How to Cut a Gambrel Roof Truss

Lay a common rafter on its edge spanning across the wall partition and ridge board. Toenail the common rafter into place so that it won't move while you lay out the ridge and seat cuts; a ridge cut allows the butt end of the common rafter to sit flush against the ridge board, while a seat cut allows the rafter's lower butt end to sit horizontally flush on the wall's top plate.

Set the edge of a level against the ridge board's inside face and mark a plum line across the broad face of the common rafter to indicate the ridge plum cut.

Move to the partition wall and set the level edge on the partition wall's outside face. Place a plum line across the broad face of the common rafter, which is a two-by-four or larger width of lumber, for the cut off line. Move the level so that it sits horizontally against the common rafter's broad face, and match it up with the cut off line. Check to make sure the line's level and make a level mark to represent the seat cut.

Move the level flush to the inside face of the partition wall. Trace up across the common rafter until the line meets with the seat cut line to create the common rafter's seat cut. Draw a plum line across the broad face of the common rafter so it matches the first line.

Position a lower slope common rafter, with the bottom edge resting on top of the subfloor and partition wall. Lower slope common rafters are two pieces of lumber that make up the upper half of the four-piece gambrel roof. Toenail the common rafter into place so that you can make layout lines. Use a level to mark the plum cut line at the partition wall end plum with the outside face of the exterior wall.

Place the level flush against the exterior wall's inside face and slide it up until it spans the gap between the top plate and bottom edge of the common rafter. Place a mark where it meets the common rafter's bottom edge; this should be the exact thickness of the exterior wall. Hold the level horizontally against the common rafter and trace a line from the mark you just made to the heel plumb cut line for the seat cut layout.

Pull the upper and lower common rafters down and use a circular saw to cut out the seat cut and layout lines. Make a layout line using a layout tool such as a speed or carpenter's square for reference.

Set the common rafters into place and move them up and down to make sure they fit properly. Use the common rafters as a template for the rest of the gambrel upper and lower slope rafters. Upper slope common rafters are two pieces of lumber that make up the upper half of the four-piece gambrel roof.