A rafter is one of the sloped structural members that run from the ridge (the peak of the roof) down to the wall plate or eave, carrying the roof sheathing, roofing and snow above it. Rafters are set side by side at a regular spacing, usually 12, 16 or 24 inches on center in wood framing, and together they form the slope of a stick-framed roof.
This guide explains how a rafter works, the parts and cuts it has, how it differs from a truss and a joist, how rafter spans are read from span tables, and what a “rafter” means in a steel building.
What a Rafter Does
Every roof has to collect its own weight, the weight of snow and the push of wind, and pass all of it down into the walls. A rafter is the first structural member in that path. The sheathing spans between rafters, each rafter spans from the ridge to the wall, and the wall carries the load to the foundation.
Because a rafter is sloped, it does more than bend. A pair of rafters meeting at the ridge also pushes outward at the bottom, the same way the legs of a stepladder spread apart if nothing holds them. Something has to resist that outward thrust, and how a roof does it is the main difference between the framing systems described below.
Parts and Cuts of a Rafter
Carpenters name each end and notch of a common rafter, and those names show up on plans and in code books:
- Plumb cut (ridge cut). The angled cut at the top end, cut so it sits vertical against the ridge board or ridge beam.
- Birdsmouth. A triangular notch near the bottom, made of a short level “seat cut” and a “heel” plumb cut, so the rafter sits flat on the wall’s top plate.
- Tail. The part that runs past the wall to form the eave overhang. It is often finished with a plumb or square cut for the fascia.
- Run. The horizontal distance the rafter covers, from the outside of the wall to the center of the ridge.
- Rise. The vertical height the rafter climbs over that run.
- Pitch (slope). Rise per 12 inches of run, written as 4:12, 6:12, 8:12 and so on.
The birdsmouth should leave enough wood above the notch to carry the load; cutting it too deep weakens the rafter right where the shear is highest. Plans and local code set the limits, so follow them rather than a rule of thumb.
Types of Rafters
Most stick-framed roofs use several kinds of rafter, named for where they sit:
- Common rafter. Runs straight from the wall plate to the ridge at right angles to both. Most of the rafters on a gable roof are commons.
- Hip rafter. Runs diagonally from a corner of the building up to the ridge, forming the sloped edge (the hip) where two roof planes meet.
- Valley rafter. The inside-corner version of a hip, running along the valley where two roof planes meet in an internal angle.
- Jack rafter. A shortened common rafter that runs from the wall plate to a hip, or from a valley to the ridge.
- Cripple rafter. A short rafter that touches neither the wall plate nor the ridge, running between a hip and a valley.
How Rafters Stay Put: Ridge Boards, Ridge Beams and Ties
There are two basic ways to stop rafters from spreading the walls.
Ridge board plus rafter ties
In the most common layout, the rafters lean against a ridge board, which is only a nailing surface and carries almost no load. The outward thrust is taken by horizontal members that tie the bottoms of opposite rafters together, usually the ceiling joists themselves. Our guide to the ceiling joist explains how those joists double as rafter ties. In this layout the ceiling joists are working in tension, so they must be continuous or properly spliced across the building.
Structural ridge beam
For a vaulted or cathedral ceiling there are no ceiling joists to act as ties. Instead, the ridge is made a real beam, supported on posts at each end, and each rafter becomes a simple sloped beam hanging between the wall and the ridge beam. With nothing pushing outward, no tie is needed, but the ridge beam and its posts now carry roughly half the roof load down to the foundation. The difference is covered in detail in ridge beam vs ridge board.
Collar ties are a third, separate member: short horizontal boards higher up between rafter pairs. They mainly help hold the rafters together at the ridge against wind uplift and are not a replacement for ties at wall level. Where they go, and whether they are needed, is set by the code your jurisdiction has adopted.
Rafter vs Truss vs Joist
These three words get mixed up because they all describe repeating members spaced across a building. The quick difference:
| Member | Orientation | How it is made | Main job |
|---|---|---|---|
| Rafter | Sloped | Single piece of lumber (or engineered wood) cut and installed on site | Carries roof load from ridge to wall |
| Roof truss | Triangle of sloped top chords and a flat bottom chord | Factory-built frame of small members joined with metal plates | Carries roof load and ties itself together, so it spans wall to wall |
| Joist | Level | Single piece or engineered I-joist | Carries a floor or ceiling |
Trusses replaced rafters on most new tract houses because they are fast to set and span the whole building without interior bearing walls, but they fill the attic with webs. Rafters leave the attic open and suit custom roof shapes. For the floor and ceiling side of the comparison, see truss vs joist.
