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Rafter Length Calculator · Free calculator

20 ft Span, 4/12 Pitch Rafter Length

On a 20-foot-wide building with a 4/12 roof, each common rafter is 10' 5-11/16" from ridge to wall, or 11' 6-3/8" with a 12" overhang — cut from a 12' board. The ridge sits 3' 3-3/4" above the top plate.

Quick answer

Rafter w/ 12" overhang
11' 6-3/8"
Line length
10' 5-11/16"
Buy
12' board
Ridge rise
3' 3-3/4"
Plumb cut
18.4°
Seat cut
71.6°

▍Rafter Length Calculator

ft
/12
in
Rafter length (incl. overhang)
11' 6-3/8"
Buy: 12' board
SPAN 20'RISE 3' 3-3/4"11' 6-3/8"
Line length
10' 5-11/16"
Overhang length
1' 0-5/8"
Run (− ½ ridge)
9' 11-1/4"
Total rise
3' 3-3/4"
Plumb cut
18.4°
Seat cut
71.6°

20' span @ 4/12 — overhang options

OverhangOverhang lengthTotal rafterStock length
0"0"10' 5-11/16"12'
12"1' 0-5/8"11' 6-3/8"12'
16"1' 4-7/8"11' 10-9/16"12'
24"2' 1-5/16"12' 7"14'

20' span at other pitches (12" overhang)

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

How the math works

  1. Run = span ÷ 2 − ½ ridge (1-1/2"): 120" − 3/4" = 9' 11-1/4"
  2. Slope factor for 4/12 = √(144 + 16) ÷ 12 = 1.0541
  3. Line length = 119.250" × 1.0541 = 10' 5-11/16"
  4. Overhang = 12" × 1.0541 = 1' 0-5/8"
  5. Total = 11' 6-3/8" → next stock length 12'

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 9' 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 20 ft span with a 4/12 pitch?

10' 5-11/16" from ridge to wall line, 11' 6-3/8" including a 12" overhang (1-1/2" ridge board deducted). Buy 12' boards.

How tall is a 4/12 roof on a 20 ft wide building?

The ridge rises 3' 3-3/4" above the top of the wall plate (run 9' 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 20 ft wide roof?

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