Cutting Plywood Roof Sheathing
The field trick that survived a hundred years because it works on the deck without a calculator. Hold the Speed® Square at the corner of a plywood sheet, measure the listed distance, cut the angle that matches your roof pitch.
After reading
Cut a sheet of 4x8 plywood roof sheathing to match your roof pitch using nothing but a Speed® Square and a tape measure. One sheet, every pitch.
The trick, in one move
Lay a sheet of 4x8 plywood flat. From the far left-hand corner, measure DOWN the long edge by the distance the table tells you for your inch-rise per foot of run. Mark a line from that point to the bottom-right corner. Cut along the line. The angle of the cut matches the angle of your roof pitch, the offcut nests against the rafter, the main sheet covers the field.
How the original prose introduces it
Before the table, the framer-author lays out the full method. Every Speed® Square owner since 1982 has read this paragraph at least once.
The diagram shows a roof as is sometimes used over a door. To get the pointed end cut, the square is held in position for the plumb cut of the flat roof. Then a line running from the pivot corner of the square thru the number representing the rise of the main roof is the cut wanted.
PLYWOOD ROOF SHEATHING, when used for a roof, can be cut on the horses as follows: from the far left hand corner of a 4'x8' sheet, measure to the right the distance given for the pitch wanted (Chart B). From this point draw a line back to the near left hand corner. These measurements are for a perfectly square roof. Better check the first piece cut for any changes required.
The chart that does the work
Find your inch-rise per foot of run in the left column. The right column is the distance to measure from the corner of the plywood. Tap any value to copy it.
Chart B
Rows
15
Inch Rise per Foot Run
- Measure from corner of Plywood
- 3' 11 3/8"
Inch Rise per Foot Run
- Measure from corner of Plywood
- 3' 11"
Inch Rise per Foot Run
- Measure from corner of Plywood
- 3' 10 5/8"
Inch Rise per Foot Run
- Measure from corner of Plywood
- 3' 10 1/8"
Inch Rise per Foot Run
- Measure from corner of Plywood
- 3' 9 5/8"
Inch Rise per Foot Run
- Measure from corner of Plywood
- 3' 9"
Inch Rise per Foot Run
- Measure from corner of Plywood
- 3' 8 3/8"
Inch Rise per Foot Run
- Measure from corner of Plywood
- 3' 7 5/8"
Inch Rise per Foot Run
- Measure from corner of Plywood
- 3' 6 7/8"
Inch Rise per Foot Run
- Measure from corner of Plywood
- 3' 5 3/8"
Inch Rise per Foot Run
- Measure from corner of Plywood
- 3' 3 7/8"
Inch Rise per Foot Run
- Measure from corner of Plywood
- 3' 2 3/8"
Inch Rise per Foot Run
- Measure from corner of Plywood
- 3' 7/8"
Inch Rise per Foot Run
- Measure from corner of Plywood
- 2' 11 3/8"
Inch Rise per Foot Run
- Measure from corner of Plywood
- 2' 9 7/8"
FOR UNEVEN PITCHED ROOFS
If your roof has no hips or valleys and you have more than one pitch, cut each section separately using the number representing the pitch of that section.
For instance, if the front section is 8" rise and 12' run, you would use number 8 and find your rafter lengths under 24' width. Then we'll say the rear is 3" rise and 16' run. Use number 3 on the square, and 32' building width for your length. The top cut to fit against the ridge is plumb for both sections. Your rear plate would be 4' higher.
Identical across three editions
The plywood corner numbers have not moved since the table first appeared. Scrub through the editions below to see the same chart in three typographic eras.
Plywood corner cuts, every edition.
The same field trick across three editions: hold the Speed® Square at the corner of a sheet of plywood, measure the listed distance, cut the angle that matches your roof pitch.
| Inch rise per ft of run | Measure from corner of plywood |
|---|---|
| 2″ | 3' 11 3/8" |
| 4″ | 3' 9 5/8" |
| 6″ | 3' 6 7/8" |
| 8″ | 3' 3 7/8" |
| 10″ | 3' 7/8" |
| 12″ | 2' 9 7/8" |
Same numbers Albert Swanson printed in 1925.
Open the full 1925 TableThe 1925 chart, unedited
The plywood corner cut chart from the original Blue Book. Same values, different setting.
Inch Rise per foot run / Measure from corner of Plywood
Rows
15
Inch Rise per foot run
- Measure from corner of Plywood
- 3' 11 3/8"
Inch Rise per foot run
- Measure from corner of Plywood
- 3' 11"
Inch Rise per foot run
- Measure from corner of Plywood
- 3' 10 5/8"
Inch Rise per foot run
- Measure from corner of Plywood
- 3' 10 1/8"
Inch Rise per foot run
- Measure from corner of Plywood
- 3' 9 5/8"
Inch Rise per foot run
- Measure from corner of Plywood
- 3' 9"
Inch Rise per foot run
- Measure from corner of Plywood
- 3' 8 3/8"
Inch Rise per foot run
- Measure from corner of Plywood
- 3' 7 5/8"
Inch Rise per foot run
- Measure from corner of Plywood
- 3' 6 7/8"
Inch Rise per foot run
- Measure from corner of Plywood
- 3' 5 3/8"
Inch Rise per foot run
- Measure from corner of Plywood
- 3' 3 7/8"
Inch Rise per foot run
- Measure from corner of Plywood
- 3' 2 3/8"
Inch Rise per foot run
- Measure from corner of Plywood
- 3' 7/8"
Inch Rise per foot run
- Measure from corner of Plywood
- 2' 11 3/8"
Inch Rise per foot run
- Measure from corner of Plywood
- 2' 9 7/8"
Cutting a Common Rafter
Lay out a common rafter from rise + run + length, with the right plumb cut at the ridge and the right tail cut at the eave.
Roof PitchRoof Pitch Reference
Convert between rise / run, pitch fraction (1/4, 1/3, 1/2, full), angle in degrees, and rafter length per foot of run for any pitch from 1/12 to 24/12.