The document discusses methods for estimating the length of common rafters used in gable roof construction. It describes using a framing square to lay out a rafter for a roof with a 12 foot run and 9 foot rise, which yields a rafter length of 15 feet. It also explains using stair gauges on a framing square to lay out multiple rafters at once by stepping off unit runs. Finally, it notes that rafter tables are generated using Pythagorean theorem calculations for different roof slopes.

1. THE COMMON RAFTER
GABLE ROOFS

3. • Total span includes the wall sheathing, birdsmouth sits over the wall sheathing
• Top of the ridge board is higher than the total rise
• Actual length of rafter less than line length of rafter
• Projection shown is without the fascia board
• Rafter stand or the Height Above Plate (HAP) is dependent on rafter size: 2x4, 2x6,
etc.

5. Estimating the Length of the Common Rafter
• Using the twelfth scale on the Framing Square
• Building run of 12 feet and rise of 9 feet
• Use 12” on the body and 9” on the tongue
• Measure the diagonal of 15” which means 15 feet
• A run of 12ft 4 ins would be 12 ins and 4 increments
CARPENTRY, 2ND Canadian Ed., Vogt F. & Nauth M., Nelson Education, 2013

6. Another method of determining
the length of the common rafter
is to set stair gauges at the unit
Rise and the unit run (12”) and
step-off the number of unit runs
with the square placed on the top
edge of the rafter stock.
In the example to the left, there
are 3 unit runs, or a total run of 3
feet, or a building span of 6 feet.
After the ridge plumb cut line is
drawn, then the square is slid down
exactly 3 times and the heel plumb
cut line is struck off.
The square is then slid down the
amount of the projection and the
tail plumb cut line is struck off,
with an appropriate reduction for
the fascia board.
CARPENTRY, 2ND Canadian Ed., Vogt F. & Nauth M., Nelson Education, 2013

8. LINE # 1 OF THE RAFTER TABLES – COMMON RAFTER/UNIT RUN (12”)
e.g. For a 6 in 12 roof slope, the unit common rafter is 13.42”
For a 7 in 12 roof slope, the unit common rafter is 13.89”
for an 8 : 12 roof slope, the unit common rafter is 14.42”
Using Pythagoras, 𝑎2 + 𝑏2 = 𝑐2, 𝐶 = (𝑎2+𝑏2 = (62 + 122) = 13.42
Using Pythagoras, 𝑎2 + 𝑏2 = 𝑐2, 𝐶 = (𝑎2+𝑏2 = (72 + 122) = 13.89
Using Pythagoras, 𝑎2 + 𝑏2 = 𝑐2, 𝐶 = (𝑎2+𝑏2 = (82 + 122) = 14.42
The rest of the Framing Square Tables is created using Pythagoras’ theorem

9. CARPENTRY, 2ND Canadian Ed., Vogt F. & Nauth M., Nelson Education, 2013

10. CARPENTRY, 2ND Canadian Ed., Vogt F. & Nauth M., Nelson Education, 2013

11. CARPENTRY, 2ND Canadian Ed., Vogt F. & Nauth M., Nelson Education, 2013
The Framing Square, with the stair
gauges set at 6” & 12”, is used to mark
the ridge plumb cut line at the top
end of the rafter. The distance
between the gauges is exactly 13.42”.
[6:12 slope].
The Speed Square pivots at the top to
make the cut lines. Here it is set so
that the number ‘8’ below the slot is
on the top edge and the angle of cut
is 34 . [8:12 slope].

12. DIFFERENT TYPES OF STAIR GAUGES AND RAFTER LAYOUT JIGS
CARPENTRY, 2ND Canadian Ed., Vogt F. & Nauth M., Nelson Education, 2013