging more of the roof, you already did and adding a few more holes will not make a difference in my eyes. 5 or 10 holes = same amount of damage to me.

I tend to use stainless bolts throughout, but finding stainless lags are difficult for me, and pricey. Had good luck with the galvanised ones at Tractor Supply and about as cheap as they get.

I am addressing the low too-low issue with plastic spacers. I wanted to raise up the rail for more air under the solar panels. The plastic spacers are cut out of a synthetic decking board. Aside from more air under the panels, the spacers also elevate the array so it will clear (go over) a rooftop plumbing vent. I should have 2.25 inches of spacers on the right edge, and 1.5 inches near the center of the roof. This counteracts roof's slight curvatures, which run both laterally and longitudinally.

Below is a similar install to your challenges in height. A few more parts but might be helpful in your endeavor. This part holds 2 450w (I think) panels. Used this design for elevation and heat and to overcome the curvature of the roof. Aluminium that is sandwiched between the two angle brackets is custom made with a few holes to allow for height adjustment. The two angle brackets are Superstrut and steel - from Home Depot. I need to develop an aluminum bracket similar.


One thing I would suggest is spanning out as many bolts into the AL channel as you can. In the case here 4. You will feel the AL stretch as you tighten the bolts, thus to me a weak point. If it were steel no problem.

Another effect I'm worried about is chassis flex. Raising the 5th wheel on and off the landing legs shows how much flexing the frame undergoes. How much more stretching-and-shrinking-and-warping does that roof undergo when the RV is bouncing down the highway? I'm not sure, but it's my belief that the spacers will allow more "wobble" and allow this flexing to be better accommodated.

My personnel RV is a 42' toy hauler with two 900lb Harleys in the back. Nothing flexes or warps worse in the RV world I would imagine. Regardless I have no issues at all with the 6 480 watt panels mounted in it. Actually never gave it a thought or concern.

 

[HRTKD]

Is your plan like this?

Solar panel
Strut
Spacer
Roof

I think I would be more comfortable with this:

Solar panel
Spacer
Strut
Roof

However, with the spacer, industry standard strut/PV brackets may not work. It depends on how you place the spacer and the fastener used on the bracket. I don't like the use of the spacer at all and would find a way to avoid that if possible.

I just looked at the installation guide for my panels and there is no mention of a minimum clearance. I've heard 3" is minimum. On an RV, we're seeing 70 to 120 MPH winds, so that much clearance isn't great for the structure. But you need clearance for air movement (cooling). The issue of air movement for cooling is a good reason to avoid low profile strut.

FWIW, I do not consider fasteners into plywood to be "good enough". Mine was not actual plywood, it was OSB, and it was about 7/16" thick. Thick enough to walk on but I still try to step on the trusses. My RV trailer is a toy hauler which has a sturdier frame than a regular trailer. I doubt that flex is an issue on my rig.

If you change the angle of the panel compared to the roof then your strut brackets may not make perfect contact with the solar panel. They won't be perfectly perpendicular. Maybe close enough though.

 

[inspectorrgadget]

Is your plan like this?

Solar panel
Strut
Spacer
Roof

I think I would be more comfortable with this:

Solar panel
Spacer
Strut
Roof

However, with the spacer, industry standard strut/PV brackets may not work. It depends on how you place the spacer and the fastener used on the bracket. I don't like the use of the spacer at all and would find a way to avoid that if possible.

I just looked at the installation guide for my panels and there is no mention of a minimum clearance. I've heard 3" is minimum. On an RV, we're seeing 70 to 120 MPH winds, so that much clearance isn't great for the structure. But you need clearance for air movement (cooling). The issue of air movement for cooling is a good reason to avoid low profile strut.

FWIW, I do not consider fasteners into plywood to be "good enough". Mine was not actual plywood, it was OSB, and it was about 7/16" thick. Thick enough to walk on but I still try to step on the trusses. My RV trailer is a toy hauler which has a sturdier frame than a regular trailer. I doubt that flex is an issue on my rig.

If you change the angle of the panel compared to the roof then your strut brackets may not make perfect contact with the solar panel. They won't be perfectly perpendicular. Maybe close enough though.

Thanks. These are all great points to consider, especially the angle of interface between bracket and the panel.

Your first list is correct: My intention is to go in this order, from top to bottom:
-Solar Panel
-Hold-down Bracket,
-bracket-Bolt-to-unistrut nut,
-Lagbolt through Unistrut,
-Caulk-
lag bolt-through-plastic decking spacer,
-more caulking-
Spacer-atop-rubber roof,
-more Dicor caulk-
Plywood
Aluminum roof truss.

 

[HRTKD]

That's a lot of caulk. LOL!

 

[Bluedog225]

No. Just no.

Putting strut and panels on a roof is a big deal. People are doing dumb and dangerous stuff and posting it. This doesn’t make it safe.

That much material coming off a roof and into a windshield can easily be fatal.

Just stop.

If you are going to put that much metal and weight on the roof of a vehicle, always bolt it down. Graded and appropriately sized through-bolts with large washers, through structural members. No exceptions.

And even then, it’s a bad idea.

Sorry if this is harsh.

PS-Pretty sure (and hopefully), this type of modification will void your insurance. People need to be held personally liable for unsafe loads.

 

[time2roll]

How close to the front edge are the elevated panels? If set back 12"+ the frontal wind will generally not be going under the panels to lift them up. I don't like "lag bolts". Are these not the large screws to assemble large structural lumber? If going into the aluminum frame some self threading screws should be fine. By all means double up a bit on the leading edge.

If the front edge is going to catch the wind I would install a deflector to direct the wind over the panels.

