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Rafter Length Calculator · Free calculator

24 ft Span, 12/12 Pitch Rafter Length

On a 24-foot-wide building with a 12/12 roof, each common rafter is 16' 10-9/16" from ridge to wall, or 18' 3-9/16" with a 12" overhang — cut from a 20' board. The ridge sits 11' 11-1/4" above the top plate.

Quick answer

Rafter w/ 12" overhang
18' 3-9/16"
Line length
16' 10-9/16"
Buy
20' board
Ridge rise
11' 11-1/4"
Plumb cut
45.0°
Seat cut
45.0°

▍Rafter Length Calculator

ft
/12
in
Rafter length (incl. overhang)
18' 3-9/16"
Buy: 20' board
SPAN 24'RISE 11' 11-1/4"18' 3-9/16"
Line length
16' 10-9/16"
Overhang length
1' 5"
Run (− ½ ridge)
11' 11-1/4"
Total rise
11' 11-1/4"
Plumb cut
45.0°
Seat cut
45.0°

24' span @ 12/12 — overhang options

OverhangOverhang lengthTotal rafterStock length
0"0"16' 10-9/16"18'
12"1' 5"18' 3-9/16"20'
16"1' 10-5/8"18' 9-3/16"20'
24"2' 9-15/16"19' 8-1/2"20'

24' span at other pitches (12" overhang)

PitchRidge riseRafter lengthStock length
3/122' 11-13/16"13' 4"14'
4/123' 11-3/4"13' 7-5/8"14'
5/124' 11-11/16"14' 0-3/16"16'
6/125' 11-5/8"14' 5-9/16"16'
7/126' 11-9/16"14' 11-3/4"16'
8/127' 11-1/2"15' 6-9/16"16'
9/128' 11-7/16"16' 2-1/16"18'
10/129' 11-3/8"16' 10-1/16"18'
12/1211' 11-1/4"18' 3-9/16"20'

How the math works

  1. Run = span ÷ 2 − ½ ridge (1-1/2"): 144" − 3/4" = 11' 11-1/4"
  2. Slope factor for 12/12 = √(144 + 144) ÷ 12 = 1.4142
  3. Line length = 143.250" × 1.4142 = 16' 10-9/16"
  4. Overhang = 12" × 1.4142 = 1' 5"
  5. Total = 18' 3-9/16" → 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 11' 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 24 ft span with a 12/12 pitch?

16' 10-9/16" from ridge to wall line, 18' 3-9/16" including a 12" overhang (1-1/2" ridge board deducted). Buy 20' boards.

How tall is a 12/12 roof on a 24 ft wide building?

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

What are the rafter cut angles for 12/12?

Plumb cut (ridge and tail) 45.0°, seat cut of the birdsmouth 45.0°. On a speed square, pivot to "12" on the common scale.

How many rafters for a 24 ft wide roof?

Two per rafter pair (one each side). Pairs = building length × 12 ÷ spacing + 1 — a 20 ft long 24' wide building at 16" OC takes 16 pairs (32 rafters) of 20' stock.