Greenhouses, Sheds and Sun Rooms - calculating gambrel roof?

i am building a 10x12 shed and would like to use a gambrel roof, but have no idea how to calculate the length or angles of my boards. the walls are 6.5 ft high and i would like the roof to be around 4 foot taller. how would i go about calculating the length and angles. the pitch of each isnt extremely important.

thanks
dragan

The angles of a gambrel roof are 30 degrees for the upper section and 60 degrees for the lower.

Easiest way is to build the trusses for the roof on the slab (or wood floor, if you're going that route) before you frame the walls. Use jigs or marks on the floor to get them all the same.

Mark a line 10 ft long on the floor or driveway. At the center point of the 10 ft line, draw another line at 90 degrees about 6 ft long extending from the center of the first line.

Cut 4 pieces of scrap lumber (1x4) as follows:

2 pieces will be 48 in. long and one end of each will be cut at 18 degrees. The opposite ends will be cut at 13 1/2 degrees.

2 pieces will be 44 in. long and have the same angles as the first two.

Lay the four pieces to form the top chords of the truss. The bottom chord will be represented by the 10 ft line. The 18 degree angle of the 48 in. pieces will rest on the top of the shed wall. The 18 degree angles of the 44 inch pieces will meet at the peak of the roof. The 13 1/2 degree angles of the 44 in. and 48 in. pieces will be joined.

I hope this makes sense. The angles are not exact. The 18 degree angles are actually 18 degrees 15 min., but I doubt you will have a mitre saw calibrated in degrees and minutes.

These angles are based on 12/4 pitch for the bottom section and 4/12 pitch for the top section. This gives a slightly lower peak than using 30 and 60 degrees as suggested by Lefty. Try both and see which you like best. Hope this helps.