How To Frame Gable Roofs 5

Calculating The Length Of A Common Rafter

The line length of a common rafter can be calculated by using the Pythagorean theorem or the rafter table on the framing square which is based on this theorem. Figure 36 shows the relationship between unit rise, unit run and unit line length. The 'Length Of Com. Per Unit Run (250) is 291.5. It is the unit line length found on the rafter table.

The span is the total horizontal building width at the time of roof framing. The run is the total horizontal distance the rafter travels from the outside plate line to the centre of the ridge. The run is one-half the span.

The length of a common rafter is measured between the centre of the ridge board and the outside edge of the top plate. If the span is 4000 mm then the the run is 2000 mm. The number of units of run is 2000 divided by 250 equal to( = ) 8. The length of the common rafter is 8 times 291.5 equal to 2332 mm.

Figure 36   Length Of A Common Rafter

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1.Framing Gable Roofs: Roof Framing Is Based On The Pythagorean Theorem.
2.Framing Gable Roofs: Layout The Ceiling Joists.
3.Framing Gable Roofs: Cut The Ceiling Joist To The Slope Of The Rafter.
4.Framing Gable Roofs: Gable Roof Components.
5.Framing Gable Roofs: Calculate The Length Of A Common Rafter.
6.Framing Gable Roofs: Mark Plumb Lines For Ridge, Birdsmouth And Overhang.
7.Framing Gable Roofs: Adjust The Rafter To The Size Of The Ridge Board.
8.Framing Gable Roofs: Layout & Cut The Rafter Birdsmouth.
9.Framing Gable Roofs: Calculate Length Of The Rafter Overhang & Cut Excess.
10.Framing Gable Roofs: Layout The Collar Ties.
11.Framing Gable Roofs: Calculate The Gable End Stud Lengths & Adjustments.
12.Framing Gable Roofs: Calculate Gable End Stud Length By Ratio & Proportion.

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Written and maintained by
Ronald Hunter

All images and text are copyright Ronald Hunter 2005 to 2011