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Rafter Length Calculator · Free calculator

16 ft Span, 8/12 Pitch Rafter Length

On a 16-foot-wide building with a 8/12 roof, each common rafter is 9' 6-1/2" from ridge to wall, or 10' 8-7/8" with a 12" overhang — cut from a 12' board. The ridge sits 5' 3-1/2" above the top plate.

Quick answer

Rafter w/ 12" overhang
10' 8-7/8"
Line length
9' 6-1/2"
Buy
12' board
Ridge rise
5' 3-1/2"
Plumb cut
33.7°
Seat cut
56.3°

▍Rafter Length Calculator

ft
/12
in
Rafter length (incl. overhang)
10' 8-7/8"
Buy: 12' board
SPAN 16'RISE 5' 3-1/2"10' 8-7/8"
Line length
9' 6-1/2"
Overhang length
1' 2-7/16"
Run (− ½ ridge)
7' 11-1/4"
Total rise
5' 3-1/2"
Plumb cut
33.7°
Seat cut
56.3°

16' span @ 8/12 — overhang options

OverhangOverhang lengthTotal rafterStock length
0"0"9' 6-1/2"10'
12"1' 2-7/16"10' 8-7/8"12'
16"1' 7-1/4"11' 1-11/16"12'
24"2' 4-7/8"11' 11-5/16"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 8/12 = √(144 + 64) ÷ 12 = 1.2019
  3. Line length = 95.250" × 1.2019 = 9' 6-1/2"
  4. Overhang = 12" × 1.2019 = 1' 2-7/16"
  5. Total = 10' 8-7/8" → 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 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 8/12 pitch?

9' 6-1/2" from ridge to wall line, 10' 8-7/8" including a 12" overhang (1-1/2" ridge board deducted). Buy 12' boards.

How tall is a 8/12 roof on a 16 ft wide building?

The ridge rises 5' 3-1/2" above the top of the wall plate (run 7' 11-1/4" × 8/12), not counting rafter depth.

What are the rafter cut angles for 8/12?

Plumb cut (ridge and tail) 33.7°, seat cut of the birdsmouth 56.3°. On a speed square, pivot to "8" 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 12' stock.