Question 1

How do you calculate rafter length?

Accepted Answer

Rafter length equals the square root of run squared plus rise squared. The run is half the building width minus half the ridge board thickness. The rise is run multiplied by pitch divided by 12. For a building 24 ft wide with a 6/12 pitch, run is 12 ft, rise is 6 ft, and rafter length is sqrt(144 + 36) = 13.42 ft (13 ft 5 in). Add overhang along the rafter for the full lumber length needed.

Question 2

What is the multiplier method for rafter length?

Accepted Answer

The multiplier method gives you a length factor per foot of run. For each pitch, the multiplier is sqrt(1 + (rise/12) squared). For 6/12 pitch the multiplier is 1.118, so a 12-ft run produces a 13.42 ft rafter. Common multipliers: 3/12 = 1.031, 4/12 = 1.054, 6/12 = 1.118, 8/12 = 1.202, 12/12 = 1.414. Multiply your run in feet by the multiplier to get rafter length.

Question 3

What is a hip rafter and how is it calculated?

Accepted Answer

A hip rafter runs diagonally from the corner of the building to the ridge in a hip roof. Its length is the run multiplied by sqrt(2 + (pitch/12) squared). For a 6/12 pitch with a 12-ft run, the hip rafter is 12 x 1.5 = 18 ft. Hip rafters always need to be longer and stronger than common rafters because they support more load and span a longer diagonal distance.

Question 4

What is a bird's mouth cut?

Accepted Answer

A bird's mouth (or seat cut) is a notch cut into the bottom edge of a rafter where it sits on the top plate of the wall. It has two cuts: a level (horizontal) cut that bears on the top plate and a plumb (vertical) cut at the outside of the wall. The notch depth should not exceed 1/3 of the rafter width to maintain strength. For 2x6 rafters, the bird's mouth is typically 3.5 inches wide and 1.5 inches deep.

Question 5

What is the difference between rise and run?

Accepted Answer

Run is the horizontal distance from the outside of the wall to the center of the ridge, equal to half the building span. Rise is the vertical height from the top plate to the top of the ridge. Pitch describes the slope as rise per 12 inches of run. For example, a 6/12 pitch rises 6 inches for every 12 inches of horizontal travel. Run, rise, and rafter form a right triangle.

Question 6

How long should rafter overhangs be?

Accepted Answer

Standard residential overhangs are 12 to 24 inches. A 12 to 18 inch overhang protects the siding and walls from water. Larger 24 to 36 inch overhangs reduce solar heat gain on south-facing walls and protect doors and windows from rain. Overhangs longer than 30 inches require structural bracing. Always measure overhang along the rafter, not horizontally, when ordering lumber lengths.

Question 7

What pitch should my roof be?

Accepted Answer

Low-slope roofs (2/12 to 4/12) are common in modern designs and require special sealed underlayment for shingles. Standard pitches (4/12 to 8/12) work for most residential construction. Steep pitches (8/12 to 12/12) are traditional, shed snow well, and create attic space but require fall protection during construction. Anything above 12/12 needs scaffolding or roof jacks. Higher pitches use 10 to 50 percent more material due to greater roof area.

Pitch	Common Rafter	Hip / Valley	Angle
3:12	1.031	1.436	14°
4:12	1.054	1.453	18°
6:12	1.118	1.500	27°
8:12	1.202	1.564	34°
10:12	1.302	1.642	40°
12:12	1.414	1.732	45°

Span (horizontal)	16" OC	24" OC
Up to 10 ft	2x6	2x8
10 to 14 ft	2x8	2x10
14 to 18 ft	2x10	2x12

Rafter Length Calculator

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Overhang

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How to Calculate Rafter Length

Common Rafter Formula

Multiplier Method

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Ridge Board vs. Ridge Beam

Overhang and Tail Cut Styles

Rafter Size Selection (Quick Reference)

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Building Geometry

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Rafter Geometry