How to cut a curved roof

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.
What's the trick to doing this?
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loprabbit@_______.com wrote:
| 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. | | What's the trick to doing this?
Exactly what you described. Lay it out on a sheet of plywood (or whatever) and make a template that you can use on your 2x8 parts.
Always glad to give your solution back to you. :-D
If you want to calculate the radius of the curve, I have a web page at the link below with an algebra/geometry refresher.
-- Morris Dovey DeSoto Solar DeSoto, Iowa USA http://www.iedu.com/DeSoto/CNC/cove_geom.html
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Morris Dovey wrote:
| If you want to calculate the radius of the curve, I have a web page | at the link below with an algebra/geometry refresher.
...and just in case you wonder why there's an angular width calculation at the bottom of the page - it's there because you may want to use the angle and radius to figure out the arc length of your roof top (which will, of course, now be greater than the original 48")
arc length = radius * angle in radians
-- Morris Dovey DeSoto Solar DeSoto, Iowa USA http://www.iedu.com/DeSoto/CNC/cove_geom.html
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Tue, Jul 31, 2007, 10:16pm (EDT-1) loprabbit@_______.com comes in and mumbles: I have a small enclosed camping trailer that has a flat roof which always leaks. <snip> What's the trick to doing this?
The "trick" is coat the roof with roof sealer.
JOAT I do things I don't know how to do, so that I might learn how to do them. - Picasso
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loprabbit@_______.com wrote:

Standard camber layout.
Take a look at Fred Bingham's book, Practical Yacht Joinery.
Step by step walk thru to graphically lay out cambers.
SFWIW, biggest one I've had to lay out was 16 ft long.
Have fun.
Lew
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Lew Hodgett wrote:

I have got to buy that book!
You recommend it so often, it's got to be worth getting.
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As long as you are using an arc and not an elipse then find a description of a tool called a trammel (maybe one M) with instructions it will be easy enough. Otherwise here are step-by-steps for using a true arc (portion of a circle).
1. Decide what Radius that would be. You can do this on grided paper, drawing to scale and using a drawing compass.
2. Lets assume you find a 12 foot radius is what you want.
3. Draw a line down the center of a piece of 4 x 8 ply, dividing the 4 into two sections.
4. Draw a line across the plywood 2 feet from one end (6 feet from the other. Lets call the place where the two lines cross the center point.
5. Lay your 2 x 8 rafter across the end of the ply aligned to one edge so the edge is now 6 feet from the center point
6. Take a loose strip of wood 2" wide by 1" thick by 6 foot 6 inches
7. Drill a hole in it the size of a pencil 3 inches from one end and push a pencil into it so the lead just hangs out the back side
8. Start hammering a 16 penny nail through the other end of it, 3 inches from the end but stop after it hangs out about 1 " So the pencil and the nail are 6 feet apart.
9. Put a spacer block the same thickness as the rafter board about half way between the rafter board and the center point
10 Lat the strip of wood over the spacer block and hammer the nail into the ply at the center point
11. Scribe the arc line on the rafter with the pencil by swinging the strip of wood (trammel) as needed.
12. Remove rafter, place new one dow, rinse and repeat.
On Jul 31, 8:16 pm, loprabbit@_______.com wrote:

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On Jul 31, 11:16 pm, loprabbit@_______.com wrote:

A simple way would use a piece of thin piece of flexible molding/wood (toekick?) longer than your board to be cut find the center on it. find the center on the board to be cut drive a finish nail on both ends of the board to be cut a little above the the finished height place the flex board under these nails then pull it to the desired hight in the center keeping centers aligned. This should give a nice curve. Cut one and use as template. I hope it makes sense
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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.
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