 

[MrThisIsME]

No. Just no.

Putting strut and panels on a roof is a big deal. People are doing dumb and dangerous stuff and posting it. This doesn’t make it safe.

That much material coming off a roof and into a windshield can easily be fatal.

Just stop.

If you are going to put that much metal and weight on the roof of a vehicle, always bolt it down. Graded and appropriately sized through-bolts with large washers, through structural members. No exceptions.

And even then, it’s a bad idea.

Sorry if this is harsh.

PS-Pretty sure (and hopefully), this type of modification will void your insurance. People need to be held personally liable for unsafe loads.

Not harsh, just silly as it shows you have no experience with such nor been on the roof of an RV.
Through-bolts are made for concrete, not RVs.

 

[inspectorrgadget]

No. Just no.

Putting strut and panels on a roof is a big deal. People are doing dumb and dangerous stuff and posting it. This doesn’t make it safe.

That much material coming off a roof and into a windshield can easily be fatal.

Just stop.

If you are going to put that much metal and weight on the roof of a vehicle, always bolt it down. Graded and appropriately sized through-bolts with large washers, through structural members. No exceptions.

And even then, it’s a bad idea.

Sorry if this is harsh.

PS-Pretty sure (and hopefully), this type of modification will void your insurance. People need to be held personally liable for unsafe loads.

You have posed a theoretical risk which in reality seems to never arise.

We witness folks with solar panels affixed to the roofs of Vans and RV's all the time. I've not read a single account of any such failures.

I don't carry insurance on my RV, because I own it outright. But, I do have experience with a "missile" coming off a pickup. When I was a teenager, a sheet of plywood flew off my pickup and into the windshield of an oncoming car. What could have been a fatality resulted in only a few cuts to the driver's face. My dad's pickup insurance covered it completely, as it was a "missile." My current pickup insurance applies to the stuff I tow.

Some folks are so srisk-averse that they stay home all the time, and are double-masked and quadruple-vaxed if out about town their car. I'm not one of them.

 

[HRTKD]

Thinking about this some more, the strut should be directly on the roof. I would create/weld a custom bracket that provides the necessary elevation off of the roof. Even then, it's a bit sketchy. My brackets would be fabricated out of solid aluminum.

I have a section of my roof that I would really like to use for a solar panel. But the refrigerator vent (10"x18", maybe) is in the way. I think I could remove the top part of the vent and with the solar panel taking its place rain won't get down there. But rain that is coming in horizontal because I'm driving 70 MPH in the rain? My point is that I'm not inclined to sacrifice functionality just to get more panels on the roof.

 

[inspectorrgadget]

How close to the front edge are the elevated panels? If set back 12"+ the frontal wind will generally not be going under the panels to lift them up. I don't like "lag bolts". Are these not the large screws to assemble large structural lumber? If going into the aluminum frame some self threading screws should be fine. By all means double up a bit on the leading edge.

If the front edge is going to catch the wind I would install a deflector to direct the wind over the panels.

The leading panel will be set back about 4 feet from the leading edge. And, I'm going to use these sorts of plastic mounting brackets for the leading panel:

https://tinyurl.com/5h3zycv6

 

[HRTKD]

You have posed a theoretical risk which in reality seems to never arise.

We witness folks with solar panels affixed to the roofs of Vans and RV's all the time. I've not read a single account of any such failures.

I don't carry insurance on my RV, because I own it outright. But, I do have experience with a "missile" coming off a pickup. When I was a teenager, a sheet of plywood flew off my pickup and into the windshield of an oncoming car. What could have been a fatality resulted in only a few cuts to the driver's face. My dad's pickup insurance covered it completely, as it was a "missile." My current pickup insurance applies to the stuff I tow.

Some folks are so srisk-averse that they stay home all the time, and are double-masked and quadruple-vaxed if out about town their car. I'm not one of them.

There is at least one post on the forum of a panel going "missing" during a drive. The forum member doesn't know what happened to it though. There are stories outside the forum of panels getting loose and flying into a vehicle behind. It does happen.

 

[MrThisIsME]

You have posed a theoretical risk which in reality seems to never arise.

Take a look at how simple and lightweight the RV manufactures mount their panels. I am amazed they never fly off. Does give me confidence though as the channel is overkill compared to OEMs.

 

[inspectorrgadget]

In Will Prouse's book, he advocates for use safety line made of stainless steel interlaced through all the solar panels. This is above and beyond what RV OEM's do. Probably a safety line for vehicle-mounted solar panels should be mandated by law. Of course I will be employing one once the panels are mounted to the roof.

 

[MrThisIsME]

In Will Prouse's book, he advocates for use safety line made of stainless steel interlaced through all the solar panels. This is above and beyond what RV OEM's do. Probably a safety line for vehicle-mounted solar panels should be mandated by law. Of course I will be employing one once the panels are mounted to the roof.

I have thought about safety lines as you mentioned. To me it seems if the panel is loose and up and moving in the wind at 65mph what kind of line and attachment will hold it?

If you have an Alliance RV dealer around take a look at their panel installs. They seem to be the leader in mounting solar to fifth wheels. I think some come with 1500w on the roof. It will amaze you how simple they mount them. I do not even think they are hitting rafters and just rely on plywood.

 

[Bluedog225]

What’s the calculated uplift force? You don’t know.

What’s the hold down capacity of your fastener. You don’t know.

What is the uplift capacity of your stick and staple “old and crusty” roof. You don’t know.

Conclusion: It must be ok.

 

[Bluedog225]

RV manufacturers rank slightly above homless in cardboard boxes in terms of construction quality.