How to Use the Swanson Speed Square for Seat Cuts

Determine the width of the roof, the thickness of the ridge, the length of the overhang and the pitch of the roof from the blueprints. If remodeling or adding to an old building you may need to measure these dimensions.

Calculate the rafter length along the top of the rafter using a hand held calculator or online calculating site.Since hand held calculators cost $50 to $100 and pencil calculations require an understanding of trigonometry, the online site is recommended. Just plug in the width of the ridge, span of the roof (wall plate to wall plate), length of overhang and the pitch (rise and run) and then press compute and you will receive several pieces of pertinent information, the most important being the length of the rafter.

Place a framing board on the saw horses and measure the length, which must exceed the length of the rafter by at least one foot.

Check the board for a crowned side. No board is perfectly straight, so you need to look down the length of the timber to determine which outside edge bows away from the center. Mark the side with a pencil, for this will be the top of the roof rafter.

Mark the point for the first plumb cut at one end. Choose a point about an inch from the end of the board and make sure this point is on the top side, which was just determined in the last step.

Place the pivot point of the Swanson Speed Square on the mark while the lipped edge of the square lies flush against the edge of the board. Then pivot the speed square so the pitch of the roof is found along the band of "common" numbers. For example if your roof has a pitch of 6-over-12 you will pivot the speed square until the number "6" lines up with the edge of the board. Mark the line along the square, non-lipped edge of the speed square and then extend it across the width of the board.

Cut off the end using a circular saw.

Measure the calculated length of the rafter from Step Two. Make a mark, extend your cutting line across the width of the board, as in Step Six and cut off the end. Both cuts should show the exact same angle. Now you are ready to make the "birds mouth" or seat cut.

Find the length of the rafter tail on the blueprint and measure the distance beginning at the tail end of the rafter. Make a mark at this point. Be sure to measure along the top side of the rafter as determined in Step Three.

Place the Speed Square pivot point on the mark and make sure the lipped edge runs along the top of the rafter toward the tail end. Adjust the square until the number "6" on the common row of the square lines up with the top edge of the rafter. Mark a line at the thin edge of the square and continue it across the face of the rafter. Do not cut.

Find the diamond imprint on the edge of the Speed Square at the 3-1/2-inch mark and place it on the line you drew in the last step. Place the diamond mark on the line so that this edge of the Speed Square is perpendicular to the plumb line. While keeping the square at a right angle to the plumb line, slide the 3-1/2-inch mark on the square along the line and away from the bottom edge of the rafter. When the pivot point of the square meets the bottom edge of the rafter, you can mark the rest of the seat cut.

Cut the lower portion of the board out using a crosscut saw. This will be a small area shaped like a right triangle located between the intersection of the two lines and the bottom edge of the board. One side will be 3-1/2-inches long and the other side will be even shorter unless you have a steep roof.