Ember screens and BAL ratings for hinterland homes, explained

What a BAL rating is

Australian Standard AS 3959, Construction of buildings in bushfire-prone areas, sets out how homes in designated bushfire-prone areas should be built. A site's bushfire attack level, or BAL, is worked out from the type of vegetation nearby, how far away it is and the slope of the land. The higher the BAL, the more heat and ember attack the building is designed to withstand.

Bushfire-prone areas on the Gold Coast are mapped by the City of Gold Coast and the Queensland Government. On the northern side they take in parts of Upper Coomera, Maudsland and Wongawallan, and other pockets where homes back onto bushland. A BAL is normally set for new homes and extensions as part of the building approval, and your certifier or a bushfire assessment report will show it.

Why embers matter more than flames

Most homes lost in bushfires are set alight by embers rather than by the fire front itself. Embers can be carried a long way ahead of a fire, land in gutters, on decks and against windows, and get inside through gaps and open windows. A house can catch fire well after the front has passed.

That is why AS 3959 pays so much attention to small openings. Screens on openable windows and doors stop embers getting inside, and at the higher ratings screens and shutters also help shield glass from heat and flying debris.

What AS 3959 asks of screens

The exact requirements depend on the rating and on the window or door, and the standard is detailed, so treat this as a general guide rather than a specification.

BAL-12.5 and BAL-19

Openable windows and doors generally need screens made of metal mesh or perforated sheet with openings of no more than 2 mm, in corrosion-resistant steel, bronze or aluminium. The frame and fixings must hold the screen closely, so embers cannot get around the edges.

BAL-29

The same 2 mm limit applies, but aluminium is no longer acceptable for the mesh: it must be corrosion-resistant steel or bronze. Other parts of the window and door construction step up as well.

BAL-40 and BAL-FZ

At these levels, windows and doors generally need bushfire shutters, or a window or door system that has been tested to the relevant fire tests. A screen only counts if it is part of a system tested for that rating. This is specialist territory, and your certifier will want to see the test evidence.

Can one screen do both jobs?

Often, yes. Many woven stainless steel security meshes have openings well under 2 mm, and some security screen systems have been tested for use at ratings up to BAL-40. A screen like that keeps intruders and insects out every day and stops embers on the worst day of the year.

It has to be the right product, with the paperwork to match. Perforated aluminium and diamond grille are aluminium, so they are not an option at BAL-29 and above, and at the lower ratings their openings must still be no more than 2 mm. Because an ember screen must sit tight to its frame, the installation matters as much as the mesh.

Existing homes and upgrades

AS 3959 applies to new homes and new building work, such as extensions, in bushfire-prone areas, so an existing home does not have to be upgraded. Even so, ember screens on openable windows and doors are one of the most practical upgrades for a home on the bushland edge, alongside clearing gutters, sealing gaps and keeping the yard near the house clear of fuel.

Queensland's fire services and the City of Gold Coast both publish bushfire preparation advice, and your certifier can tell you whether new windows or doors in a renovation need to meet a BAL.

Cost and what to expect

As a starting point, our window security screens are typically $290 to $620 per window, and a hinged 316 stainless security door $790 to $1,250, supplied and installed. Expect a BAL-tested version of the same opening to cost more, because of the product testing and the closer-fitting frame. Your written quote shows the rating each product is tested to.

Questions