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Rafter Length Calculator · Free calculator

28 ft Span, 8/12 Pitch Rafter Length

On a 28-foot-wide building with a 8/12 roof, each common rafter is 16' 9" from ridge to wall, or 17' 11-7/16" with a 12" overhang — cut from a 18' board. The ridge sits 9' 3-1/2" above the top plate.

Quick answer

Rafter w/ 12" overhang
17' 11-7/16"
Line length
16' 9"
Buy
18' board
Ridge rise
9' 3-1/2"
Plumb cut
33.7°
Seat cut
56.3°

▍Rafter Length Calculator

ft
/12
in
Rafter length (incl. overhang)
17' 11-7/16"
Buy: 18' board
SPAN 28'RISE 9' 3-1/2"17' 11-7/16"
Line length
16' 9"
Overhang length
1' 2-7/16"
Run (− ½ ridge)
13' 11-1/4"
Total rise
9' 3-1/2"
Plumb cut
33.7°
Seat cut
56.3°

28' span @ 8/12 — overhang options

OverhangOverhang lengthTotal rafterStock length
0"0"16' 9"18'
12"1' 2-7/16"17' 11-7/16"18'
16"1' 7-1/4"18' 4-1/4"20'
24"2' 4-7/8"19' 1-7/8"20'

28' span at other pitches (12" overhang)

PitchRidge riseRafter lengthStock length
3/123' 5-13/16"15' 4-3/4"16'
4/124' 7-3/4"15' 8-15/16"16'
5/125' 9-11/16"16' 2-3/16"18'
6/126' 11-5/8"16' 8-7/16"18'
7/128' 1-9/16"17' 3-1/2"18'
8/129' 3-1/2"17' 11-7/16"18'
9/1210' 5-7/16"18' 8-1/16"20'
10/1211' 7-3/8"19' 5-5/16"20'
12/1213' 11-1/4"21' 1-1/2"22'

How the math works

  1. Run = span ÷ 2 − ½ ridge (1-1/2"): 168" − 3/4" = 13' 11-1/4"
  2. Slope factor for 8/12 = √(144 + 64) ÷ 12 = 1.2019
  3. Line length = 167.250" × 1.2019 = 16' 9"
  4. Overhang = 12" × 1.2019 = 1' 2-7/16"
  5. Total = 17' 11-7/16" → next stock length 18'

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 13' 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 28 ft span with a 8/12 pitch?

16' 9" from ridge to wall line, 17' 11-7/16" including a 12" overhang (1-1/2" ridge board deducted). Buy 18' boards.

How tall is a 8/12 roof on a 28 ft wide building?

The ridge rises 9' 3-1/2" above the top of the wall plate (run 13' 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 28 ft wide roof?

Two per rafter pair (one each side). Pairs = building length × 12 ÷ spacing + 1 — a 20 ft long 28' wide building at 16" OC takes 16 pairs (32 rafters) of 18' stock.