Cutting and Setting Rafters

The triangle you are actually cutting

A rafter is not a stick of timber with three cuts on it. It is the hypotenuse of a right triangle whose base is the horizontal run and whose height is the rise, and every mark you make on that timber is a consequence of those two numbers. Get the triangle right and the ridge cut, the birdsmouth and the tail all fall into place. Get it wrong and no amount of shimming at the plate will hide it.

Run is measured from the centre of the ridge to the outside face of the wall plate — not to the inside face, not to the sheathing line. Rise is measured over that same run. Pitch expresses the relationship between the two, conventionally as rise in inches over 12 inches of run in imperial practice, or as a ratio or angle in metric practice. All three describe the same triangle. A 6:12 roof, a 1:2 ratio and 26.57 degrees are one thing wearing three coats.

Everything downstream is a subtraction or a rotation of that triangle. The plumb cut at the ridge is the triangle's own angle read off vertical. The seat cut is that same angle read off horizontal — its complement. The tail cut, whether plumb, square or a combination, repeats one of those two angles at the other end. Hip and valley members introduce a second triangle nested inside the first, but the logic does not change: a longer run, the same rise, a shallower resulting angle.

Before a saw comes out, settle four numbers on paper or on the phone: total span, run per side, rise at the ridge, and the theoretical rafter length. Confirm they agree with each other. A span that does not divide cleanly into the run you were given usually means the wall plates are not where the drawing says they are, and finding that out with a tape is cheap compared with finding it out with a stack of miscut 2x8s.

Deriving run and rise from what the drawing gives you

Drawings rarely hand over the two numbers you want. They give a pitch and an overall span, or a ridge height and a wall plate level, or an angle in degrees with no ratio at all. Each of those is solvable, but each hides a different trap.

Span-and-pitch is the common case. Halve the span to the outside faces, subtract half the ridge thickness measured on the horizontal, and you have the true run for a common rafter. That ridge deduction is the single most commonly forgotten step in the whole exercise. On a nominal 2x ridge it is a small number; on an engineered ridge beam or a doubled LVL it is enough to push the seat cut past the inside face of the plate, which changes the bearing and, in an inspected job, changes whether the roof passes.

Ridge-height-and-plate-level is the retrofit case, common when you are tying into existing work. Measure the plate level and the underside or top of the ridge with a laser, take the difference, and account for whether the stated ridge height refers to the top of the ridge, the underside, or the theoretical intersection of the rafter top edges. Those three points can be 200 mm apart on a steep roof. Ask, and if nobody can answer, measure the existing rafters you are matching and work backwards.

Degrees-only drawings arrive from architects who model in CAD and never think in twelfths. Convert once, write the ratio on the drawing in pencil, and work in ratio thereafter — a framing square, a speed square and a rafter table all speak ratio, and converting repeatedly at each cut invites a rounding error that compounds down a long rafter.

Theoretical length, and the three deductions that shorten it

The length that comes out of the triangle is the theoretical length: measured along the top edge of the rafter, from the centreline of the ridge to the outside face of the plate. No rafter is ever cut to that number. Three deductions stand between the theory and the timber.

First, the ridge deduction already mentioned — half the ridge thickness measured horizontally, then projected back along the rafter's slope. Second, the seat cut, which does not shorten the rafter along its top edge but does determine where the plate sits relative to that edge, and therefore the height above plate that governs your ridge height. Third, the tail, which lengthens rather than shortens, and which you add after the birdsmouth is located, never before.

Height above plate is the number that ties the roof to the building. It is the vertical distance from the seat cut up to the top edge of the rafter, measured on the plumb line through the outside face of the plate. Two rafters with identical length and identical angles but different seat depths will produce two different ridge heights. When a roof comes in low, the birdsmouth is usually the culprit, cut deeper on some rafters than on the pattern.

Cut one rafter, then stop. Take it up, try it against the ridge and the plate at both ends of the building, and check the top edge lands where the fascia wants it. Only when that single rafter sits without persuasion does it become the pattern. Trace it, do not measure from it — a pencil line traced from the pattern carries the pattern's angles; a re-measured line carries your tape's errors again.

The seat cut: the joint that carries the roof

The birdsmouth is where geometry meets load path. Its horizontal face bears on the plate; its vertical face locates the rafter against the plate's outer edge. The horizontal face is cut at the complement of the plumb angle, which is why a single square setting gives you both cuts on a common rafter if you understand the relationship.

