Rafters vs Trusses
Rafters are single sloped boards cut and framed on site, sized from the building code’s span tables. Trusses are triangulated frames built in a plant, each engineered for its span and load. Rafters leave an open attic and are built on site; trusses span wall to wall without interior support and, the truss industry says, close a house in faster. Here is how each works, the rules for both, and which suits which roof.
Rafter framing
Rafters meet at a ridge board; ceiling joists or rafter ties hold the walls in. Open attic.
Truss framing
Webs triangulate the frame; it spans wall to wall. Webs fill the attic.
Schematic, not to scale. Size your own: rafter length calculator or roof truss calculator.
Put numbers on either one
Both calculators take your span, pitch and overhang and show the arithmetic.
Rafters vs trusses at a glance
| Rafters | Trusses | |
|---|---|---|
| How they are made | Cut and assembled on site from dimensional lumber | Built in a plant from lumber and metal connector plates, delivered whole |
| How they are sized | From the IRC rafter span tables, R802.4.1, for standard loads | Engineered for each job to ANSI/TPI 1, with a design drawing, R802.10 |
| At the ridge | A ridge board at least 1 inch nominal, or a designed ridge beam | No ridge board; the truss is one piece |
| Outward thrust | Resisted by ceiling joists or rafter ties | Resisted by the truss’s own bottom chord |
| Interior walls | May be needed to support ridges, hips and long spans | Can span wall to wall with no interior bearing walls, says SBCA |
| Attic | Open, usable for storage or a room | Filled with webs, unless attic trusses are used |
| Changes later | Altered under the code's normal framing rules | No cutting or altering without a design professional |
How rafter framing works
A rafter roof is a series of sloped boards in pairs. Under the 2024 International Residential Code, rafters are framed opposite each other to a ridge board, offset no more than 1 1/2 inches, and joined with a collar tie, a ridge strap or a gusset plate (R802.4.2). The ridge board must be at least 1 inch nominal thick and no shallower than the cut end of the rafters (R802.3).
Rafters push outward on the walls as the roof is loaded, and something has to hold the bottom together. The span tables assume ceiling joists or rafter ties at the bottom of the attic; where they do not provide a continuous tie across the building, the ridge has to be carried by a wall or a ridge beam designed by engineering instead. Below a 3:12 pitch, the IRC requires ridges, hips and valleys to be designed as beams (R802.4.4). Hip and valley rafters must be at least 2 inches nominal thick and as deep as the cut end of the rafters (R802.4.3), and purlins braced down to bearing walls may be used to shorten a rafter’s span (R802.4.5).
How far rafters span
Rafter size comes from Tables R802.4.1(1) through (8), which cover different live loads, dead loads and ceiling conditions; spans are measured horizontally, not along the slope. The first table, for a 20 psf roof live load, 10 psf dead load and no ceiling attached to the rafters, gives these spans for No. 2 Douglas fir-larch:
| Rafter spacing | 2×4 | 2×6 | 2×8 | 2×10 | 2×12 |
|---|---|---|---|---|---|
| 12 in on center | 10 ft 10 in | 16 ft 10 in | 21 ft 4 in | 26 ft 0 in | over 26 ft |
| 16 in on center | 9 ft 10 in | 14 ft 7 in | 18 ft 5 in | 22 ft 6 in | 26 ft 0 in |
| 24 in on center | 8 ft 2 in | 11 ft 11 in | 15 ft 1 in | 18 ft 5 in | 21 ft 4 in |
Other species, grades, snow loads and attached ceilings give different spans, so use the table that matches your roof, and the AWC span tables the IRC points to for anything outside them. Where ceiling joists or rafter ties sit higher in the attic, the spans are reduced by the adjustment factors in Table R802.4.1. The rafter length calculator gives the length to cut once the size is chosen.
How truss framing works
A truss carries the roof with a triangle of top chords, a bottom chord and webs joined by metal plates, so the bottom chord takes the outward thrust that ceiling joists take in a rafter roof. Each one is engineered: Section R802.10 requires design to ANSI/TPI 1, a design drawing showing span, spacing, joints, bearings and loads, approval by the building official before installation, and bracing as the drawings require. The roof truss guide shows the common types and shapes.
