Roof Frame & Tie-Down Local Practice Solution

Current at: 20 July 2010

Project Background

In 2008 tornadoes hit houses Shoalwater Bay and Roleystone. In each incident house lost their roofs or suffered significant damage from the wind forces and flying debris.

Building Commission asked TimberED to provide a report explaining why the roof frames failed. The report suggested the failures resulted from roof framing and tie-down practices that were not sufficient to resist the wind forces (Refer to Tech Report No 54: www.jcu.edu.au/cts/research_reports/index.htm).

So what does the Building Code of Australia (BCA) prescribe?

For local (metropolitan) practice where “stick” roof framing is constructed on masonry walls, the BCA falls short – it does not provide a practicable construction solution.

Unlike other parts of the BCA, Part 3.4.3 Timber Framing does not contain a description of Acceptable Construction Practice, it only provides an Acceptable Construction Manual option that references AS 1684.2 Residential timber framed construction. This standard does not include any descriptions for the use of engineered timber and steel beams that are common to local practice, and it silent on several other aspects of local stick framing practices – especially how to connect a timber roof frame to masonry walls.

Whilst BCA Part 3.3.3 Masonry Accessories provides a description for roof to wall connection, it only applies to cavity walls and does not describe fixings over windows openings and the like. This details is also limited to a 10 metre roof span (most project home exceed this span) and an N2 design wind speed (coastal locations can be N3 and N4).

Building Commission asked As-Built Learning Exchange to develop a local practice solution that they could seek to have adopted into the BCA.

Developing a Local Practice Solution 

ABLE Local Practice Solutions provide building professionals with recognised building solutions tailored to local practices. In the case of roof framing and tie-down, the objective was to develop a set of timber-on-masonry construction details to fill the gaps between the findings of the TimberED report and the BCA local practice shortfalls.

Tie-down is a critical issue for builders because wind incidents are a high risk event. Shoalwater, Roleystone and other events suggest that unless building practices change, damage will occur, flying debris will damage adjacent houses and, whilst it has not happened yet, flying debris is likely to cause severe injury to an occupant or neighbour. In a world where civil litigation is increasing, the builder of a house that fails could see themselves being held responsible for damage to the house, damage to adjacent homes, and for personal injury.

This project provided the opportunity to create a framing and tie-down solution that integrated tie-down connections with work already done to resolve “stick” framing issues.

The starting point was to compare local practice with BCA and AS 1684.2 provisions and then to review building process from roof take-off to construction. Comparing practice and codes identified details where local practice does not comply but should, details where local practice does not comply but has a verifiable performance history, and areas where local practice is not described. From the review of building processes it was discovered that schedulers did not have appropriate resources for working out the roof supporting frame (beam, strut and underpurlin) locations when taking-off material quantities and this that often contributed to framing non-compliance issues found on-site.

The next step was to develop a simple tie-down construction system that would work for design wind speeds up to the N3 limit of other BCA provisions (metal roof cover is affected for N1, N2 and N3 wind speeds, tiled cover in only affected for N3 wind speeds).

Tie-down is an upside down version of span. For construction and dead loads (downward) rafters are designed for the load they carry (tiles or metal cover) and the distance they span between supports. For wind loads (upward) rafters are designed for wind loads (uplift). The critical difference is end support. For downward loads the rafter sits on top the wall plate and underpurlin and against the ridge – gravity holds it in place. For upward loads, if the rafter is only strapped to the supporting member at one end the other end is effectively unsupported. When wind loads are applied the rafter fails and the roof follows.

A simple and robust construction systems is to connect roof members to their supporting members and then to tie the supporting members (plate, underpurlin and ridge) to the masonry walls below. With this system you can achieve the greatest span between tie-down points, tie-down is independent of strutting, masonry tie-downs can be pre-positions and built in (no chasing or surface fixing), there is plenty of flexibility and, for the supervisor, the system is highly visible and is easier to verify from the ground.

To prove that the system was simple and robust a series of site trials were undertaken. Carpenters trialled and tested the practicability of the framing and tie-down details, Midland Brick undertook the development of a tie-down maxi block, and then bricklayers trialled and tested tie-down prototypes and provided feedback on what was needed to easily located and built-in tie-down rods and straps.

Roof Framing & Tie-down Local Practice Solution

The Roof Framing & Tie-down Local Practice Solution is now in the final stage of expert verification.

Once this work has been completed Building Commission will review the technical evidence and when satisfied, issue an Advisory Note recognising the Solution as being equivalent to BCA construction.

The final Solution will include framing details verifying: timber to steel beam, timber to masonry wall, hanger to masonry wall and hanger to steel beam connections; under-blocking and bracing for engineered timber and steel beams on masonry walls; uneven spacing of multiple underpurlins; cantilevered underpurlins supporting hip rafters; strut to underpurlin connections; and that a chock is not required behind inclined struts.

Tie-down details will verify: member to member connections for roof framing, strapping to connect the plate to external masonry walls, strapping to connect underpurlin and ridge to internal masonry walls, and the placement and type of built in connections to internal and external masonry walls.

ABLE will also be publishing several other roof framing resources.

A Stick Roof Strut & Beam Take-Off Calculator will enable schedulers to verify beam to underpurlin off-sets at take-off stage to ensure compliant strutting angles can be achieved and it will enable them to work out masonry tie-down locations so the rods and straps can be shown on drawings. The Stick Roof Strutting Ready-Reckoner will provide Supervisors with an on-the-run chart to verify strutting angles and key roof member spans. And the Roof Framing Work Practice Description will provide a document template from which can be used to create a technical description for inclusion in roof carpenter sub-contract agreements.