What Is a Rafter? Definition, Anatomy & Types

What is a rafter?

A rafter is a sloped structural member that frames a roof, running from the top of the exterior wall (the wall plate) up to the peak (the ridge) and carrying the roof sheathing, underlayment, and shingles. Rafters are cut in pairs — one member per roof plane, mirrored across the ridge — and spaced 16 or 24 inches on center along the building length. Each rafter is one piece of dimensional lumber (a 2x6, 2x8, 2x10, or 2x12) cut to the roof pitch on site. The cut at the top (plumb cut) meets the ridge board; the cut at the wall (birdsmouth) seats onto the wall plate; the overhang past the wall (tail) carries the fascia and soffit.

A rafter beam is the larger, beam-class sister of a single rafter used when one 2x is not deep enough for the span. Rafter beams are typically built up from multiple 2x plies (bolted or plywood-gusseted) or use a single engineered LVL/PSL member. The ridge beam of a vaulted-ceiling roof is the most common rafter beam: where a ridge board (a non-structural spacer) would be too thin, a ridge beam carries the peak load and spans between posts, allowing the rafters to push outward less against the walls.

Stick-framed rafters are the alternative to engineered roof trusses. The two systems share the same physics (bending moment, deflection, bearing) but differ on labor, lead time, span, and attic flexibility — see the matrix above for the full trade.

Rafter anatomy — five named parts

Anatomy reference table

Five rafter types — comparison matrix

The five rafter types all derive from the common rafter. The hip, valley, and jack add an additional compound or cheek cut; the lean-to (shed) is the common rafter without a ridge. Use the filter to focus on a single type — the matrix highlights the location, cut complexity, plumb cut angle, and length formula for each.

The Recommended badge marks the rafter types a beginner should master first (Common and Lean-to). The High Risk badge marks a type where a beginner is most likely to make a layout error (none here — all five are within reach, but the compound-cut types are easy to get wrong).

Rafter types — type-by-type deep dive

Common rafter

The common rafter is the reference rafter: a single 2x cut to the roof pitch, framed in pairs from wall plate to ridge. Its plumb cut meets the ridge board at 90° minus the pitch angle (so a 6/12 rafter plumb cut is 63.43° from the top edge of the board). Its birdsmouth seats on the wall plate with a level seat cut equal to the plate width. Its length is Run ÷ cos(pitch) for the line length, plus the overhang along the rafter for the total. Every other rafter type is a modification of the common rafter.

Hip rafter

The hip rafter runs diagonally from an outside corner up to the ridge, in plan at 45°. Because it crosses 45° in plan, its horizontal run is √2 times longer than a common rafter run for the same span — but the line length is multiplied by the plan-angle cosine factor (cos 45° = 0.7071). The hip rafter needs a double cheek cut (single bevel with both cheeks mitered) at the ridge, and a compound birdsmouth at the corner wall plate. The hip rafter is the structural spine of a hip roof; all jack rafters frame into it.

Valley rafter

The valley rafter is the inside-corner mirror of the hip. Where two roof planes meet at an inside corner (a T, L, or cross intersection), the valley rafter runs from the corner up to the ridge at 45° in plan, carrying the lower rafters of both intersecting planes. Same plan-angle factor (0.7071), same compound cuts at both ends, same cheek cuts as the hip. The valley is also the drainage line: water from both slopes converges there, which is why valley flashing and underlayment are critical.

Jack rafter

Jack rafters fill the triangle between a hip or valley and the wall plate (or the ridge). A hip jack runs from the wall plate up to the hip rafter; a valley jack runs from the valley rafter up to the ridge; a hip-to-valley jack (the rarest) runs between the two. Each jack is shorter than the previous by a constant increment equal to the length-per-foot-of-run times the spacing. Layout is on a 16- or 24-inch baseline along the wall (or the ridge). The jack rafter needs a plumb cut at the high end (matching the common rafter) and a single cheek cut at the hip/valley end (a compound cut beveled to match the hip/valley side cut).

Lean-to / shed rafter

The lean-to (shed) rafter is the geometric simplest rafter: a single slope, full span, no ridge. The high end bears on a higher wall plate; the low end bears on a lower wall plate. The plumb cut at the high plate is 90° − pitch; the plumb cut at the low plate is 90° + pitch (an inverted plumb). The length formula is the simplest: Span ÷ cos(pitch). When the same rafter is attached to a higher wall (typically against a main building), it is called a lean-to rafter; when both ends are on freestanding walls of equal or unequal height, it is a shed rafter.

Rafter sizes and notation

Rafter sizes use nominal lumber notation — the “2x” series — but the actual, dressed dimensions are 0.5 inches smaller per side. A 2x8 rafter is 1.5 inches by 7.25 inches actual, not 2 by 8. The depth (the larger dimension) is what governs both bending capacity (section modulus scales with depth squared) and the birdsmouth notch depth, so the actual depth — not the nominal size — is what you use in every span and HAP calculation.

A rafter sizing chart lists, for each nominal size, species, grade, and on-center spacing, the maximum allowable span under a given roof live load (typically 20 psf live + 10 psf dead for asphalt shingles in the US). For the full rafter sizing chart by species and spacing, see our rafter lumber size calculator and the rafter span and size guide.

The role of rafters in the roof structure

In a rafter-framed roof, the rafters are the primary load path from the sheathing to the walls. They carry three loads: dead load (the weight of the sheathing, underlayment, and shingles — typically 10 psf); live load (snow or maintenance crews — typically 20–40 psf depending on climate); and wind uplift (negative pressure on the roof surface during high-wind events). Each rafter sees a tributary strip equal to its on-center spacing, so the linear load is the psf load times the spacing in feet.

The rafters push outward against the top of the wall at the birdsmouth; that thrust is resisted by ceiling joists or collar ties tying the rafter pairs together at or below the mid-height of the attic. Without that tie, the rafters would spread the walls outward under load. A ridge beam — when used in place of a ridge board — carries the peak load down to posts and a foundation, eliminating the outward thrust entirely and allowing vaulted ceilings with no ceiling joists.

The five rafter types work together: the common rafters carry the bulk of the roof plane; the hip and valley rafters carry the diagonal loads at the corners; the jack rafters fill the triangles the commons cannot reach. Cut and placed correctly, a rafter-framed roof is a triangulated structure that resists wind, snow, and gravity for the life of the building.

Pitfalls when choosing or cutting rafters

Rafter definition — frequently asked questions

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