EstimatorKitFree rafter cut list
Rafter calculator — every mark on one common rafter
Give a span and a pitch and this hands back the marks you make on the board: the ridge plumb cut in degrees off square, the half-thickness taken out for the ridge, the birdsmouth seat and heel, the tail past the plate and the overall length between the two plumb lines. It then counts the rafters for the whole roof at 12, 16, 19.2 or 24 inch centers and names the stock length that covers one in a single piece. Spans go in as they read on the tape, and the notch is measured against the quarter-depth limit the IRC prints for an end notch.
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- No signup
- Feet, inches and fractions
- Notch checked to R802.7.1
- Cut list copies as text
Outside face of one top plate to the other. 24' 6" and 7.4 m both read.
Half pitches are fine — type 6 1/2 and it reads as 6.5:12.
The overhang is the horizontal projection past the wall, not the length of the tail — the tail is longer, and by how much is one of the numbers below.
19.2 in gives five bays to an 8 ft sheet, which is why it turns up on rafter tables next to the round numbers.
Cut each common rafter to
14' 10 1/16"
14.8 ft · 178 in, long point to long point between two parallel plumb cuts
The marks, in the order you make them
- Ridge plumb cut, saw off square
- 26.6°
- Deduction, a single 2× board on edge
- 0.84 in off the length
- Ridge cut to heel cut
- 13' 4 3/16"
- Heel cut to the tail
- 1' 5 7/8"
- Seat cut, saw off square
- 63.4°
- Ridge board top, above the plate
- 6' 6 3/8"
On a framing square that is 6 on the tongue and 12 on the blade; the plumb line follows the tongue and the seat follows the blade.
Birdsmouth
- Seat, level
- 3.5 in
- Heel, plumb
- 1.75 in
- Depth taken out of the rafter
- 1.57 in (22%)
- Height above plate
- 6.36 in
IRC R802.7.1 limits a notch at the end of a sawn rafter to one quarter of the member depth; this notch removes 22%. That is a check against a published limit, not an approval — the span, the species and the grade are still someone else's call.
Order for the whole roof
50 2 × 8 × 16 ft
- Per side, at 16 in centers
- 25
- Offcut from each stick
- 1' 1 15/16"
- Total rise over the run
- 6'
Depths are the surfaced dry sizes — surfaced dry depth of a nominal 2 × 8 under PS 20, and surfaced dry width of a nominal 2 × 4 top plate under PS 20. Rough-sawn and full-dimension stock measures its nominal size, which moves every notch figure above.
How to lay out a common rafter
Three steps, and the first one decides whether the other two are worth doing.
Measure across the plates, outside face to outside face
The span this page wants is the horizontal width of the building at the top of the walls, not the distance along an existing rafter and not the width of the foundation. If you are copying an old rafter down, measure the level distance it covers rather than the board itself: the tail is part of the board and is not part of the run, and forgetting that is the single most common way a replacement rafter comes out a foot long.
Deduct the ridge before you mark, not after
A run measured to the middle of the building ends at the centerline of the ridge, and the ridge board occupies half its own thickness on each side. Choose the ridge you are actually using — a 2x board, a 1 3/4 in LVL, a doubled 2x, or none at all — and the run drops by half of it before the length is worked out. That deduction is taken along the slope, so at 8:12 a 3/4 in shortening of the run takes 0.90 in off the board rather than 0.75.
Cut one, hang it, then trace the rest
Every rafter on one side of a gable is identical, so the first one is a pattern and the other eighteen are traced from it. Hang the pattern against the ridge and the plate before you cut anything else: a birdsmouth that is 1/8 in shallow shows up immediately as a gap at the seat and is invisible on paper. Cut the plumb ends with a circular saw and finish the inside corner of the birdsmouth with a handsaw, because a round blade cannot reach a square corner without overcutting the face behind it.
Technical specifications
| Pitch input | Any rise per 12 in of run above zero, with quick picks from 4:12 to 12:12; half pitches read as typed, so 6 1/2 is a valid entry |
|---|---|
| Ridge options | 1 1/2 in board, 1 3/4 in LVL, 3 in doubled 2x, or no ridge — half the thickness leaves the run in every case |
| Rafter stock | 2 x 6, 2 x 8, 2 x 10 and 2 x 12 at their surfaced dry depths of 5 1/2, 7 1/4, 9 1/4 and 11 1/4 in |
| Bearing | 3 1/2 in and 5 1/2 in top plates; the seat equals the plate width and the heel equals the plate width times the pitch |
| Notch check | One quarter of the member depth, the end-notch limit printed in IRC R802.7.1 |
| Spacings | 12, 16, 19.2 and 24 in on center — 19.2 puts five bays under an 8 ft sheet |
| Board order | The shortest of 8, 10, 12, 14, 16, 18, 20 and 24 ft that covers one rafter whole, with the offcut named |
| Data kept | None; the span, the pitch and the lumber sizes never leave this tab |
Frequently asked questions
Do I take the ridge board off the span or off the run?
