There are many websites available for stringer and rafter calculations to help
with your projects. The folowing are only examples, mark and cut stringers and rafters
to your specifications and needs.
Example Instructions for Stair Stringers
- To build stair stringers, you need to get an exact measurement from the bottom floor to the top floor.
- Suppose the distance from bottom floor to top floor including the thickness of the second floor is 108".
Divide 108" by 7 (7 is an average height of a step) and you will get 15.428, that means you will need 15 risers.
Now divide 108 by 15 and you will get around 7" or the exact height each riser needs to be, the treads need to be about 10".
- The amount of treads will be one less than the amount of risers, so to find the total run or length of the stairs,
you multiply 10 by 14 and that equates to 140".
- Now that you know the stringers will need to be cut with a 10" tread and a 7" riser,
simply set the 3n12 to 7 on the rise and 10 on the run (see below), tighten knobs and mark all the stringers.
Unlike the framing square, the 3n12 allows you set the dimensions one time and mark all of your stringers
without having to remeasure or readjust.
- Mark the 2x12 and remember to deduct the tread thickness from the bottom riser.
Example Instructions for Rafters
- Constructing rafters for 12' x 16' building with a 5/12 pitch on the roof. 5/12 means for every 5" of rise you have 12" of run. With a 5/12 pitch your rafter length would be 6'6" from the top plumb cut to the outside wall plumb mark which is also where the birds mouth will be cut. If you want an overhang for the rafters of 12", you will make the rafters 6'6" plus 12".
- To mark the rafters, adjust the 3n12 to 5 on the Rise and 12 on the run (picture on left below) and make the top plumb mark (peak of the roof) measure down 6'6" and make a mark for the outside wall and measure down 12" and mark for the overhang, if using a ridge board, deduct 3/4" from the 6'6" length to allow for the ridge board. This would make it 6' 5 1/4" from the top plumb mark to the outside wall mark.
- Turn the 3n12 around 180 degrees to make the mark for the birds mouth (picture on right below)