THE COMMON RAFTER

GABLE ROOFS

• 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.

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

Another method of determining the length of the common rafteris to set stair gauges at the unitRise 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

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, ,

Using Pythagoras, ,

Using Pythagoras, ,

The rest of the Framing Square Tables is created using Pythagoras’ theorem

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

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

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].

DIFFERENT TYPES OF STAIR GAUGES AND RAFTER LAYOUT JIGS

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