How Far Can a Rafter Span?
Allowable rafter span depends on the lumber species and grade, the size (2×6, 2×8 and so on), the spacing, the roof live or snow load, the dead load of the roofing, and the deflection limit. That is why there is no single answer, and why codes refer to span tables rather than one number.
The American Wood Council’s Span Tables for Joists and Rafters, 2024 Edition (checked November 2026) is the standard reference. A few points from its explanation section are worth knowing before you open any table:
- Rafter tables are grouped by load: the R-series tables run from 20 psf live load plus 10 psf dead load upward, all with an L/240 deflection limit.
- For sloping rafters, the tabulated span is measured along the horizontal projection, not along the rafter itself. A rafter on a steep roof is physically longer than its span.
- The tables assume at least three rafters spaced no more than 24 inches on center, sheathed and nailed along the top edge.
- Spans apply to standard dressed lumber: a 2×8 is actually 1-1/2 x 7-1/4 in, a 2×10 is 1-1/2 x 9-1/4 in, and a 2×12 is 1-1/2 x 11-1/4 in.
- You also need the lumber’s design values (E and Fb), which come from a companion design-values supplement, not from the stamp alone.
WoodWorks keeps an index of the current and past span tables and design values for joists, rafters, headers and beams (checked November 2026), which helps when a jurisdiction is still on an older code edition.
Span tables cover ordinary rafters on a uniform load. Anything outside that, such as a rafter carrying a point load from a dormer, solar racking, a mechanical unit, or a rafter cut back for a skylight, has to be sized and checked by a licensed structural engineer or the building department.
Rafters in Steel Buildings
In steel framing the word “rafter” means something larger. In a pre-engineered metal building, each main frame is made of two columns and two sloped rafters, usually tapered built-up I-sections, joined with a moment connection at the ridge and at the knees. These frames are set far apart, in bays measured in feet rather than inches, and lighter members called purlins span between them to carry the roof panels. So the steel rafter does the job of a ridge-to-wall beam, but at a much wider spacing than wood rafters.
In conventional steel buildings, sloped roof beams are also called rafters, and they are often rolled wide-flange shapes. Sizing them follows the same steel beam design rules as any other beam: bending, shear, deflection and lateral support of the compression flange. Our steel beam span chart shows how depth and weight affect span for wide-flange beams, as a starting point before an engineer’s design.
Common Rafter Problems
- Sagging ridge or roof. Often a sign that rafters are spreading because ties were removed, cut or never installed, or that the rafters are undersized for the span.
- Walls bowing outward at the top. The classic symptom of missing rafter ties. It needs an engineer’s assessment, not just new ties pulled tight.
- Cut or notched rafters. Rafters notched for ducts, vents or skylights lose strength. They usually need headers, doubled rafters or reinforcement.
- Removed ceiling joists. Opening a ceiling to vault it removes the rafter ties. That turns a ridge board roof into one that needs a structural ridge beam, which is a structural change, not a finish change. If walls below are also coming out, start with how to tell if a wall is load bearing.
Rafter FAQ
What is the difference between a rafter and a beam?
A rafter is a type of beam: a repeating, usually sloped member that carries the roof deck. “Beam” on its own normally means a heavier member that carries other members, such as a ridge beam that supports the top ends of rafters.
How far apart are rafters spaced?
In wood framing, 16 or 24 inches on center is typical, with 12 inches used for heavy loads. Span tables are built around these spacings, and the roof sheathing must be rated for whatever spacing is chosen.
Is rafter span measured along the slope?
No. In the AWC span tables, the span of a sloping rafter is its horizontal projection. You then work out the actual rafter length from the pitch, plus the overhang.
Can I remove rafter ties or collar ties?
Not without a replacement load path. Removing ties can let the rafters push the walls outward. A licensed structural engineer needs to design the alternative, such as a structural ridge beam.
Are rafters stronger than trusses?
Neither is stronger as a rule. Each is designed for its span and load. Trusses span farther with smaller lumber because of their triangulated webs; rafters leave usable attic space and are easier to alter later.
Reference information only, not engineering advice. Disclaimer.