Depth is limited, and the limit is a structural one rather than a matter of taste. Removing too much of the rafter's depth at the seat cuts into the section exactly where shear is highest. Prescriptive limits on notch depth at a bearing appear in the timber framing provisions of the building code in force — the International Residential Code in much of the United States, the National Construction Code and AS 1684 Residential Timber-Framed Construction in Australia, or the relevant national annex to Eurocode 5 in Europe. The permitted proportion differs between them, and it differs again for engineered lumber, where the manufacturer's own installation literature governs and often prohibits any notch at all on the tension edge. Read the applicable document rather than carrying a remembered fraction from another jurisdiction.

Bearing length matters as much as notch depth. The horizontal seat should sit fully on the plate, not perched on its outer corner with air behind. On a steep roof with a shallow plate this becomes genuinely tight, and the usual resolution is a raised heel or an engineered connector rather than a deeper notch. On a low-pitch roof the opposite problem appears: the seat is long and shallow, and a slightly over-cut plumb face leaves the rafter able to creep outward under load.

Overcutting at the internal corner of the birdsmouth is the defect inspectors look for. A circular saw kerf that runs past the intersection puts a stress raiser in the rafter at its most loaded point. Stop the saw short and finish the corner with a handsaw or a multi-tool. It takes seconds per rafter and it is the difference between a joint that passes and a joint that gets flagged across an entire roof.

Where the triangle changes: hips, valleys and dormers

A hip or valley rafter runs diagonally across the plan, so its horizontal run is longer than the common rafter's run over the same rise. The rise has not changed — the ridge is where it is — so the resulting angle is shallower and the member is longer. That single fact explains why hip stock is usually deeper than common stock and why the plumb cut on a hip looks wrong to an eye trained on commons.

Cheek cuts are the second complication. Where a hip meets the ridge it must be cut on two faces, each at an angle derived from the plan geometry rather than from the roof pitch alone. On an equal-pitch, square-cornered roof these settle into predictable values; on an unequal-pitch roof or a bay that is not square, they do not, and each one has to be worked individually. Bevelling both cheeks from a single centreline mark keeps the hip's own centreline where the jacks expect it.

Jack rafters are commons with the top end shortened and cheek-cut to land on the hip or valley. Their lengths form an arithmetic progression — each successive jack shorter than the last by a constant amount determined by the spacing and the pitch — which means a single error propagates visibly. Cut them in pairs, left and right, and offer the first pair up before committing the rest.

Dormers introduce a third condition: a smaller roof intersecting a larger one, with valley members whose run is set by the dormer's width and whose rise is set by the main roof's plane. The dormer rafters themselves are commons on the dormer's own triangle, but their bottom ends land on a sloping surface rather than a level plate, so the seat cut is not the familiar complement of the plumb cut. Set the valley boards first and let the dormer rafters be scribed to them where the geometry is fiddly — a scribed cut that fits beats a calculated cut that nearly does.

Setting the roof, and where it goes out of true

Set the ridge to a stringline, not to the two end rafters. A ridge propped at each end and left to find its own level will sag by an amount you can see from the ground once the tiles are on. Prop at intervals, string it, and adjust before any intermediate rafters go on, because once they are nailed the ridge is locked to whatever shape it was in.

Opposing rafters go up in pairs so the ridge is never pushed sideways. Nail one side fully only when its opposite is bearing, and check plumb on the gable-end pair frequently — the whole roof will follow the first bay, and a roof that starts a degree out of plumb stays a degree out of plumb all the way along, with the discrepancy showing up at the far gable where there is no adjustment left.

Crown every rafter before it is cut and mark the crown edge up. Timber has a bow; a roof made of rafters with their crowns arranged randomly reads as a wavy plane under sheathing, and the waviness is permanent. This costs nothing at the cutting station and is impossible to fix afterwards.

Spreading is the failure mode of a ridge board roof without a proper tie. A ridge board is a spacer, not a beam: it aligns the rafters but does not carry them, and the rafters' thrust must be resisted by ceiling joists or collar ties acting in tension at or near plate level. Where the design calls for a structural ridge beam instead, the load path reverses — the beam carries the rafters and the walls take vertical load only — and the beam and its posts must be sized and supported accordingly. Confusing the two is the most consequential error on this list, because the roof will look correct for years before the walls start to move.

Check the plane of the roof before sheathing by stringing diagonally across the rafter tops and sighting along the run. A rafter sitting proud is usually one whose birdsmouth is shallow; one sitting low has been cut deep. Both are correctable at this stage with a shim or a recut, and neither is correctable once the deck is down and the felt is on.