To build a Grambrel truss, use regular 2" x 6" lumber for the trusses and oriented strand board, a type of plywood, for the joining pieces (called "straps") and the interior support frame. Build a square assembly framework jig with three scrap wood stops to hold pieces straight while building. Assemble half trusses by laying cut lumber pieces flat in jig and attaching OSB straps with nails or screws. Join half-trusses, and attach top frame pieces.
Calculate the size and number of trusses needed. Build the assembly jig of 2" x 6" lumber laid flat in a square, cut 3 stops of OSB, and use the jig for assembly. Laying two pieces of lumber at the desired angle, use the conjunction to make a cardboard pattern for joining straps; cut straps from OSB.
Build half-trusses on the jig, using two lumber pieces per half-truss. Cut ends of lumber at the desired angles to form a miter joint. The two joined pieces form a dogleg. Standard angle for the doglegs is 30 degrees. Nail together using OSB straps. Lay two doglegs flat together to form an arch, and mark the angles to be joined for the peak; use this as a template to cut OSB straps for the peak. Mark and cut all doglegs at the correct angle for peak miter joints. Standard peak angle for a gambrel roof is 60 degrees. Join doglegs to form complete trusses by nailing together at the peak with OSB straps. Mark and cut OSB frame pieces to join trusses together at 16" (or desired spacing); join frame pieces to trusses.