Rafter Length Calculator

How this rafter length calculator works

A common rafter is the hypotenuse over half the building. The calculator runs the pitch multiplier over the half-span, converts your level overhang to its slope length, and adds them:

The answer comes back in carpenter’s feet-and-inches to the sixteenth, with the plumb-cut angle and the shortest standard board (8–20 ft) that covers the cut. At the defaults — a 24 ft building at 6:12 with a 12 in overhang — that’s 14′-6 7/16″ out of a 16 ft board; the full chain is in the cut math section below.

The parts of a rafter, plate to ridge

One board, five names along its length — the vocabulary is the layout, so it’s worth thirty seconds:

Plumb cut (ridge cut)

The top cut, vertical when the rafter stands at pitch — sawn at the pitch angle, 26.6° for a 6:12. It lands against the ridge board, which is why the centerline deduction below matters.

The ridge deduction

This calculator’s length runs to the ridge centerline. A ridge board has thickness, so deduct half of it — ¾ in for 2× stock — measured square off the plumb cut. Skip the deduction and every rafter lands ¾ in long, which the walls notice.

Birdsmouth (seat and heel)

The notch that seats the rafter on the wall plate: a level seat cut and a plumb heel cut, laid out where the rafter crosses the outside of the plate. Keep at least two-thirds of the board’s depth above the notch — over-notching is the classic layout sin.

Tail and overhang

Everything past the birdsmouth. You enter the overhang as level projection — how far the eave sticks out in plan — and the calculator stretches it along the slope, because a 12 in level overhang at 6:12 is 13 7/16 in of actual board.

Collar ties and rafter ties

The horizontal members that keep rafter pairs from spreading the walls (rafter ties, low) or the ridge from opening in wind (collar ties, high). A ridge board roof needs rafter ties or a structural ridge beam — that decision is framing design, not cut length.

What lumber for rafters?

Length is this calculator’s job; depth is the span table’s. IRC R802.4 sizes rafters by four things at once — species, grade, spacing and snow load — so the same 2×8 that spans a garage in Georgia comes up short in Buffalo. How to buy once the table has spoken:

• Read the span table first: your county’s ground-snow load plus the AWC/IRC rafter tables give the required depth for each spacing — the calculator’s length then names the stock to buy.
• #2 grade is the framing standard; pick boards straight along the edge, and crown every rafter up — the roof load will flatten it, never the reverse.
• Past 20 ft of cut length, single sticks run out: go two-piece with a scabbed splice over a purlin, step up to LVL, or switch to trusses.
• Price the stack as volume: the board foot calculator turns a rafter tally into board feet and dollars.

Rafter lengths by span and pitch

Table 2 runs the common spans through the engine with a 12 in overhang — cut length first, the board to buy in parentheses.

How to measure for rafters

Three numbers off the plans or the plates, one pattern rafter, and the rest is production. Dimension fields take feet-inches (24, 24 ft 6 in) or metric (7.5 m).

1. Measure the span — outside of wall plate to outside of wall plate, the full building width the roof covers. Measure it at both ends; old walls wander.
2. Set the pitch from the plans, or match an existing roof with the roof pitch calculator’s level-and-tape method.
3. Enter the overhang as level projection — how far the eave sticks out in plan. The calculator handles the slope stretch.
4. Read the cut length and board — then deduct half the ridge thickness (¾ in for 2× stock) square off the plumb cut.
5. Cut one pattern rafter and test it both ends of the building before cutting the stack — plates that spread an inch turn a perfect pattern into firewood.
6. Count the sticks: building length ÷ spacing + 1, times two sides — and a few spares for crowns you reject at the pile.

Worked example: rafter length for a 24 ft span

The defaults, worked line by line — 24 ft building, 6:12 pitch, 12 in level overhang:

Note what the overhang did: 12 level inches became 13 7/16 in of board, because the tail runs down the slope like everything else. And the 1½ ft of “spare” board past the cut is not waste — it’s the crown trim, the plumb-cut kerf, and the margin that lets you re-cut one bad end instead of re-buying one bad rafter.

What this rafter calculator doesn’t decide

Cut length is geometry; the rest of a roof frame is structure. Before the saw starts, these come from tables, plans or an engineer:

• Rafter depth and spacing: IRC R802.4 span tables, entered with your snow load, species and grade.
• Ridge board vs ridge beam: a board needs rafter ties; a structural beam needs sizing and posts. Cathedral ceilings force the beam conversation.
• Hip and valley rafters: they run diagonally at their own 17-in unit run — different length, different layout, not this common-rafter math.
• Uplift and hardware: hurricane ties, blocking and connection schedules per wind zone.
• The truss alternative: long spans and simple roofs often price better engineered — quotes are free, cranes are not.
• Permits and inspection: roof framing is inspected framing almost everywhere.

Sources and standards

The geometry here is exact trig; the framing rules around it come from the standards inspectors frame to:

• IRC R802: roof framing — rafter span tables (R802.4), ties and connections.
• AWC Span Tables for Joists and Rafters: the species-and-grade tables behind every depth decision.
• Framing-square practice: the unit-run tradition (12 in common, 17 in hip/valley) this page’s layout language comes from.
• Right-triangle trigonometry: lengths and angles are computed, not looked up — the table above is a printout of the same math the calculator runs.

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