no-fastener roof seam?

[nrhmaine]

We'll have two athwartships seams in our roof material - 0.040 anodized aluminum.

I see there have been a number of ways this is done - flat trim and screws - screws without trim - and I think I remember someone using VHB tape without any other fasteners.

That would be my preference if it's doable, and if there were a problem later, I could certainly put a flat-trim over that seam in the future.

The idea would be to install the first piece of exterior, and put a 6" swath of 4200 under the end of that sheet - strap it until set. Then overlap the second sheet, using VHB (and maybe 6" of 4200 to attach to the roof-plywood as well?) Roll the seam to get good contact pressure with the VHB.
Then I'll mask and seal the seam with Lexel or something like that.

I'd appreciate any advice on this plan. Would you put screws along that top-edge of the first sheet? Would you use 4200 to attach the second sheet to the roof plywood? Would you abandon this plan and get some flat trim?

Thanks,
Nate

[noseoil]

I think if you use a bonding system instead of mechanical fasteners, it can work well enough, but I would use caution when making the bond. Remember the wind/rain direction (fish scales) for shedding rain & blowing water on the highways, so the lap joint is oriented correctly. Surface prep will be essential to having it last to get a proper bond.

Adhesives are used in the aircraft industry for fastening skins & structural members, but this is done under very strict conditions of heat, dust & pressure. Surface prep!

That having been said, I would most likely use some type of mechanical fastening along with the bonding agent. Locate the spars to coincide with the seams & run stainless steel screws into the spars, to make sure there is no failure at a later date. I tend to over-do things & use extra caution when I'm working on something new which needs to last, so that's just me.

You can always do a mock-up to simulate the joint for curvature & the overlap. My greatest concern with this type of system would be a curvature of the skins as they lap together, so they work against each other. The seam can act like a fulcrum to start a separation, due to the tension of the joint over repeated heating & shrinking from extremes of temperature, on the two curved skins which are actually flat.

[twisted lines]

This is what hold's my trailer body to the frame, With minimal back up.
It's also commercially used for large aluminum panels in Transporting application's!
Sikaflex-252 Polyurethane Elastic Adhesive

[nrhmaine]

I think if you use a bonding system instead of mechanical fasteners, it can work well enough, but I would use caution when making the bond. Remember the wind/rain direction (fish scales) for shedding rain & blowing water on the highways, so the lap joint is oriented correctly. Surface prep will be essential to having it last to get a proper bond.

Adhesives are used in the aircraft industry for fastening skins & structural members, but this is done under very strict conditions of heat, dust & pressure. Surface prep!

That having been said, I would most likely use some type of mechanical fastening along with the bonding agent. Locate the spars to coincide with the seams & run stainless steel screws into the spars, to make sure there is no failure at a later date. I tend to over-do things & use extra caution when I'm working on something new which needs to last, so that's just me.

You can always do a mock-up to simulate the joint for curvature & the overlap. My greatest concern with this type of system would be a curvature of the skins as they lap together, so they work against each other. The seam can act like a fulcrum to start a separation, due to the tension of the joint over repeated heating & shrinking from extremes of temperature, on the two curved skins which are actually flat.

You make a good point about the tension in the sheets tending to want to open the joints. Mine will be on nearly flat sections (~15’ radius) but still, screws would make me feel more certain.

I think my resistance is mainly to having a trim strip break up the lines, and the potential for leaks that is introduced by more holes.

Maybe a compromise would be to use screws without a trim strip, to decrease the visual impact.

I’ll feel out how much flexion the sheets are under at the joints, once I’ve gotten to the skinning stage.


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[noseoil]

No need for an extra strip of aluminum on the seam, if there are spars backing up the joint. Typically, when running the screws into a spar, you would pre-drill pilot holes across the joint, then use a dab of caulk to bed each fastener as it's run into the spar to prevent leaks & seal the hole from water. The real problem is from the dis-similar materials against each other & heating/cooling cycling. The aluminum will expand in the heat, the wood will not.

With a 15' radius, the amount of leverage on the joint would be fairly small. I think I would still do a mock-up of the joint with the correct adhesive & radius, just to see how it holds up. Put it in the sun, then the freezer a few times & watch the edges to see if there's too much lifting. It will creep on you, but the question remains, how much will it move, where & will it try to open the seam to let water in? Water in a seam with freezing = lifted edge at some future date.

I would like to see a test of this system with a mock-up, just to see how things work with the adhesive & repeated stress cycling.

[aggie79]

We no longer have our teardrop, but I did use multiple sheets of aluminum on the roof and hatch. I used 3m 4200 as an adhesive/sealant and stainless steel screws as fasteners. I pre-drilled the fastener spacing for the top aluminum sheet. The bottom sheet was installed first bedding the perimeter and edges in 3m 4200 and fastened with narrow crown screws. Then I placed the top sheet to overlap the bottom. Then the It is important to have a substantial lap on the joint to keep the overlapping sheet from trying to rise at the edge. The line of screws were in line with the roof spar. I bedded the top sheet edges and lap in 3m 4200 and then used temporary screws to secure while the sealant cured. The edges were tacked in place with staples. Later, I removed the protective film and temporary screws, squirted some 3m 4200 into the screw holes, and installed the permanent stainless steel fasteners. These joints have remained leak-free for 15+ years. While I agree with the previous commenter about the direction of the lap, I decided it was better to not pond water and installed the lap so that it would gravity drain. Here are a few pics:

[nrhmaine]

Thanks very much for the detailed description and pictures!


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[MickinOz]

In 1973, in high school sheet metal class, I was taught this seam. 50 years later, I have absolutely no idea what the seam is called, nor the name of the tool used to close/set it.

seam.JPG (15.12 KiB) Viewed 340 times

With the red line and black like representing the edges of two sheets of aluminium, and the blue picture representing the grooved punch used for closing and sealing the seam, the seam kinda works like a hurricane hinge. The seam I made using the punch closed up tight.
In a vertical face situation as drawn, water would run down the black sheet, over the seam, and continue down the red sheet.
Horizontally on a roof, the black sheet would be used to form the leading half of the seam. the red the trailing half. Again it would work similarly to a hurricane hinge.

I've always thought it would make a good join, but I cannot for the life of me recall the names of it and the tool.

Edit: It's called a grooved seam, and the tool is a grooving iron.
http://wiki.dtonline.org/index.php/Grooved_Seam_Joint

[nrhmaine]

Thanks Mick. That looks great! If you want to revive your high-school shop skills, and fancy a trip to Maine (the Australia of the northeast), you'd be welcome to come up here and do my seams for me!

Nate

[Al in PA]

Len Daddona posted a tutorial on this forum a number of years ago. He used an aluminum brake to make the joint that Mick described. Then he used a roller (similar to a vinyl flooring roller) to press the joint together. I can't find his tutorial in the Design Resources anymore.

[MickinOz]

Thanks Mick. That looks great! If you want to revive your high-school shop skills, and fancy a trip to Maine (the Australia of the northeast), you'd be welcome to come up here and do my seams for me!

Nate

Ah. Maine. If I had a bucket list, the Maine lobster boat races would be on it.

[nrhmaine]

Yep, it’s a real scene. Plus, no venomous snakes in the whole state!


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[nrhmaine]

I may investigate the folded seam idea. I expect a sheet metal shop may be able to create two such seams for me.


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