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Rafter Length Calculator · Free calculator

16 ft Span, 4/12 Pitch Rafter Length

On a 16-foot-wide building with a 4/12 roof, each common rafter is 8' 4-3/8" from ridge to wall, or 9' 5-1/16" with a 12" overhang — cut from a 10' board. The ridge sits 2' 7-3/4" above the top plate.

Quick answer

Rafter w/ 12" overhang
9' 5-1/16"
Line length
8' 4-3/8"
Buy
10' board
Ridge rise
2' 7-3/4"
Plumb cut
18.4°
Seat cut
71.6°

▍Rafter Length Calculator

ft
/12
in
Rafter length (incl. overhang)
9' 5-1/16"
Buy: 10' board
SPAN 16'RISE 2' 7-3/4"9' 5-1/16"
Line length
8' 4-3/8"
Overhang length
1' 0-5/8"
Run (− ½ ridge)
7' 11-1/4"
Total rise
2' 7-3/4"
Plumb cut
18.4°
Seat cut
71.6°

16' span @ 4/12 — overhang options

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

16' span at other pitches (12" overhang)

PitchRidge riseRafter lengthStock length
3/121' 11-13/16"9' 2-9/16"10'
4/122' 7-3/4"9' 5-1/16"10'
5/123' 3-11/16"9' 8-3/16"10'
6/123' 11-5/8"9' 11-15/16"10'
7/124' 7-9/16"10' 4-3/16"12'
8/125' 3-1/2"10' 8-7/8"12'
9/125' 11-7/16"11' 2-1/16"12'
10/126' 7-3/8"11' 7-5/8"12'
12/127' 11-1/4"12' 7-11/16"14'

How the math works

  1. Run = span ÷ 2 − ½ ridge (1-1/2"): 96" − 3/4" = 7' 11-1/4"
  2. Slope factor for 4/12 = √(144 + 16) ÷ 12 = 1.0541
  3. Line length = 95.250" × 1.0541 = 8' 4-3/8"
  4. Overhang = 12" × 1.0541 = 1' 0-5/8"
  5. Total = 9' 5-1/16" → next stock length 10'

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 7' 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 16 ft span with a 4/12 pitch?

8' 4-3/8" from ridge to wall line, 9' 5-1/16" including a 12" overhang (1-1/2" ridge board deducted). Buy 10' boards.

How tall is a 4/12 roof on a 16 ft wide building?

The ridge rises 2' 7-3/4" above the top of the wall plate (run 7' 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 16 ft wide roof?

Two per rafter pair (one each side). Pairs = building length × 12 ÷ spacing + 1 — a 20 ft long 16' wide building at 16" OC takes 16 pairs (32 rafters) of 10' stock.