How to construct roof trusses and rafters

What is the difference between roof trusses and site-cut rafters in Australia?

🪚 Constructing Roof Trusses and Rafters — Professional Framing Guide

Roof trusses and site-cut rafters are primary structural elements and must be set out, fabricated and installed in strict accordance with the engineered design, NCC requirements and relevant Australian Standards. For Australian trade professionals, the key distinction is that prefabricated trusses are generally engineered and manufactured off-site, while cut roofs and conventional rafters may be measured, cut and assembled on site to approved plans.

This guide covers the professional process for interpreting roof framing documentation, setting out trusses and rafters, fabricating conventional members, and installing them safely and accurately. AS 1684 Residential Timber-Framed Construction, engineer’s details and manufacturer’s installation documentation should always take precedence over generic site practice.

What skill level is required for constructing roof trusses and rafters?

Advanced — Estimated time: 1-3 days for small conventional roof framing, 1 day for truss installation on a standard residential structure, depending on roof complexity, access and crew size

What essential tools are needed for roof truss and rafter construction?

Materials Required

Safety Requirements

Step-by-Step Guide

Step 1: Review the Structural Documentation

1️⃣ Confirm the design intent

Obtain and review the full set of approved documents before any cutting or lifting begins. This includes architectural plans, structural engineering drawings, truss layout plans, truss design certificates, tie-down schedules and any manufacturer installation details.

Check:

For conventional framing, identify whether members such as rafters, underpurlins and struts are nominated under AS 1684 span tables or by engineer’s design. For trussed roofs, verify truss type, orientation and set-out against the wall layout.

💡 Never fabricate from architectural elevations alone. Use structural and truss documentation as the controlling set.

Step 2: Set Out and Check Supporting Structure

2️⃣ Prepare the wall plates and bearing points

Before roof framing starts, ensure all external and internal loadbearing walls are plumb, straight and fully fixed. Check top plates for bow, level and alignment.

Mark out:

Use a string line or laser to confirm the building is square and that supporting walls line through correctly. Any inaccuracy in wall framing will transfer directly to the roof.

For truss roofs, mark each truss position on both top plates so the crew can stand and brace progressively. For conventional cut roofs, mark rafter centres and any beam, ridge or underpurlin locations before cutting members.

Step 3: Select and Grade the Timber

3️⃣ Use compliant structural timber

Confirm all framing timber matches the nominated grade and treatment level. Common roof framing members are MGP10, MGP12 or F-grade hardwood, typically in 90x45 mm, 140x45 mm, 190x45 mm or 240x45 mm depending on span and load.

Inspect every piece for:

Crown all rafters and long members consistently. Reject pieces with excessive deformation, particularly where straight roof lines and sheet roofing tolerances are critical.

💡 Store framing timber clear of the ground and protected from weather. Wet timber movement can throw out roof plane alignment.

Step 4: Fabricate Conventional Rafters

4️⃣ Cut plumb cuts, birdsmouths and tails accurately

Where the roof is conventionally framed, begin by calculating the rafter length from the wall plate to ridge, accounting for:

Use a framing square or rafter square to mark:

Keep the birdsmouth within allowable limits so the rafter is not structurally weakened. As a rule, do not overcut the seat depth beyond design allowance. Use the first accurately cut member as a pattern only after checking it on the structure.

For larger roofs, fabricate pairs and stack-label them by location. If hips and valleys are involved, cut commons first, then jack rafters, hip rafters and valley rafters to suit the set-out.

Step 5: Assemble Conventional Roof Components

5️⃣ Install ridge, rafters, ceiling ties and supports

Stand the ridge board or structural ridge beam in accordance with the plans. If using a non-structural ridge board, remember it acts as a fixing line only and relies on opposing rafters and ceiling ties for stability.

Install:

Check each pair for plumb and spacing as you go. Temporary brace the frame immediately to prevent roll or spread. Ensure struts bear correctly onto loadbearing walls or strutting beams below.

Where the roof is engineered conventionally, all props, underpurlins and tie connections must match the engineer’s nominated fixing schedule.

Step 6: Install Prefabricated Roof Trusses

6️⃣ Stand, line and brace trusses to manufacturer details

For trussed roofs, confirm delivery sequence and identify each truss by tag or layout number. Start with the end trusses, plumb them and install temporary braces before standing intermediate trusses.

Typical process:

Install all temporary bracing immediately as the trusses are erected. Once the full run is stood, install permanent bracing in accordance with the truss manufacturer’s layout, including:

⚠️ Never rely on partial fixing or roof battens alone to stabilise newly stood trusses.

Step 7: Fix Tie-Downs and Structural Connectors

7️⃣ Complete all uplift and lateral restraint connections

Every roof frame requires a complete load path from the roof cladding down to the footings. Install all specified tie-down components, including:

Match fastener type and quantity exactly to the fixing schedule. Connector performance depends on the correct nails or screws being used in every hole required by the manufacturer.

In cyclonic or high-wind regions, additional hold-down measures may apply. Check the wind classification and regional framing requirements before final sign-off.

💡 One missing connector can compromise the load path. Treat the tie-down schedule as critical structural work, not a finishing item.

Step 8: Straighten, Plane and Finalise Roof Lines

8️⃣ Check line, level, plane and spacing

Once the structural frame is secure, straighten the roof. Use string lines over top chords or rafter backs to identify highs and lows. Plane or pack where permitted to bring members into a consistent plane for battens or sarking.

Check:

This stage is essential for sheet roofing, tiled roofs and exposed eaves work. Small framing errors become very visible once battens, fascia and cladding are installed.

Step 9: Install Battens, Fascia Backing and Secondary Members

9️⃣ Prepare the roof for cladding

After the primary structure has been signed off internally by the lead carpenter or site supervisor, install secondary roof framing components such as:

Set batten spacing to the roofing manufacturer’s specification. Ensure fascia lines are consistent and ready for gutter installation. If the roof includes boxed eaves, frame outriggers and soffit support accurately to avoid visible wave or sag.

Step 10: Inspect, Certify and Handover

🔟 Complete final compliance checks

Carry out a final inspection against plans, engineering and AS 1684 requirements. Confirm:

On larger or regulated works, coordinate inspection with the building surveyor, certifier or engineer before cladding conceals the frame.

💡 Photograph tie-downs, girder truss connections and bracing before roof wrap or cladding goes on. It is valuable for certification and defect prevention.

Common Mistakes to Avoid

Pro Tips

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