Rafters Types: Roof Rafter Types, Uses & Installation
A field-ready reference on rafters types — what each one is, why it exists, how it is sized and installed, and whether it is safe for your roof.
Why Are Rafters Used in Roof Construction?
Why not just place a flat panel over the walls?
A flat, unsupported roof panel would sag and fail under its own weight almost immediately at any meaningful span. Rafters solve this by turning the roof into a triangulated, sloped structural system. Here is why rafters remain a default choice in residential and light commercial construction:
- Load distribution: Rafters spread roof loads evenly across the wall plates instead of concentrating them at a few points.
- Water shedding: The pitch created by rafters lets rain and snow run off instead of pooling.
- Design flexibility: Rafters can be cut on site to fit hips, valleys, dormers, and irregular roof plans that pre-fabricated systems struggle with.
- Attic usability: Because rafters do not need the internal webbing that trusses use, the space beneath the roof stays open and usable.
- Repairability: Individual rafters can be inspected, repaired, or replaced without disturbing the whole roof structure.
Types of Rafters in Civil Engineering
Rafters are classified mainly by their position and function within the roof frame. Below are the seven recognized rafters types used across residential and light commercial roof framing.
Common Rafters
Run at right angles from the wall plate to the ridge board, evenly spaced along the full length of the roof. They form the main sloped surface of a simple gable roof.
Principal Rafters
Heavier, more widely spaced rafters that carry secondary members such as purlins. Common in traditional roof trusses and larger timber-framed structures.
Hip Rafters
Run diagonally from the corner of the wall plate up to the ridge, forming the sloped external corner (hip) seen on hip roofs.
Valley Rafters
The mirror of a hip rafter — they sit at the internal intersection of two roof slopes, collecting and directing water into a valley gutter.
Jack Rafters
Shorter rafters that run between a wall plate and a hip or valley rafter rather than reaching the ridge directly. Sub-types include hip jacks and valley jacks.
Cripple Rafters
Rafters that run between two hip or valley rafters without touching either the wall plate or the ridge — typically used around dormers or openings.
California Rafters
Shorter rafters laid over a continuous sub-roof deck to create an intersecting secondary roof plane, a technique often used for adding a gable onto a hip roof.
| Rafter type | Position | Typical roof shape |
|---|---|---|
| Common rafter | Wall plate to ridge | Gable roof |
| Principal rafter | Wall plate to ridge (heavier) | Trussed roof |
| Hip rafter | Corner to ridge | Hip roof |
| Valley rafter | Internal corner to ridge | L-shaped / intersecting roof |
| Jack rafter | Wall plate to hip/valley | Hip or valley roof |
| Cripple rafter | Between hip and valley | Roof with dormer or opening |
| California rafter | Over a sub-roof deck | Intersecting roof planes |
Types of Rafters by Material
Beyond position, rafters are also grouped by the material they are cut from — each with a different balance of span capacity, cost, and workability.
- Timber rafters: The traditional choice — sawn softwood such as pine or fir, easy to cut and fasten on site.
- Engineered wood rafters: Laminated veneer lumber (LVL) or I-joists, chosen for longer spans and dimensional stability with less warping.
- Steel rafters: Cold-formed or hot-rolled steel sections used for long-span industrial and commercial roofs where fire resistance or strength-to-weight ratio matters.
- Concrete rafters: Precast reinforced concrete members occasionally used in heavy industrial or high-durability structures.
How to Choose the Right Rafter Type
How do you decide which rafters types fit your roof?
- Start with the roof shape. A simple gable needs only common rafters; a hip roof needs hip and jack rafters; an L-shaped roof needs valley rafters.
- Check the span. Longer spans may require principal rafters, engineered wood, or steel instead of standard sawn timber.
- Factor in local loads. Snow load, wind uplift, and seismic zone all affect the rafter size and spacing required by code.
- Consider attic use. If the space beneath the roof needs to be usable, rafters are generally preferable to trusses.
