Easy Wood Calc

Rafter length for a 12 ft wide shed with a 4/12 roof

Short answer

A 12 ft wide shed with a 4/12 gable roof and a 12 inch overhang needs rafters about 87-3/4 inches (7 feet 3-3/4 inches) long — 8 ft boards. A 12×16 shed at 16 inches on center takes 26 rafters.

How to work it out

  1. Run = 12 ft ÷ 2 − ½ × 1.5 in ridge = 71.25 in
  2. Slope factor √(1 + (4/12)²) = 1.0541 → 71.25 in × 1.0541 = 75.10 in
  3. + overhang 12 in × 1.0541 = 12.65 in → 87.75 in = 87-3/4 in (7 ft 3-3/4 in)
  4. Board: 87.75 + 1.83 plumb-cut allowance (5.5 in × 4/12) = 89.59 in, + 1 in trim = 90.59 in → buy 8 ft boards
  5. Plumb cut 5.5 in × 1.0541 = 5.80 in; heel 3.5 in seat × 4/12 = 1.17 in; HAP = 5.80 in − 1.17 in = 4.63 in = 4-5/8 in
  6. Top of ridge board = HAP 4.63 in + run 71.25 in × 4/12 = 28-3/8 in (2 ft 4-3/8 in) above the plates
  7. Birdsmouth depth = heel 1.17 in × cos 18.4° = 1.11 in — limit ¼ × 5.5 in = 1.38 in → OK

A 12×16 gable shed is one of the most-built backyard projects, and at 4/12 every 2×6 rafter comes out of a single 8 ft board with room for the plumb-cut tail. The rise is just 2 ft over the 6 ft run — the top of the ridge board ends up about 28-3/8 in above the plates — which keeps the roof low but leaves little storage overhead.

Steeper pitches shed snow better and give a loft, but they eat board length: at 6/12 the rafters grow to about 7 ft 9 in, and a 2×6 plumb-cut tail leaves less than ¼ in to spare on an 8 ft board — buy 10 ft. Many shed plans frame at 24 in on center; switch the spacing to see the count drop.

Adjust for your project

ft
Measure across the tops of the walls, outside edge to outside edge.
in 12
Rise in inches for every 12 in of level run (6 = 6/12). A roofer’s pitch gauge or a level and tape will read it.
in
Measured level from the outside of the wall to the end of the tail. Shed roof: the low wall.
The board’s actual depth — a 2×6 is 5½ in deep. It sets the plumb cuts and the birdsmouth.
ft
More options: Ridge · Seat · Tail …
in
Half is deducted from each rafter. A 2× ridge is 1½ in thick.
in
Usually the full width of the top plate: 3½ in on a 2×4 wall, 5½ in on a 2×6 wall. The code minimum is 1½ in of bearing on wood (IRC R802.6).
in
How far the roof runs past the end walls. Adds fly (barge) rafters and lengthens the ridge and sheathing.
Rafter length (top edge, with overhang)87-3/4 in (7 ft 3-3/4 in)
Board length to buy8 ft boards
Rafters needed
26 rafters
Height above plate (HAP) at the wall line
4-5/8 in
Top of ridge board above the wall plates
28-3/8 in (2 ft 4-3/8 in)
Rise at the building line (measuring line)
24 in (2 ft)
Birdsmouth heel cut (plumb height)
1-3/16 in
Birdsmouth depth, square to the rafter
1-1/8 in
Plumb cut length (ridge and tail)
5-13/16 in
Ridge board depth
2×8 or deeper (plumb cut 5-13/16 in)
Bottom of the tail below the top of the wall (fascia line)
5-3/16 in
Length without overhang
75-1/8 in (6 ft 3-1/8 in)
Rafter length (decimal inches)
87.75 in
Roof angle (plumb-cut angle from square)
18.4 degrees
Pitch
4/12 = 18.43°
Ridge board length (building plus rake overhangs)
192 in (16 ft)
Roof area to sheathe (all slopes)
234 sq ft
  • Rafter length is the line length along the top edge, from the ridge plumb cut (after the ridge deduction) to the end of the tail. Mark the birdsmouth heel cut where the rafter crosses the outside of the wall plate.
  • Birdsmouth depth 1-1/8 in (measured square to the rafter) is within ¼ of the 5-1/2 in depth (1-3/8 in) — the limit IRC Figure R802.7.1.1 shows for the seat notch.
  • Rafter size, spacing and span, ridge support, ties and uplift connections are set by your local building code and span tables (IRC R802) — this calculator only does the geometry.
Check your codeCode limits vary by jurisdiction — confirm with your building department before you cut or build. (IRC R802)

For this job · affiliate link
Speed square (7 in)Pivot a speed square to 4 on the COMMON scale and one line marks the plumb cut, the birdsmouth and the tail — the same setting on every rafter.
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Rafters for a 12 ft wide 4/12 shed, by shed length

Rafters for a 12 ft wide 4/12 shed, by shed length
Shed lengthRafters needed
8 ft14 rafters
10 ft18 rafters
12 ft20 rafters
16 ft26 rafters
20 ft32 rafters

What to know about rafters for a 12 ft shed

At 4/12 the slope factor is 1.054, so every foot of level run needs about 12.65 in of rafter. Run per side is 72 in less ¾ in for the ridge — 71¼ in — which gives 75.1 in from ridge to plate; the overhang adds about 12.65 in along the slope. The rise on the measuring line is 24 in; add the 4-5/8 in height above plate of a 2×6 with a 3½ in seat and the top of the ridge board sits about 28-3/8 in above the plates. The plumb cuts are set at about 18.4°.

A low pitch narrows your roofing choices. Asphalt shingle makers typically call for extra underlayment on shallow slopes, and 4/12 sits close to the point where standard installation starts, so read the shingle wrapper before you commit. Metal panels and roll roofing have their own minimum slopes — check the product, not a rule of thumb.

The count is 13 per side: 192 in of shed length ÷ 16 in is 12 spaces, plus one. At 24 in on center it drops to 9 per side, 18 in all. The birdsmouth on a 2×6 is shallow at this pitch: a full 3½ in seat gives a heel cut of 1-3/16 in and a notch about 1-1/8 in deep measured square to the rafter, under the ¼-depth limit of 1-3/8 in shown in IRC Figure R802.7.1.1. On 2×4 rafters the same seat would be too deep — their limit is 7/8 in.

The gable-end walls of this shed are 12 ft long — How many studs for a 12 ft wall counts them. For a garage-sized roof, see Rafter length for a 24 ft span at 6/12 pitch.

Different size? Use the full Rafter Length Calculator.

What you’ll need

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Frequently asked questions

How long are shed rafters for a 12 ft width at 6/12?

About 93-1/16 inches (7 feet 9-1/16 inches) with a 12 inch overhang — 10 ft boards, once the 2×6 plumb-cut tail and trim are allowed for. The top of the ridge board rises to 40 inches (3 feet 4 inches) above the plates.

How long is a single-slope (lean-to) rafter over a 12 ft shed at 4/12?

About 164-7/16 inches (13 feet 8-7/16 inches), because one rafter now spans the full 12 ft — 14 ft boards. A 16 ft long shed at 16 inches on center takes 13 rafters.

How many rafters for a 12×16 shed at 24 in on center?

18 rafters — nine per side — each about 87-3/4 inches (7 feet 3-3/4 inches) long.

Other common sizes

Sources