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Rafter Length Calculator · Free calculator

30 ft Span, 12/12 Pitch Rafter Length

On a 30-foot-wide building with a 12/12 roof, each common rafter is 21' 1-1/2" from ridge to wall, or 22' 6-7/16" with a 12" overhang — cut from a 24' board. The ridge sits 14' 11-1/4" above the top plate.

Quick answer

Rafter w/ 12" overhang
22' 6-7/16"
Line length
21' 1-1/2"
Buy
24' board
Ridge rise
14' 11-1/4"
Plumb cut
45.0°
Seat cut
45.0°

▍Rafter Length Calculator

ft
/12
in
Rafter length (incl. overhang)
22' 6-7/16"
Buy: 24' board
SPAN 30'RISE 14' 11-1/4"22' 6-7/16"
Line length
21' 1-1/2"
Overhang length
1' 5"
Run (− ½ ridge)
14' 11-1/4"
Total rise
14' 11-1/4"
Plumb cut
45.0°
Seat cut
45.0°

30' span @ 12/12 — overhang options

OverhangOverhang lengthTotal rafterStock length
0"0"21' 1-1/2"22'
12"1' 5"22' 6-7/16"24'
16"1' 10-5/8"23' 0-1/8"24'
24"2' 9-15/16"23' 11-7/16"24'

30' span at other pitches (12" overhang)

PitchRidge riseRafter lengthStock length
3/123' 8-13/16"16' 5-1/8"18'
4/124' 11-3/4"16' 9-5/8"18'
5/126' 2-11/16"17' 3-3/16"18'
6/127' 5-5/8"17' 9-13/16"18'
7/128' 8-9/16"18' 5-7/16"20'
8/129' 11-1/2"19' 1-7/8"20'
9/1211' 2-7/16"19' 11-1/16"20'
10/1212' 5-3/8"20' 8-15/16"22'
12/1214' 11-1/4"22' 6-7/16"24'

How the math works

  1. Run = span ÷ 2 − ½ ridge (1-1/2"): 180" − 3/4" = 14' 11-1/4"
  2. Slope factor for 12/12 = √(144 + 144) ÷ 12 = 1.4142
  3. Line length = 179.250" × 1.4142 = 21' 1-1/2"
  4. Overhang = 12" × 1.4142 = 1' 5"
  5. Total = 22' 6-7/16" → next stock length 24'

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 14' 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 30 ft span with a 12/12 pitch?

21' 1-1/2" from ridge to wall line, 22' 6-7/16" including a 12" overhang (1-1/2" ridge board deducted). Buy 24' boards.

How tall is a 12/12 roof on a 30 ft wide building?

The ridge rises 14' 11-1/4" above the top of the wall plate (run 14' 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 30 ft wide roof?

Two per rafter pair (one each side). Pairs = building length × 12 ÷ spacing + 1 — a 20 ft long 30' wide building at 16" OC takes 16 pairs (32 rafters) of 24' stock.