In article ,
wrote:
I have a small enclosed camping trailer that has a flat roof which
always leaks. I decided to replace the entire roof and put a rounded
top on it so the center is domed. The trailer is about 4 feet wide.
My plan is to take 2x8's and make the center of the roof the full
width of the 2x8 (7.5") and using a bandsaw, cut the ends down to 2
inches. Then the whole roof will be covered with tin. Sawing these
2x8's is no problem. The problem is how to lay it out so it's even.
How do I do it?

What I want is the center to be the full width of the 2x8 and the ends
to be 2", with a smooth curve from the center to each side. Thus, the
48" board will be full width at 24" and taper down to 2" on both
sides.

A string comes to mind, with a pencil tied to the end. That would
work on a sheet of plywood, but not on an 8" wide board.

Do _exactly_ what 'comes to mind'. Doesn't have to be a sheet of plywood,
just 'something' that will let you put the 'pivot point' (where anchor the
string) 'far enough' away from where you're drawing.

A 2x4 with the end temporarily tacked to the middle of your 2x8 will do
admirably.

Or just duct tape it down to the driveway, and do the same with the end of
the string.

You can use science to figure out how long the string has to be, or you can
just 'play with it', until you get things to your satisfaction.

Note: if you've had an on-going 'leak-in-the-roof' problem, you probably have
significant _other_ structural problems to deal with as well. Check *VERY*
thoroughly for (a) mold, and (b) _wood-rot_, *everywhere* the water got
past the absolute -outside- of the trailer.

Generally, a trailer that has suffered anything more than -very- minor will
cost more to return to 'good working order' than the trailer is worth.