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Rafter Length Calculator · Free calculator

12 ft Span, 8/12 Pitch Rafter Length

On a 12-foot-wide building with a 8/12 roof, each common rafter is 7' 1-5/8" from ridge to wall, or 8' 4-1/16" with a 12" overhang — cut from a 10' board. The ridge sits 3' 11-1/2" above the top plate.

Quick answer

Rafter w/ 12" overhang
8' 4-1/16"
Line length
7' 1-5/8"
Buy
10' board
Ridge rise
3' 11-1/2"
Plumb cut
33.7°
Seat cut
56.3°

▍Rafter Length Calculator

ft
/12
in
Rafter length (incl. overhang)
8' 4-1/16"
Buy: 10' board
SPAN 12'RISE 3' 11-1/2"8' 4-1/16"
Line length
7' 1-5/8"
Overhang length
1' 2-7/16"
Run (− ½ ridge)
5' 11-1/4"
Total rise
3' 11-1/2"
Plumb cut
33.7°
Seat cut
56.3°

12' span @ 8/12 — overhang options

OverhangOverhang lengthTotal rafterStock length
0"0"7' 1-5/8"8'
12"1' 2-7/16"8' 4-1/16"10'
16"1' 7-1/4"8' 8-7/8"10'
24"2' 4-7/8"9' 6-1/2"10'

12' span at other pitches (12" overhang)

PitchRidge riseRafter lengthStock length
3/121' 5-13/16"7' 1-13/16"8'
4/121' 11-3/4"7' 3-3/4"8'
5/122' 5-11/16"7' 6-3/16"8'
6/122' 11-5/8"7' 9-1/16"8'
7/123' 5-9/16"8' 0-3/8"10'
8/123' 11-1/2"8' 4-1/16"10'
9/124' 5-7/16"8' 8-1/16"10'
10/124' 11-3/8"9' 0-3/8"10'
12/125' 11-1/4"9' 9-3/4"10'

How the math works

  1. Run = span ÷ 2 − ½ ridge (1-1/2"): 72" − 3/4" = 5' 11-1/4"
  2. Slope factor for 8/12 = √(144 + 64) ÷ 12 = 1.2019
  3. Line length = 71.250" × 1.2019 = 7' 1-5/8"
  4. Overhang = 12" × 1.2019 = 1' 2-7/16"
  5. Total = 8' 4-1/16" → next stock length 10'

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 5' 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 12 ft span with a 8/12 pitch?

7' 1-5/8" from ridge to wall line, 8' 4-1/16" including a 12" overhang (1-1/2" ridge board deducted). Buy 10' boards.

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

The ridge rises 3' 11-1/2" above the top of the wall plate (run 5' 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 12 ft wide roof?

Two per rafter pair (one each side). Pairs = building length × 12 ÷ spacing + 1 — a 20 ft long 12' wide building at 16" OC takes 16 pairs (32 rafters) of 10' stock.