Tropical Cyclone Seroja

TC Seroja – 11 April 2021

TC Seroja caused severe damage to many houses and buildings in the coastal areas of the Mid-West of WA. Many houses had damage to parts of their roof due to failure of the roof tie-downs. In the areas that experienced higher wind speeds (Kalbarri and Northampton), around 10% of houses lost most of their roof.

See the Guide for Homeowners below for repairing and rebuilding houses damaged in Tropical Cyclone Seroja.

You can consult our Repairing or Rebuilding Guide for a quick overview.

Track of TC Seroja (Bureau of Meteorology)
Track of TC Seroja (Bureau of Meteorology)

TC Seroja was not an unusual event; several tropical cyclones have come south of the Mid-West latitudes in the past: TC Wally (1976), TC Alby (1978), Bruno (1982), Billy-Lila (1986), Ned (1989), Elaine (1999), Bianca (2011), and Iggy (2012). Of these, TC Alby and TC Billy-Lila were severe tropical cyclones for part of their tracks. Seroja was still a severe tropical cyclone when it crossed the coast. It produced wind speeds that were a little less than the current design wind speed for the region around Kalbarri. The wind speeds were lower elsewhere in the area and in Geraldton were around 2/3 the design wind speed.

Cyclone tracks in the south of WA 1971 to 2018 (Bureau of Meteorology)
Cyclone tracks in the south of WA 1971 to 2018 (Bureau of Meteorology)

The simple message is that TC Seroja was not really rare and similar events could happen in the region in the future. It is highly likely that your house will have winds from a cyclone and the wind speed could be as high or a little higher than the winds in Kalbarri during TC Seroja.

A Guide for Homeowners in Kalbarri, Northampton, and Port Gregory

Below is a guide for repairing and rebuilding houses damaged in Tropical Cyclone Seroja:

The communities around Kalbarri, Northampton, Port Gregory and Geraldton are in Wind Region B. The roof tie-downs for houses in these regions need to be stronger than those used for houses in Wind Region A (the southern and inland parts of WA, including Perth). These web pages provide guidance for homeowners repairing or rebuilding their houses in this region. If the roof structure is not well tied down, the roof could fail in a future wind event.

Disclaimer

This advice does not replace or override any legislative requirements, and builders following it should read it in conjunction with relevant laws, and current Codes and Standards, including:

  1. National Construction Code of Australia (current edition)
  2. AS 4055; or AS/NZS 1170.2
  3. Australian Standard AS 1684.2, or AS 1684.3, or the NASH Standard – Residential and low-rise steel framing Part 2: Design solutions.
  4. AS 4773.1 Masonry in small buildings Part 1: design
  5. Building Act 2011 and Building Regulations 2012

Roof tie-downs

Strong winds blowing over your house cause uplift on the upper surface of the roof. To stop the roofs from lifting off, the roof cladding needs to be securely anchored to many tonnes of building material. The connections between all building elements from the roof cladding through the roof structure and walls and into the ground must be strong enough to resist the huge forces that wind applies to your house. Together these elements form a tie-down chain.

The elements that form this chain include:

  1. roof sheeting and the fasteners that carry the loads from the roofing to the battens or purlins
  2. battens/purlins and fasteners that carry loads from battens/purlins to the rafters or trusses;
  3. roof structure including rafters or trusses and tie-down to the top of walls – carrying loads from the batten/purlin fasteners to the tops of the walls;
  4. uplift load transfer within the wall from the top plate to the base of the wall;
  5. uplift load transfer from the bottom of the wall to the floor system; and
  6. uplift load transfer through the floor and sub-floor systems to the ground.

If all of these elements are strong enough, the house will be resilient in future cyclones, but if any one of them is weak, then a future cyclone could cause damage to the repaired or new roof.

Ensure that repairs and reconstruction to all elements in the tie-down chain following damage in TC Seroja are appropriate for the wind speeds and loads expected in Wind Region B.

Building approval

The WA Building Act requires that all building work complies with the National Construction Code. The building regulatory process includes certification of plans and specifications, approval of the work by the local council, and sign-off by the builder to indicate that the construction followed the approved plans.

Building approval is usually required for major repairs and is mandatory for reconstruction work.

The Shire of Northampton has indicated that you must have building approval for all building work that affects the building structure including all of the elements in the tie-down chain illustrated above. (The only exceptions are minor repairs e.g., replacing a single sheet of wall cladding or a few tiles or working on non-structural elements such as gutters, trims and internal finishes.) If you have already completed some significant repairs on structural elements such as battens and rafters, you should apply for building approval retrospectively.

  • The approved work must be done by a registered builder – you can only do approved work yourself if you are a registered owner-builder.
  • The repairs or reconstruction need to be designed for the wind loads or wind classification for your house. A structural engineer usually does this and can also design the new or upgraded tie-down system.
  • To comply with the Building Act, you must engage electricians and plumbers to do any electrical or plumbing work.

