Rafter
A rafter is the sloped structural member from ridge to eaves carrying roof load. Sized from AS 1684 by stress grade, span, wind class, load width. Different from a truss.
Ask Chalkline about this →A rafter is the sloped structural member running from the ridge at the top of a pitched roof down to the top plate at the eaves. It supports the roof load (battens, sheeting or tiles, snow, wind) and transfers that load to the top plate. Rafters are sized from AS 1684 span tables based on stress grade, clear span, wind classification, and load width (also called rafter spacing). A rafter system is distinguished from a truss system, where prefabricated nail-plated assemblies replace conventional rafters and ceiling joists.
Rafter vs truss:
| Feature | Rafter (conventional) | Truss (prefabricated) |
|---|---|---|
| Where made | Site-cut by chippy | Factory-made, delivered to site |
| Roof framing | Rafters + ceiling joists separate | Combined into single truss assembly |
| Span flexibility | Limited; longer spans need underpurlins / collar ties | Long clear spans (10-15 m+) possible |
| Roof shape flexibility | Excellent; hand-cut to any geometry | Less; bespoke trusses cost more |
| Labour intensity | High; skilled chippy hours | Low; trusses lifted into place |
| Material efficiency | Less efficient (heavier sections) | Highly efficient (engineered) |
| Australian usage | Mostly heritage, retrofit, custom | Volume residential, project homes |
Most volume residential builds in Australia use trusses; rafter systems persist in custom architectural, heritage replacement, and where roof geometry is irregular.
Rafter components:
| Component | What it does |
|---|---|
| Common rafter | Standard sloped member from ridge to top plate |
| Hip rafter | Sloped member at the external corner of a hip roof (at 45° to common rafters in plan) |
| Valley rafter | Sloped member at the internal corner where two roof slopes meet |
| Jack rafter | Short rafter spanning from hip or valley to top plate, parallel to common rafters |
| Cripple rafter | Short rafter between valley and ridge or hip |
Sizing rafters (AS 1684 process):
- Determine the wind classification (N1-N6, C1-C4) from the site.
- Determine the load width = rafter spacing in mm (typically 600 mm or 900 mm).
- Determine the clear span = horizontal distance between supports (top plate to underpurlin, or top plate to ridge for short spans).
- Look up the AS 1684 span table for the wind class, load width, and stress grade (MGP10, MGP12, F-grade hardwood).
- Select the timber size (e.g. 90 x 45 MGP10, 140 x 45 MGP10, 190 x 45 MGP10).
- Confirm tie-down requirements for the wind classification.
A typical residential rafter (single-storey, N2 wind class, 600 mm centres, 3.5 m clear span) is 90 x 45 MGP10.
Bird’s mouth at the top plate:
The rafter is birdsmouth notched at the top plate to give a level seat and a vertical back. The birdsmouth is typically one-third the rafter depth maximum, to preserve strength. See birdsmouth for details.
Connection at the ridge:
| Connection | Where used |
|---|---|
| Direct nail to ridge board | Standard for residential gable / hip roofs |
| Skewed metal connector | Where higher uplift loads (N3+) |
| Underpurlin + struts | For longer rafters where the rafter alone can’t span |
Collar ties and underpurlins:
For longer rafters (typically >3.6 m clear span), AS 1684 may require:
- Collar ties (horizontal members between opposing rafters above the ceiling joist line): tie the rafters together to resist spreading.
- Underpurlins (longitudinal members under the rafters): break the span into shorter sections.
- Struts (from underpurlin to internal wall or strutting beam): transfer the underpurlin load to a structural wall.
These additions cascade design complexity; engineering input is often needed.
Common builder issues:
- Rafter undersized for wind class: caught at frame inspection.
- Birdsmouth cut too deep (>1/3 depth): structural failure.
- No collar ties on long spans: rafters spread; ridge sags.
- Rafter ends not aligned: roof line wavers.
- Wrong rafter species/grade for the table read: undersized for the actual material.
For builders:
- Confirm wind class at the start: get the site wind classification from the engineer or your AS 4055 lookup.
- Stock standard rafter sizes for the wind class you typically work in: bulk buying saves cost.
- Spec stress grade and species on the order: don’t accept ungraded timber.
- Pre-cut birdsmouths consistently: jig set-up saves time and avoids variation.
- Walk the rafters before sheeting with the chippy: confirm alignment, birdsmouth depth, tie-down.
Also known as: common rafter, sloping roof member.
Category: Framing / roof / AS 1684.
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Last updated: 2026-05-16. Verified: 2026-05-16.