Bushfires and high winds go together. How do metal and tiled roofs react?
Roofs, and the weaknesses around roofs and roof access, is probably really the one to emphasize in that there is quite a high degree of loss implication around those roofs, and as a priority getting your roof sorted out is a really good approach. Roof spaces typically contain combustible material such as framing, insulation or stored materials. Roof are typically built with many gaps that can allow embers and debris to reach the roof space. Typical sarking used in these roof spaces is not a match for the burning debris that builds up against the sarking in these roof spaces.
Fires happen to occur on very windy days. That's just one of the important aspects of a severe fire weather day.
Wind gusts that exceed 75km/hr (like on Black Saturday) are enough to potentially lift tin, or dislodge roof tiles.
That degree of weakening obviously then allows the other bushfire actions to play on the house and cause issues.
Think very carefully about how adequately your roofing is fixed. An extra pack of screws on every other ridge on a roof is an excellent consideration.
Every roof sheet has an end, either at the ridge line of a house or at the gutter line of the edge of the roof, and that's where you have significant potential ember entry.
Fortunately for a steel roof, those areas can be readily addressed by putting ember screening systems either along the ridge capping and along the gutter line.
A terracotta and cement roof tiles presents a particular problem in that they just inherently aren't ember tight. Tiles don't seal against each other, and the entire roof itself has almost a universal ember access problem.
The ridgelines certainly are, if they're well pointed and covered, but the tiles simply don't sit tightly enough over each other to prevent long term leaf, debris and build up under the tiles, and also allow embers through those same gaps.
A tiled roof needs quite a dramatic intervention to resolve the problem.
How do embers get inside the roof space and how can you stop them?
Protecting under a roof ridgeline for ember attack, both fibreglass type batt and a rockwool batt will perform adequately because there simply isn't the heat locally to actually melt out the glass batt.
If you were trying to prepare a roof for flame attack and direct like flame zone impingement, then the rockwool option would be a preferred upgrade.
Dektites aren't created equal, and you'll find that there's two types when you go into a hardware store. One is a high temperature one, which is actually designed to go around things like flues for wood heaters and gas heaters that perforate up through the roof, and they're typically a red silicon material. They're quite high temperature performing, and can resist quite a bit of debris build up against them, and the burning out of that debris.
You can clean your gutters out, but during the fire event itself the debris can build up again, to some extent in those gutters and apply localized flame attack to the roof elements adjacent.
Gutterguard essentials: Galvanized or stainless, aperture size 2mm or smaller, and fixing both to the roof and to the gutter line must be of some type of non-combustible material and flashing.
How risky are polycarbonate and fibreglass roof sheeting, skylights and evaporative coolers?
All polycarbonate materials, where they're used externally as a pergola, shade or an extension off the side of a house unenclosed, when there's a local flame source acting on it, it can melt and drip away like honey from that flame source without burning itself.
If polycarbonate is used as a skylight in the house itself, or as a window glazing element, it's going to potentially melt or fall away from a relatively small heat source and therefore open up the house and not act as a barrier for embers to get into things.
The only times we've really seen it burn and act as a flame source is when it's formed quite a significant wet molten puddle on the ground and then continued to be heated by an adjacent source until you finally get that puddle to burn.
In contrast Fiberglass (that typically used a polyester resign) skylights and translucent panels are extremely ignitable and will, in themselves, carry a fire from a gutter line all the way to a house.
If these fibreglass elements are present, or used in your house, they should be removed and replaced by glass or polycarbonate alternative.
Skylights come in every shape and size but, unfortunately, they create an inconsistency in the roof profile...a place for debris to build up on and prop against...another potential place where debris can accumulate and localized flames can start to play on the adjacent sky lighting elements. So we really have to put that on the table as a sensitive or vulnerable element.
Evaporative coolers are quite problematic in fires. They burn prolifically unless specifically designed with fine meshes and non-combustible filters, and the whole box is essentially non-combustible.