Even if building approval is not required, your builder should still ensure that any repairs comply with the minimum standards in the NCC.

The NCC specifies the minimum requirements for buildings, so you must comply with all of the requirements. However, it is possible to exceed the minimum requirements and to make your house more resilient in a future cyclone by asking your builder to use some construction details usually specified for Wind Region C (e.g. for Denham or Broome).

Asbestos

If your house was built before the mid-1980s, there might be some asbestos in the roof or walls. Broken asbestos is a risk to your health and must be removed by licenced operators. More information is available on this website:

https://www.commerce.wa.gov.au/worksafe/asbestos-frequently-asked-questions

Major work on the roof structure

Major work includes major repair or replacement of structural elements in the roof or replacing the roof cladding with a different roof cladding. You should engage a builder to do the new work and repair any other parts of your house that have been damaged. You need to get the following:

  • Wind classification or evaluation of wind loads
  • Design of tie-downs
  • Plans and specifications
  • Certification and building approval
  • Building to the approved drawings
  • Appropriate documentation on completion

Determining the wind classification of your site is the first step in the structural design of your house.

The Australian Standard for wind loads on housing (AS 4055) defines wind classifications for houses. There are size limitations on the houses that can be evaluated using this Standard. Wind loads on houses that are larger than those size limitations will need to be evaluated by an engineer using a different Standard (AS/NZS 1170.2).

The wind classifications are based on the likely wind speed at the house site and take account of:

  • How close your block is to open country, the ocean or other large bodies of water
  • Whether your house is ‘shielded’ by other similar sized houses that are close by
  • If your house is located on the top of a hill or on flat ground.

N classifications and C classifications

Australia has a system of two wind classifications:

  • N-classifications for wind regions A and B – inland parts of WA and coastal parts from Kalbarri south; and
  • C-classifications for wind regions C and D – the coastal parts of WA north of Shark Bay.

The N-classifications assume that all doors and windows can resist the design wind forces and won’t be damaged by wind-borne debris.  The C-classifications take into account wind-borne debris impact that may break doors and windows. Where this happens, the high windward wall pressure is let into the house and presses up on the underside of the ceiling. The C-classifications therefore require stronger tie-downs in the roof structure than the N-classifications with the same wind speed at the house site.

Assumed wind pressures in N wind classifications
Assumed wind pressures in C wind classifications

Wind classifications in Kalbarri, Northampton and Port Gregory

In Wind Region B, the lowest wind classification anywhere is N2, but is usually N3 or higher. AS 4055 is used to determine the wind classification for most houses. However, if your house is too large to fit within the size limitations in AS 4055, then an engineer can determine an equivalent wind classification using a different wind loading standard.

The engineer who evaluates the wind classification of your block uses measures of the exposure of the house to wind. In general, if a house has a good view, it will be more exposed to wind and will need a higher wind classification.

For example, the following maps give an indication of the wind classifications for houses in Kalbarri. (Please note that the maps are only a guide and site wind classifications need to be evaluated by an appropriately trained person who visits the site). The first classification in the key is the one based on strict application of the Standard for wind loads on housing. The classification in brackets represents a classification with an equivalent wind speed, but designed to resist the internal pressures that could occur in some houses during tropical cyclones.

Wind Classifications to AS 4055 for house sites in the more established, northern part of Kalbarri

This part of Kalbarri is almost fully developed, so it is likely that any vacant blocks might be built on during the next 5 years. The wind classifications have been evaluated using the assumption that any vacant block may have a house on it within 5 years.

Wind Classifications to AS 4055 for house sites in southern subdivisions of Kalbarri

This part of Kalbarri was released for development more than 10 years ago. As of April 2021, there are no houses in the subdivision that are surrounded on all sides by two rows of houses. As it is unlikely the subdivision will be fully developed in the next 5 years, the wind classifications have been evaluated using the current shielding of each block.

The minimum standard of construction in Wind Region B is the appropriate N-classification – N2 or higher. However, you can choose to build your house to an equivalent C-classification, which will mean your house has more resilience in future tropical cyclones when there may be a lot of wind-borne debris, as there was during TC Seroja. The stronger tie-downs in your roof will minimise the chance of roof damage if windows or doors are damaged by wind-borne debris.

The design wind speed for C1 wind classification is the same as the design wind speed for N3, but the design pressures for C1 are higher as they assume that a window or door could have been broken by wind-borne debris. The following table gives equivalent C wind classifications for each of the N wind classifications e.g., if the wind classification for your house is N4, you could ask your engineer and builder to design your roof to C2 specifications. The design wind speed is the same for both classifications, but the C classification means the roof structure will be strong enough to cope if a window or door breaks during a cyclone.

