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Rafter Length Calculator · Free calculator

20 ft Span, 8/12 Pitch Rafter Length

On a 20-foot-wide building with a 8/12 roof, each common rafter is 11' 11-5/16" from ridge to wall, or 13' 1-3/4" with a 12" overhang — cut from a 14' board. The ridge sits 6' 7-1/2" above the top plate.

Quick answer

Rafter w/ 12" overhang
13' 1-3/4"
Line length
11' 11-5/16"
Buy
14' board
Ridge rise
6' 7-1/2"
Plumb cut
33.7°
Seat cut
56.3°

▍Rafter Length Calculator

ft
/12
in
Rafter length (incl. overhang)
13' 1-3/4"
Buy: 14' board
SPAN 20'RISE 6' 7-1/2"13' 1-3/4"
Line length
11' 11-5/16"
Overhang length
1' 2-7/16"
Run (− ½ ridge)
9' 11-1/4"
Total rise
6' 7-1/2"
Plumb cut
33.7°
Seat cut
56.3°

20' span @ 8/12 — overhang options

OverhangOverhang lengthTotal rafterStock length
0"0"11' 11-5/16"12'
12"1' 2-7/16"13' 1-3/4"14'
16"1' 7-1/4"13' 6-9/16"14'
24"2' 4-7/8"14' 4-3/16"16'

20' span at other pitches (12" overhang)

PitchRidge riseRafter lengthStock length
3/122' 5-13/16"11' 3-5/16"12'
4/123' 3-3/4"11' 6-3/8"12'
5/124' 1-11/16"11' 10-3/16"12'
6/124' 11-5/8"12' 2-3/4"14'
7/125' 9-9/16"12' 7-15/16"14'
8/126' 7-1/2"13' 1-3/4"14'
9/127' 5-7/16"13' 8-1/16"14'
10/128' 3-3/8"14' 2-7/8"16'
12/129' 11-1/4"15' 5-5/8"16'

How the math works

  1. Run = span ÷ 2 − ½ ridge (1-1/2"): 120" − 3/4" = 9' 11-1/4"
  2. Slope factor for 8/12 = √(144 + 64) ÷ 12 = 1.2019
  3. Line length = 119.250" × 1.2019 = 11' 11-5/16"
  4. Overhang = 12" × 1.2019 = 1' 2-7/16"
  5. Total = 13' 1-3/4" → next stock length 14'

Rafter size (2x6 / 2x8 / 2x10 / 2x12) depends on spacing, species and snow load — check the IRC rafter span tables (Section R802) for your run.

Trade pro tips

  • The horizontal run for this roof is 9' 11-1/4" — that is the number to look up in the IRC rafter span tables.
  • Cut one pattern rafter, test it at both ends of the building, then use it to mark the rest.
  • Measure line length along the top edge from the ridge plumb cut to the birdsmouth heel.

Frequently asked questions

How long are the rafters for a 20 ft span with a 8/12 pitch?

11' 11-5/16" from ridge to wall line, 13' 1-3/4" including a 12" overhang (1-1/2" ridge board deducted). Buy 14' boards.

How tall is a 8/12 roof on a 20 ft wide building?

The ridge rises 6' 7-1/2" above the top of the wall plate (run 9' 11-1/4" × 8/12), not counting rafter depth.

What are the rafter cut angles for 8/12?

Plumb cut (ridge and tail) 33.7°, seat cut of the birdsmouth 56.3°. On a speed square, pivot to "8" on the common scale.

How many rafters for a 20 ft wide roof?

Two per rafter pair (one each side). Pairs = building length × 12 ÷ spacing + 1 — a 20 ft long 20' wide building at 16" OC takes 16 pairs (32 rafters) of 14' stock.