- Confirm with a span table or engineer. Always verify the final size against your local building code span table or a structural engineer’s calculation.
How to Install Rafters (General Process)
- Mark the layout. Lay out rafter positions on the wall plate at the required spacing, commonly 400 mm, 450 mm, or 600 mm center to center.
- Cut the ridge and bird’s mouth. Cut the top ridge angle and the bird’s mouth notch that seats the rafter onto the wall plate.
- Set the ridge board. Temporarily brace the ridge board at the correct height before fixing rafters to it.
- Fix common rafters first. Nail or bolt common rafters in pairs, working from one end of the roof to the other.
- Add hip, valley, and jack rafters. Fit these once the common rafters define the main roof plane.
- Install bracing and fasteners. Add collar ties, hurricane straps, or wind bracing as required by code before sheathing.
- Sheath and inspect. Fix the roof deck and have the frame inspected before covering it with roofing material.
Is It Safe? Rafter Safety Considerations
Is a rafter roof safe? Yes — a correctly designed rafter roof is a proven, code-compliant structural system used on the vast majority of houses worldwide. Safety depends on getting a few fundamentals right rather than on the framing method itself.
Key safety checkpoints
Rafters must be sized for the actual span and load using a verified span table or engineering calculation — undersized rafters are the most common cause of sagging or failure.
Spacing, bird’s mouth depth, and fastener type must follow local building code, and hurricane ties or wind bracing should never be skipped in high-wind or seismic regions.
During construction, rafters should be temporarily braced until sheathing is fixed, and workers should always follow fall-protection practices when working at roof height.
Advantages and Disadvantages of Rafters
Advantages
- Full use of attic space for storage or living area
- Adapts easily to complex, irregular roof shapes
- Can be modified, repaired, or replaced individually
- Materials are widely available and easy to source
- Lower equipment needs — no crane required for lifting
Disadvantages
- More on-site labor and cutting time than pre-built trusses
- Requires a skilled carpenter for accurate compound cuts
- Slower overall installation for large or repetitive roofs
- Quality depends heavily on site workmanship and supervision
- Can be less cost-efficient at very large scale
Common Uses of Rafters
Rafters remain the default roof framing choice across a wide range of building types because of their flexibility and repairability.
Rafters vs. Trusses
| Factor | Rafters | Trusses |
|---|---|---|
| Attic space | Fully usable | Restricted by webbing |
| Installation speed | Slower, built on site | Faster, lifted into place |
| Design flexibility | High — good for irregular roofs | Lower — fixed at the factory |
| Labor skill required | Skilled carpentry | Basic assembly labor |
| Best for | Custom, small to mid-size roofs | Large, repetitive roof runs |
Frequently Asked Questions About Rafters Types
Rafters are sloped structural members that run from the ridge or hip of a roof down to the wall plate, forming the main framework that supports the roof deck, covering, and imposed loads.
A rafter is a single sloped member built and positioned on site, while a truss is a pre-engineered triangular assembly manufactured off-site and lifted into place as one unit.
Seven commonly recognized types by position: common, principal, hip, valley, jack, cripple, and California rafters.
Yes, when it is designed by a qualified engineer, correctly sized for span and load, spaced per code, and properly braced against wind and lateral movement.
It depends on span, spacing, pitch, snow and wind load, and material grade — always confirm against a local span table or an engineer’s calculation.
Most residential roofs use 400 mm, 450 mm, or 600 mm center-to-center spacing, depending on rafter size, roof covering weight, and local code.
Full attic usability, easy adaptation to complex roof shapes, and the ability to modify members on site.
More site labor, need for skilled carpentry on compound cuts, and generally slower, less cost-efficient installation at large scale.
Yes — steel and engineered wood rafters are used where longer spans, higher loads, fire resistance, or dimensional stability are required.
Using run, rise, and pitch through the Pythagorean relationship — rafter length equals the square root of run squared plus rise squared, adjusted for the overhang.