N classification

C Classification

N2*

C1

N3

C1

N4

C2

N5

C3

N6

C4

*There is no direct equivalence between N2 and a C-classification, but it is conservative to use C1 for N2 house sites.

Retrofitting older buildings to C2 wind classifications

The Weather the Storm web site presents a number of retrofits that are designed for C2 wind classifications. This classification has a wind speed equivalent to N4 and is therefore either appropriate or conservative for any house in Kalbarri.

The following items are important in the tie-down chain. Although the specification of these items is the responsibility of the builder, here is some general guidance on the minimum construction requirements.

Cladding

The metal sheeting must be fixed to the battens according to the manufacturer’s recommendations that include the correct batten spacing, type of fasteners, and the correct number of fasteners per sheet for the sheet profile and thickness.

Tie-down of battens to trusses or rafters

These connections often can’t be checked once the roofing is installed. If your roof is being repaired, the right time to check them is before the new roofing is put on.

Timber-framed roof

Bugle head Type 17 screws, framing anchors or straps as per AS 1684. (Nails on their own do not generally have enough capacity in wind region B.)

Steel-framed roof

Appropriate thickness battens and truss top chords fastened with the correct size Tek screws as per the NASH standard. (If the batten or truss wall thickness is too small, then the metal may tear around the connection even if the right screw has been used.)

Tie-down of the roof structure to walls

Many of the more conspicuous roof failures in Kalbarri were due to failures in the tie-down of roof structure to walls. In new construction, it is important that there are enough of these connections and that they are strong enough. In repairs, it may be necessary to fit more of them.

Tie-down of timber or light weight steel roof structure to brick walls

Sheet roofs are light and need to be effectively tied into the heavy brickwork to keep them in place under the high uplift loads from wind region B winds.

Usually this is achieved by steel straps embedded in the brickwork as per AS 4773.1 but for Wind Region B, in new brick walls, the straps should be embedded at the base of the wall to mobilise enough weight in the inner leaf of bricks. (See left diagram below.)

When repairing roofs where TC Seroja broke or damaged straps, extra straps may need to be installed into existing brick walls. This can be done by passing a steel rod through the brickwork across the cavity inside a looped strap as shown in the right diagram below.

In either case, the straps need to be pulled up tight and vertical when fixing to the roof structure to stop the roof from moving on top of the walls.

Embedded strap at base of wall
Looped strap around rod through cavity

Tie-down of timber roof structure to timber frame walls

A number of tie-down options are presented in AS 1684.

Tie-down to steel frame walls

As per the National Association of Steel frame Housing (NASH) Standards. Pay particular attention to the thickness requirements for steel members or details such as washer size and thickness.

Ridge and hip capping

Ridge and hip capping must be mechanically fastened with clips, nails or wire. Repoint capping as the wind loads may have dislodged or cracked the existing capping.

Fixing tiles to tile battens

AS per AS 2050:2018. Note that in more exposed locations, fixing of every tile may be required.

Fixing tile battens to rafters or trusses

AS 2050 gives the minimum size of nails required. If fixing to existing hardwood trusses, it is good to predrill the rafters to avoid splitting them.

Replacing roof cladding with a new material.

Several homeowners in the areas damaged by TC Seroja indicated that they planned to repair their house using a different roof cladding material. A building permit is required whenever the material used for roof cladding is changed. A redesign of the roof structure is needed to accommodate the new loads. Extra connections or some strengthening may be required.

  • Replacing a tile or AC sheet roof with a metal sheet roof: The new metal sheet roof will be lighter than the tile roof, so stronger tie-down connections are needed in the roof structure to carry the higher net uplift forces down to the ground. The building work should include strengthening the roof to wall connections and even connections between the top of the wall to the ground. Here are some educational videos on re-roofing.
  • Replacing a metal sheet roof with tiles: The much higher weight of the new tile roof means that the roof trusses will have to be strengthened to carry the weight forces onto the walls. The building work should include strengthening or replacing the roof trusses.

Pre-drilling hardwood timber for installing new screws

Hardwood timber in older homes will be well seasoned (dried) and may be prone to splitting when new screws are installed. When the house was first built, these hardwood members would have been ‘green’ or unseasoned and able to accommodate screws and nails without splitting, but now they will be very dry and newly driven nails or screws may split the timber, particularly where they are driven quite close to each other. Pre-drill holes for screws to avoid splitting the timber. The pre-drilled hole should be about 80% of the fastener diameter.

Demolition work

If your whole house will be removed, a demolition permit must be obtained. If only part of your house will be demolished as part of the reconstruction, the building permit for the new work should also include the partial demolition.

Heritage listed buildings

Work on heritage-listed buildings may require separate planning permission. If you think your building may be heritage-listed, then contact the Shire office for advice.

Split, decayed or insect-damaged timber

Replace any timber that is split, or damaged by rot or insects.

Damage to Roofs Caused by TC Seroja