Worried about roof skin

At this point, I would do anything you can to get the roof end radius to lay flat. You are not going to have many options once you begin to lay the aluminum roof skin and it puckers in the corners. I do not know how you "sistered" the aluminum to the rafters, but maybe just hit it with a belt sander to knock off the high points. I keep a "quality brand" course 36 grit belt for jobs like this.

You can use 3/4" Styrofoam, and using a straight edge slice through the skin on one side with a single edge razor blade. This will allow the material to break at the cuts. It's like how you kerfed the plywood. You want exterior cuts at about one inch apart to let the material follow your radius. After you get the foam to lay down tape the seams with the aluminum HVAC shinny tape.

Basically, you want to be able to place a straight edge from one sidewall to the other and not have anything bow it up in the middle. Note, if you want the roof vent to have a raised area, do it after you lay the roof skin.

I like to use a vapor barrier house wrap between the foam and the skin. It's arguably not needed if you take the time to tape all of your foam joints, but it will not hurt.

After you roll out the new roof skin you want to lay some 2 by 4's over the roof skin. The number of boards will be determined by the trailer's length, and the radius shape. I think maybe every two feet is a good spacing. Make sure you have nice smooth boards, not a lot of knots to damage your new roof skin.

I use 3/4" nylon webbing straps. If you have anyone that works for your utility company, tell them you need some salvaged 2500 pound Mule Tape. They use it for pulling new power lines, and they are only allowed to use it one time. They throw miles of it away. It's perfect for pulling down the roof skin. I use a wide beam placed under the trailer tongue supported on two jack stands. This gives me something to tie the Mule Tape and run it over the roof and 2 by 4's. I build into my trailer frames a rear receiver hitch for a bicycle rack. I use this receiver point as a place to install a draw bar to support the rear tie point. I use ratchet straps to provide a way to adjust the tension on the Mule Tape straps. You need four (or more) straps, to allow you to place different tension across the roof. Usually it takes more tension in the middle to get the metal to lay flat.

This next step is the "most important step" in laying down the skin, and will be the "most difficult for me to explain" in words. I could demonstrate the technique in less than two minutes, it's really simple. Yes, you do want to start in the middle, and work your way out to the ends. You also need to work back and forth from side to side. You do not want to work one side continuously only to find out you have a big pucker in the four corners. It helps to bend down just one small area, then staple it with 1/4" crown staple. Then skip about a foot to the rear and repeat the bend and staple. Then skip to about a foot in front of your first staple and bend and staple. At this point you have one side with three tacked down points. Go to the other side of the trailer and repeat the three tack points. Continue the side to side, front to back tacking until you get the roof skin secured. You will need to use a 12" long 2 by 4 with your left hand holding down on the roof skin, and use a large mallet to hit down on the skin edge. Leather, plastic, hard rubber is what you need for this first bend. I finish the bend with a auto body hammer. The cut length of material you leave to bend over is very important, because it will determine the width of material the rain rail will cover.

The reason you skipped around, and do not run a continuous tacked down edge, is that any "excess roof material" is spread out over the entire distance, and not bunched up at the four corners.

Staple's? Your going to be going through .030" side skins, AND also .040" roof skin. You might need to find a happy combination to get the job done. I use lots of HF 1/4" crown staples, and I use a cheap HF air stapler. One thing that I pay close attention to is the air pressure at the gun. I use an adjustable regulator to control the pressure. Too much, and you blow through the metal, too little pressure and you bend the staplers. The angle you hold the stapler to the metal is also important. You are only talking about .010" net thicker. It's not a big deal.  The amount of pressure you hold the stapler against the surface is also important.  Basically, you need to find "your happy combination", and it best to experiment on some scrap materials.

How will you end the front and rear seams? The down side to using the truck skin aluminum is that you do not have the option of using the normal "S" lock seam.

All of this advice has been "learned" from years of making mistakes, it is not a just an internet copy and paste.

John

The short answer is "no", because I only use the new "factory assembled" RV roof's.  They are assembled with four foot sections and joined together with ACME seams (and a butyl tape inside the seam).  They are a nice, trouble free system.

Old style mobile home, travel trailer, and camper builds were done with aluminum (sometimes galvanized, or bare steel) sheets in nominal sizes, usually four feet.  They were joined with a lap seam system of "black tar/gooey" butyl tape (my guess) and nails, or screws into a rafter.  It's hard to argue with a design that has lasted for seventy years, but IMO the metal factory seamed roofs available today are a better choice.

In WisconsinJoe's case, he's doing something entirely different.  He's using a "one piece" Semi Truck Trailer roof skin which is 103" wide, .040" thick aluminum available in any length off a roll.  The good news is it's locally available in most areas of the country, you can save a lot of money on shipping, which makes it very attractive.  The down side is you are usually forced into some kind of a lap style joint at the ends, which can still be workable. 

It's good to make your plan, then sleep on it.  I think in my case it's 1/3, 1/3, and 1/3!  1/3 of my work hour is spent "studying, thinking and designing"  That is usually done by tossing and turning in bed at night.  The next 1/3, is spent sourcing the required materials, traveling to stores, time on the computer searching for just the right hardware piece, etc.  The final 1/3 of the work hour is spent actually doing the work, and making saw dust.  The bad part about being retired is you do not get a scheduled break time.

John

Thanks John, again, for your thoughtful reply. You confirmed my fear about the bulging radius. At least there seems to be a relatively painless fix. I can remove the aluminum box beams since they are merely screwed through the 2x2s on the side. Then I can snap straight lines on both sides of the wood, and use a handheld surface planer to bring to flat. I don't expect the interior ceiling wood will push them farther outward. Rather, it's just that the bent plywood has too much strength for the rafters to overcome.

I think I will also be using that 0.040 metal for the sides, as well as the roof. Other than the problem of creating nice brake patterns (as you informed in a previous post), I am too tempted by the low cost. Do you anticipate any problems with shooting staples through two layers of the aluminum? Do you suggest finding "chisel point" staples as opposed to generic HF style staples?

Copper would be exotic.  Maybe a little heavy, compared to aluminum.

Where I live, they strip the copper wiring out of vacant buildings and houses to sell to the recycler's.  The current sell price on aluminum is only 20 cents a pound, which nets you about $40 for a entire truck load of aluminum, not even worth the effort to recycle.  But if you skinned your trailer in copper sheet, I would be concerned that they would steal your trailer to salvage the copper because they could make hundreds of dollars by recycling it.

The things we think about in the New World, LOL.

John