Replacement roofs which have not been adequately tied down, have been put up without a permit or have been installed by unqualified people have emerged as a serious concern when it comes to wind damage from severe weather events such as cyclones or storms, the latest analysis suggests.

Following several cases where replacement roofs have been dangerously lifted off within the state, the Western Australian building regulator Building and Energy worked with James Cook University’s Cyclone Testing Station to investigate cases where buildings and houses which had been re-roofed had fully or partially lifted off.

The investigation comes amid ongoing concerns about the number of roofs which have been replaced have been damaged of lifted off during severe weather events.

Within the Perth metropolitan area, for example, these replacement roofs account for at least half of all roofs which have been lost and subject to Building and Energy investigations. Many of these cases involved replacement of roof tiles with sheet metal cladding.

When it comes to roofs, these are subject to forces as wind flowing over and around a building puts positive pressure (pushes) on the windward wall and negative pressure (pulls) on the roof and other walls, as shown in Figure 3-1(a).

The pull or suction on the roof is in an upward direction.

This is especially prevalent where there are openings on the wall facing the wind, such as doors or windows which are open or which are broken. This creates pressure on the underside of the roof.

When combined with the upward suction, this pressure can see the uplift potentially double when compared with cases where no opening is present and wind instead flows around the building.

In cases where the roof structure and cladding are not sufficiently tied down, this can cause all or part of the roof to lift off the building.

Meanwhile, the cladding on the roof may need to be replaced over time.

Where this is done with like-for-like material, the report says that this should not alter the net uplift of the roof structure – albeit with a check of the roof structure and tie-downs being advised to ensure that these remain fit for purpose.

Where different materials are used, however, the overall push and pull forces acting on the roof will be altered.

This commonly happens where heavier tile roof cladding is replaced with lighter metal sheet cladding.

This increases the net uplift and may necessitate heavier new battens with stronger batten-to-rafter ratios.

As far as Western Australia is concerned, building permits mare not usually required if the cladding is to be replaced with the same material, and the replacement works will not affect the roof structure or tie-down systems.

Approvals are required, however, where roof cladding is replaced with different materials. Approvals may also be needed even where the same materials are used where the thickness, profile or unit lengths of the replacement differ from that of the original material as this may necessitate upgrades to the roofing structure.

During their investigation, however, Building and Energy and JCU found that poor practice lay behind roof failures in many cases.

Among these failures:

• In many cases, there was no record of any approval to replace previous cladding with a different material. Where approval documents were found, these were often incomplete or lacking in information such as wind classification and tie-down details. As a result, roof tie-downs were not checked or upgraded.
• In some cases, original tile battens were used as battens to the new metal sheet roof. Where this happened, the spacing of the roof fasteners was too large and the capacity of the tile battens was insufficient to resist higher level of uplift associated with the lighter roof.
• Structural elements such as the timber rafters and roof connections had deteriorated.
• In cyclonic areas, over-battens and tie-downs were removed or cut. An alternative tie-down system was not installed.

Consequences are serious.

In some cases, large steel or timber beams have been propelled up to 100 metres over houses, power lines and trees before becoming embedded into the ground, fences or other buildings.

In a guide for owners, Building and Energy said several steps are recommended.

These include:

• Ensuring relevant approvals have been obtained.
• Ensuring that required strengthening to the roof is sufficiently documented.
• Using competent people, such as a structural engineer and/or a registered builder with relevant qualifications and experience, to check the existing roof structure and any proposed upgrades.
• Ensuring that the roof covering is installed to the manufacturer’s installation instructions (which must also meet applicable building standards).
• Using a suitably qualified person, such as a registered building surveyor, builder or civil engineer, to check the roof gutters, downpipes and sub-surface drainage are sufficient to direct rainwater away.
• Having builders ensure that additional tie-downs have sufficient capacity and engage all parts of the roof as necessary.

Building and Energy Executive Director Saj Abdoolakhan said the importance of diligence when undertaking roof replacements should not be underestimated.

“The loss of a roof could cause serious injury or death to people sheltering inside or hit by wind-borne debris …,” he said.

“… Events like Tropical Cyclone Seroja highlight why changing a roof covering from one material to another requires careful planning and expert knowledge to ensure a lighter roof doesn’t lift off during a wind event.”