NCC Roof Structure for Australian Homes: Part H1, AS 1684 and AS 4055

What it is

Roof structure on a Class 1 residential job is covered by NCC 2022 Volume Two Part H1 Structure. The Deemed-to-Satisfy pathway calls up AS 1684 for timber framed houses in non-cyclonic and cyclonic areas, AS 4055 for wind classification, and AS 1720.5 for the design of nailplated timber roof trusses.

The role of the roof structure is to carry dead load (roof tiles or sheet, sarking, insulation, ceilings), live load (maintenance access), wind uplift, and any snow load in alpine areas. It also has to be tied back to the wall frame and the slab so the entire load path is continuous from the ridge to the footing.

The standards stack

AS 4055 wind classification

AS 4055 sets the wind classification for housing. Non-cyclonic regions use N1, N2, N3 or N4. Cyclonic regions use C1, C2 or C3. The classification depends on the wind region (A, B, C or D), terrain category, topographic class and shielding. AS 4055 has geometric limits: eaves height not exceeding 6.0 m, ridge height not exceeding 8.5 m, building width not exceeding 16.0 m, length not exceeding five times width, and roof pitch not exceeding 35 degrees. Outside those limits the design has to follow AS/NZS 1170.2.

AS 1684 timber framing

AS 1684.2 covers non-cyclonic timber framing for N1 to N4. AS 1684.3 covers cyclonic for C1 to C3. AS 1684.4 is a simplified non-cyclonic version for N1 and N2. The 2021 editions of Parts 2 and 3 supersede the 2010 versions and are the documents currently called up by NCC 2022.

The span tables, beam sizes, rafter and ceiling joist spacings and tie-down schedules all come from AS 1684. A builder framing under AS 1684 reads the span tables for the wind classification and roof type.

AS 1720.5 truss design

Most modern Australian houses use prefabricated nailplated trusses rather than cut-and-pitched rafters. AS 1720.5 sets the design rules. The truss manufacturer issues a layout, a truss schedule, a bracing plan and a tie-down schedule. That documentation is what gets inspected.

Where roofs fail an inspection

Tie-down missing or undersized

Cyclone rods, framing anchors and triple-grip plates need to be installed to the AS 1684 or truss manufacturer schedule. The classic defect is wall-plate-to-rafter tie-down that stops at the top plate and never connects through to the studs and the bottom plate. The load path is broken and the audit picks it up by checking continuity on photos.

Roof bracing not installed or temporary bracing left in

AS 1684 requires speed brace, T-brace or strap brace to the roof plane based on wind classification. On many jobs the temporary erection bracing stays in and the permanent bracing never gets installed. The roof sheets the bracing visually but does not carry the lateral load.

Truss layout deviates from the engineered drawing

A girder truss gets rotated. A multi-ply girder loses a ply at the fabricator. A hip end is reframed on site without the engineer signing off. Each of those breaks the design assumption in the truss schedule.

Cyclone rods missed at openings

In C1 to C3 areas, AS 1684.3 requires continuous tie-down at openings and corners. These are often the items missed when the framing crew is unfamiliar with cyclonic detailing.

Inspection hold points

For a TradeLens roof inspection the hold points are frame completion, tie-down installation, roof bracing completion, and pre-sheeting. The pre-sheeting check is the load-bearing one. The auditor checks truss spacing against the layout, tie-down at each truss-to-wall connection, bracing to plan, and any modifications signed off in writing. Cyclone areas add a corner and opening tie-down check.

Where rafters are cut and pitched instead of trussed, the span and size are checked against AS 1684 span tables for the wind class and roof load width.

Why an auditor cares

Roof failure modes are dramatic. A loss of roof in a high wind event is a Category One defect and an insurance claim that will name the builder. Even short of failure, sagging ridges and cracked ceilings trace back to undersized members and missing tie-down. The audit pattern is consistent. Is the wind classification on the plan correct for the site. Is the truss layout the one that was fabricated. Is each connection tied down to the schedule. Is the bracing installed.

Each of those traces back to an AS 1684 or AS 4055 clause, and each defect on a finished house is harder to fix than it was to build.