Off the run, and only half of it. The ridge sits centered on the building, so each pair of rafters gives up half the ridge thickness — 3/4 in for a 2x board, 7/8 in for a single-ply LVL, 1 1/2 in for a doubled 2x. Taking the whole thickness off one side leaves a gap you will find when the second rafter refuses to meet the first, and taking it off the span before halving does the same thing twice as badly.
My miter saw will not reach the seat cut angle. What do I do?
Cut it with a circular saw instead — the seat angle is the complement of the plumb angle, so a 6:12 rafter needs 63.4 degrees off square and a miter saw stops somewhere between 50 and 60. This is not a limitation you work around by tilting the stock; the birdsmouth is two cuts that meet at an inside corner, and a blade of any diameter overcuts one face while undercutting the other. Framers mark both lines with a square, run the circular saw to the intersection on the visible face, and finish the last half inch with a handsaw or a jigsaw.
What does HAP mean and why does the framer keep asking for it?
Height above plate: the plumb distance from the top of the wall plate to the top edge of the rafter, measured at the outside face of the wall. It matters because the fascia, the soffit and the intersection with the wall sheathing are all set from it, and because two rafters of different depth cut to the same seat width give different HAPs — a 2x10 at 6:12 on a 2x4 plate lands 2 1/4 in higher than a 2x8 does. Where a roof meets an existing structure, the HAP is usually the number that has to be matched, and the seat width is adjusted to get it.
Can I cut the birdsmouth deeper to lower the roof?
Not much, and not without checking. The notch is at the bearing point, which is the worst place on the member to lose material, and the IRC limits an end notch in a sawn rafter to a quarter of the member depth. The page reports the notch as a percentage for exactly this reason: a 5 1/2 in seat on a 2x6 wall at 8:12 already removes more than a quarter of a 2x8. When the number comes back over, the usual answers are a deeper rafter, a narrower bearing, or a raised-heel detail — and an engineer signs off on which, not a web page.
How many rafters does a 32 ft long gable roof need?
Fifty at 16 in centers: twenty-four spaces plus one to close the run gives twenty-five per slope, doubled. The arithmetic is the building length divided by the spacing, rounded up, plus one — the plus one is the rafter at the far end of the last bay, and leaving it out is how a roof ends up with a bay and no rafter to close it. Barge rafters over a rake overhang, ridge blocking and any doubled rafters at a chimney or skylight are on top of that count.
Does the overhang change the run?
No — the run stops at the outside face of the wall, and the overhang is measured from that face outward, level. The tail is longer than the overhang because it is the same horizontal distance measured on the slope: a 16 in overhang on a 9:12 roof is a 20 in tail. Enter the overhang as the horizontal projection you want past the wall, which is also how a plan dimensions it and how the soffit width is set.
The math checked out but my rafters are all long by the same amount. Why?
Almost always the plumb cut is being measured from the wrong corner. Every length here runs between two parallel plumb lines, so it is the same on the top edge, the bottom edge and anywhere between — but a tape hooked on the long point of the ridge cut and read at the short point of the tail cut adds the depth of the board times the pitch. Pick one edge, mark both plumb lines against it, and the error disappears.
About laying out a common rafter
A common rafter is four decisions, and only the first of them is a length. The run stops at the ridge centerline, so the board has to come back half a ridge thickness. The birdsmouth has to be deep enough to sit flat on the plate and shallow enough to leave the member carrying, which puts the seat width and the rafter depth in tension with each other. The tail is dimensioned level on the plan and cut on the slope. And the two plumb cuts at either end are parallel, which is what makes the whole board measurable from a single edge. Get any one of those wrong and the arithmetic is still correct — it is the rafter that does not fit.
The birdsmouth is where this page earns its keep, because it is the one place a reasonable-looking layout quietly breaks a rule. The seat is normally cut to the full width of the bearing, so a 2x6 wall takes 5 1/2 in of it, and the plumb heel that goes with that seat is the seat width multiplied by the pitch. The material actually removed is neither of those two figures: it is the seat width times the sine of the roof angle, measured square to the rafter, and on a 2x8 over a 2x6 wall at 6:12 that comes to 2.46 in of a 7 1/4 in member — thirty-four percent, where the code text for an end notch says twenty-five. Most rafter calculators never mention it. Raising the heel, deepening the rafter or narrowing the bearing are the ways back under the line, and the building department decides which of them applies to your roof.
When all you want is the number to mark, the rafter length calculator skips the cuts and adds the hip, valley and jack lengths instead. If the pitch is something you have to go and measure rather than something the plan tells you, start at the roof pitch calculator, and keep the roof pitch chart open for the degrees and the slope factors. Once the roof is framed, the walls under it take off through the stud calculator and the framing calculator, and the covering through the shingle calculator.
Where the span and the pitch go
Every number on this page is worked out by JavaScript running in the tab you are reading it in. Nothing you type — measurements, quantities, the prices your supplier quoted you — is uploaded, logged or kept, which is also why the calculators carry on working on a site with no signal.
The cut list the Copy button produces is assembled in the tab and written straight to your clipboard. It is not posted anywhere first, and closing the page is enough to lose it.