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Rafter Length Calculator · Free calculator

32 ft Span, 8/12 Pitch Rafter Length

On a 32-foot-wide building with a 8/12 roof, each common rafter is 19' 1-7/8" from ridge to wall, or 20' 4-1/4" with a 12" overhang — cut from a 22' board. The ridge sits 10' 7-1/2" above the top plate.

Quick answer

Rafter w/ 12" overhang
20' 4-1/4"
Line length
19' 1-7/8"
Buy
22' board
Ridge rise
10' 7-1/2"
Plumb cut
33.7°
Seat cut
56.3°

▍Rafter Length Calculator

ft
/12
in
Rafter length (incl. overhang)
20' 4-1/4"
Buy: 22' board
SPAN 32'RISE 10' 7-1/2"20' 4-1/4"
Line length
19' 1-7/8"
Overhang length
1' 2-7/16"
Run (− ½ ridge)
15' 11-1/4"
Total rise
10' 7-1/2"
Plumb cut
33.7°
Seat cut
56.3°

32' span @ 8/12 — overhang options

OverhangOverhang lengthTotal rafterStock length
0"0"19' 1-7/8"20'
12"1' 2-7/16"20' 4-1/4"22'
16"1' 7-1/4"20' 9-1/16"22'
24"2' 4-7/8"21' 6-11/16"22'

32' span at other pitches (12" overhang)

PitchRidge riseRafter lengthStock length
3/123' 11-13/16"17' 5-1/2"18'
4/125' 3-3/4"17' 10-1/4"18'
5/126' 7-11/16"18' 4-3/16"20'
6/127' 11-5/8"18' 11-1/4"20'
7/129' 3-9/16"19' 7-5/16"20'
8/1210' 7-1/2"20' 4-1/4"22'
9/1211' 11-7/16"21' 2-1/16"22'
10/1213' 3-3/8"22' 0-9/16"24'
12/1215' 11-1/4"23' 11-7/16"24'

How the math works

  1. Run = span ÷ 2 − ½ ridge (1-1/2"): 192" − 3/4" = 15' 11-1/4"
  2. Slope factor for 8/12 = √(144 + 64) ÷ 12 = 1.2019
  3. Line length = 191.250" × 1.2019 = 19' 1-7/8"
  4. Overhang = 12" × 1.2019 = 1' 2-7/16"
  5. Total = 20' 4-1/4" → next stock length 22'

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 15' 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 32 ft span with a 8/12 pitch?

19' 1-7/8" from ridge to wall line, 20' 4-1/4" including a 12" overhang (1-1/2" ridge board deducted). Buy 22' boards.

How tall is a 8/12 roof on a 32 ft wide building?

The ridge rises 10' 7-1/2" above the top of the wall plate (run 15' 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 32 ft wide roof?

Two per rafter pair (one each side). Pairs = building length × 12 ÷ spacing + 1 — a 20 ft long 32' wide building at 16" OC takes 16 pairs (32 rafters) of 22' stock.