Because each truss spans the full width, SBCA notes that houses can be framed without interior bearing walls, leaving one open space whose partitions can later be moved. The trade-off is that the truss is finished: Section R802.10.4 forbids cutting, notching, drilling or splicing any member without a registered design professional’s approval.
Attic space and vaulted ceilings
A rafter roof with ceiling joists leaves the attic open between the joists and the rafters, which is the main reason to choose rafters where storage or a future room matters. Standard trusses fill that space with webs. Two truss shapes close the gap: room-in-attic trusses, which leave a rectangle open and have a bottom chord designed for floor-type loads, and scissor trusses, whose sloping bottom chord makes a vaulted ceiling. With rafters, a vaulted ceiling usually means dropping the ties and carrying the ridge on a designed ridge beam.
Cost and build time
SBCA, which represents truss makers, lists the advantages it claims for trusses: a building closed in sooner, less waste from cutting errors, less expensive carpentry labor, framing delivered as one package, and no interior bearing walls or the foundations under them. HomeGuide puts roof trusses at $60 to $700 each for materials, and replacing them at $5 to $14 per square foot installed. None of our sources publishes a comparable price for framing the same roof with rafters, so we do not claim one is cheaper by a set amount.
Which one to use
- You want attic storage or a future room: rafters, or room-in-attic trusses designed for it.
- You want open rooms without interior bearing walls: trusses, which span wall to wall.
- A shed, porch or small addition you are framing yourself: rafters sized from the IRC tables are within reach of a careful builder, while trusses are ordered from a truss maker.
- A vaulted ceiling: scissor trusses, or rafters on a designed ridge beam.
- A complicated roof line: either can work; get a truss quote and a framing quote and compare.
Whichever you choose, your building department approves the plans, and anything beyond the code tables needs engineering.
Sources
- 2024 International Residential Code, Chapter 8: ridge boards (R802.3), rafter sizing, framing, hips and valleys, supports and purlins (R802.4), Table R802.4.1(1) spans and footnotes, and wood trusses (R802.10).
- Structural Building Components Association, Roof Trusses: the benefits SBCA claims for trusses.
- Structural Building Components Association, Truss Configurations: attic and scissor trusses.
- HomeGuide roofing labor cost guide, dated November 7, 2025: truss material and replacement cost.
- All read on November 2, 2026. Every source is listed on the methodology page.
Related calculators and guides
Rafters vs trusses FAQ
What is the difference between rafters and trusses?
Rafters are single sloped boards cut and assembled on site, sized from the span tables in the building code, with a ridge board at the top and ceiling joists or rafter ties at the bottom. Trusses are triangulated frames built in a plant from lumber and metal plates, each engineered for its span and load and delivered ready to set.
What is a truss roof?
A roof framed with prefabricated trusses instead of rafters. Each truss carries the roof from one exterior wall to the other, so the webs fill most of the attic but the rooms below can be laid out without interior bearing walls.
How far can a 2×6 rafter span?
It depends on the wood species and grade, the spacing and the loads. In the 2024 IRC table for a 20 psf roof live load and 10 psf dead load with no ceiling attached, a No. 2 Douglas fir-larch 2×6 spans 14 feet 7 inches at 16 inches on center and 11 feet 11 inches at 24 inches, measured horizontally. Snow country uses other tables.
Are trusses cheaper than rafters?
SBCA, the truss industry association, says trusses save on carpentry labor, cutting waste and site time. HomeGuide puts trusses at $60 to $700 each for materials. None of our sources publishes a like-for-like price for rafter framing, so this page does not claim a figure for the difference.
Do rafters need ceiling joists or rafter ties?
Something has to stop the rafters pushing the walls outward. The IRC rafter span tables assume ceiling joists or rafter ties at the bottom of the attic; where they do not tie the structure together, the ridge must be supported by a wall or a designed ridge beam instead of a plain ridge board.
Can you turn a truss attic into living space?
Not by cutting the webs out: the IRC forbids altering truss members without a registered design professional's approval. For usable space, plan room-in-attic trusses, whose bottom chord is designed for floor loads, or frame the roof with rafters.