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AS 1720.5 (nailplated trusses): the design code behind the certificate

AS 1720.5 is the design standard for nailplated timber roof trusses. The truss engineer designs to it; the certificate handed to the builder is its output.

By Dan Waring. Last updated 16 May 2026. Verified 16 May 2026.

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In plain English

AS 1720.5:2015, Timber structures, Part 5: Nailplated timber roof trusses, is the design-side Australian Standard for prefabricated nailplated trusses. It sits alongside AS 1720.1 (general timber design) and AS 4440 (truss install). For a builder, the design certificate that arrives with the trusses is the AS 1720.5 output. The truss manufacturer’s engineer runs the loading analysis under AS 1720.5, sizes each member, designs the connector plates, and issues the certificate. The builder relies on the certificate, follows the layout drawing, and installs per AS 4440.

The standard is engineer-only territory: a builder does not design a truss. But understanding what the certificate is covering, and what it is not covering, prevents the design-vs-install errors that crop up at frame inspection.

What it requires

For the truss-design engineer working to AS 1720.5:

  1. Connector plate capacity. The nailplate joint between members has a calculated capacity in tension, shear, and bending. The plate size, gauge and tooth pattern are designed to AS 1720.5’s tabulated values for the proprietary nailplate system used (e.g. MiTek, Pryda, Multinail).
  2. Member sizing. Each chord, web and webbing-pitch member is sized for combined axial and bending forces under the worst-case load combination.
  3. Web layout geometry. Web positions are set to deliver the load efficiently into the plate joints; web angles outside the standard’s bounds reduce plate capacity.
  4. Loading combinations. Dead, live, wind (per AS 1170.2 and AS 4055 wind classifications), and snow where relevant. The truss carries the worst case.
  5. Deflection limits. Bottom chord and top chord deflections are limited so the truss does not bow at install or sag in service.
  6. Certification. A signed design certificate per truss type, named to the project and the truss tags on the layout drawing, must be issued by a qualified engineer at the truss manufacturer or their consultant.

What it doesn’t cover

  • Truss installation. That is AS 4440. AS 1720.5 stops at the truck.
  • Stick-framed roof construction. AS 1684 (residential timber-framed construction) is the install standard for stick framing.
  • Steel trusses. Cold-formed steel sits under AS/NZS 4600.
  • Glued laminated timber. That’s AS/NZS 1328.
  • Loading determination. Dead and live loads come from AS/NZS 1170 series. AS 1720.5 uses the loadings; it doesn’t define them.

Practical implications

  • The design certificate is non-substitutable. A builder cannot accept a verbal “she’ll be right” from the truss yard. If a truss arrives without a design certificate, the certifier will not pass frame inspection.
  • Re-design is required for site modifications. Drilling through a chord, notching a web, or shortening a bottom chord changes the AS 1720.5 inputs. The original certificate is no longer valid. The builder either: (a) gets a re-certification from the engineer, or (b) replaces the modified truss.
  • Service penetrations have to be designed in. A 100 mm chase for HVAC ducting through a web is the kind of detail that needs to be on the truss design before the truss is built; cutting it on site is a defect. Coordinate with the services design at the truss-order stage.
  • Wind classification drives capacity. A truss certified for an N3 wind region (suburban Sydney) is not the same truss as one certified for C2 cyclonic (Cairns). When a builder uses a generic plan in a higher-wind region, the trusses need re-certification.
  • Camber is a design output, not an install option. Some trusses are designed with a built-in upward camber at the top chord, so that under service load they deflect to a flat profile. Setting the truss “flat” against the camber is a defect; install per the drawing.

References

See also

Last updated: 2026-05-16. Verified: 2026-05-16. Quarterly review for currency.

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