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Rafter Length Calculator · Free calculator

36 ft Span, 4/12 Pitch Rafter Length

On a 36-foot-wide building with a 4/12 roof, each common rafter is 18' 10-7/8" from ridge to wall, or 19' 11-9/16" with a 12" overhang — cut from a 20' board. The ridge sits 5' 11-3/4" above the top plate.

Quick answer

Rafter w/ 12" overhang
19' 11-9/16"
Line length
18' 10-7/8"
Buy
20' board
Ridge rise
5' 11-3/4"
Plumb cut
18.4°
Seat cut
71.6°

▍Rafter Length Calculator

ft
/12
in
Rafter length (incl. overhang)
19' 11-9/16"
Buy: 20' board
SPAN 36'RISE 5' 11-3/4"19' 11-9/16"
Line length
18' 10-7/8"
Overhang length
1' 0-5/8"
Run (− ½ ridge)
17' 11-1/4"
Total rise
5' 11-3/4"
Plumb cut
18.4°
Seat cut
71.6°

36' span @ 4/12 — overhang options

OverhangOverhang lengthTotal rafterStock length
0"0"18' 10-7/8"20'
12"1' 0-5/8"19' 11-9/16"20'
16"1' 4-7/8"20' 3-3/4"22'
24"2' 1-5/16"21' 0-3/16"22'

36' span at other pitches (12" overhang)

PitchRidge riseRafter lengthStock length
3/124' 5-13/16"19' 6-1/4"20'
4/125' 11-3/4"19' 11-9/16"20'
5/127' 5-11/16"20' 6-3/16"22'
6/128' 11-5/8"21' 2-1/16"22'
7/1210' 5-9/16"21' 11-1/16"22'
8/1211' 11-1/2"22' 9-1/8"24'
9/1213' 5-7/16"23' 8-1/16"24'
10/1214' 11-3/8"24' 7-13/16"24'+
12/1217' 11-1/4"26' 9-3/8"24'+

How the math works

  1. Run = span ÷ 2 − ½ ridge (1-1/2"): 216" − 3/4" = 17' 11-1/4"
  2. Slope factor for 4/12 = √(144 + 16) ÷ 12 = 1.0541
  3. Line length = 215.250" × 1.0541 = 18' 10-7/8"
  4. Overhang = 12" × 1.0541 = 1' 0-5/8"
  5. Total = 19' 11-9/16" → next stock length 20'

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 17' 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 36 ft span with a 4/12 pitch?

18' 10-7/8" from ridge to wall line, 19' 11-9/16" including a 12" overhang (1-1/2" ridge board deducted). Buy 20' boards.

How tall is a 4/12 roof on a 36 ft wide building?

The ridge rises 5' 11-3/4" above the top of the wall plate (run 17' 11-1/4" × 4/12), not counting rafter depth.

What are the rafter cut angles for 4/12?

Plumb cut (ridge and tail) 18.4°, seat cut of the birdsmouth 71.6°. On a speed square, pivot to "4" on the common scale.

How many rafters for a 36 ft wide roof?

Two per rafter pair (one each side). Pairs = building length × 12 ÷ spacing + 1 — a 20 ft long 36' wide building at 16" OC takes 16 pairs (32 rafters) of 20' stock.