Construction glossary · Wood framing
What is rafter in construction?
A rafter is a sloped structural framing member that runs from the ridge or wall plate down to the eave, carrying the roof deck and loads in a stick-framed roof. Rafters are spaced at a set on-center dimension, usually 16 in or 24 in, and cut to length along the roof pitch. They are the field-built alternative to manufactured trusses.
Updated June 2026 · Reviewed by the Ruh construction team
Common rafter on a 6:12 roof
Estimating wood framing scope this week? Watch Ruh take off a real plan in 30 minutes.
Book a walkthroughIn US light-frame work, a rafter is a dimensional lumber member (commonly 2x8, 2x10, or 2x12) installed at an angle to form the roof slope. The top end bears at the ridge board or ridge beam, the bottom end notches over the top plate with a birdsmouth and runs out to form the eave overhang. Rafters resist gravity, snow, and wind uplift, and they work against ceiling joists or collar ties to keep the walls from spreading. Spacing is driven by the roof load, the rafter size, and the species and grade in the span tables. A stick-framed rafter roof is slower to build than trusses but gives you a clear attic and easier field adjustment on odd geometry. The estimator counts rafters off the plan and develops the true rafter length from the run and the pitch, because the plan length is the horizontal run, not the sloped length you actually cut and pay for.
Rafters are counted as a quantity (each) and priced by length. Read the roof framing plan for spacing (16 in or 24 in OC), the building width to get the horizontal run, and the roof pitch called out as rise over 12 (for example 6:12). Count rafters along each roof plane: divide the plane length by the spacing and add one for the starting member, then double for both slopes of a gable. Convert plan run to sloped length with the pitch multiplier (line length factor). Pull rafter size and the overhang dimension from the wall section and eave detail. Account for the ridge, hip, and valley members separately. Order lumber in the next standard length above the cut length so you have stock for the birdsmouth and tail.
Worked example
A 30 ft wide gable building, rafters at 24 in OC, 6:12 pitch. Horizontal run per side is half the width, so 30 ft / 2 = 15 ft run. The 6:12 line length factor is the square root of (6 squared + 12 squared) divided by 12, which is sqrt(180) / 12 = 13.4164 / 12 = 1.1180. Sloped rafter length = 15 ft x 1.1180 = 16.77 ft, which is 16 ft 9 in (rounding 0.77 ft x 12 = 9.24 in down to the nearest inch for the body of the rafter). Add a 1 ft overhang and you order 18 ft stock. Count per slope on a 40 ft long roof: 40 ft / 2 ft spacing = 20, plus 1 = 21 rafters per side, x 2 sides = 42 rafters at 18 ft.
Try Ruh on a real bid. 100% money-back guarantee if you are not satisfied.*
*Scoped delivery, terms apply. Read the guarantee terms
How Ruh handles rafter
Ruh reads the roof framing plan, picks up the spacing, run, and pitch, and does the takeoff: it counts rafters per plane, applies the pitch multiplier to convert run to true sloped length, adds the overhang, and rounds each member up to the next stock length. It then prices the rafters on your own price book by size and species. The estimator reviews the pitch factor, the overhang call, and the count, then signs off.
See framing estimating softwareRafter: frequently asked questions
Why is the rafter longer than the building is wide?+
The plan shows horizontal run, not slope. A rafter follows the pitch, so its true length is the run times the line length factor. On a 6:12 roof that factor is about 1.118, so a 15 ft run cuts at roughly 16 ft 9 in before the overhang.
How do I get rafter count from spacing?+
Divide the roof plane length by the on-center spacing in feet and add one for the starting rafter. A 40 ft plane at 24 in OC gives 40 / 2 + 1 = 21 rafters per slope. Double it for both slopes of a gable.
Rafters or trusses for this roof?+
Trusses are faster and engineered off-site for clear spans. Stick-framed rafters cost more labor but give a usable attic and handle odd geometry without a truss package. Span, attic use, and schedule decide it.
Still measuring by hand? Your next takeoff can run while you review this one.
See it on your plansRelated terms
Keep going: read the full guide or explore framing estimating software.
See Ruh price a bid from your own drawings.
AI takeoff and estimating on your price book, your estimator signs off.
Figures on this page are illustrative. Construction estimates depend on project-specific conditions, source documents, market pricing, and professional judgment. Ruh's AI assists the estimator and does not replace professional review: your team reviews, validates, and approves every estimate, bid, and pricing decision.