On 22/01/13 22:35, Allan Duncan wrote:
On 22/01/13 15:10, Rohan McLeod wrote:
Assembled cognoscenti; it being the bush-fire season my thoughts turned to the practicability of protecting buildings , using external water-spray systems. [...]
The term "bushfire protection" needs to be better defined before you can move on to numbers on a page.
If there is heavy vegetation right up to the house, then radiant heat and flame impact is the operating mode, and the figures for burn-over protection of a fire truck applies (mentioned by another reply).
If you have done a proper preparation and the house is more than three times the tree height away from the bush, then the operating mode is ember attack. CSIRO recommended (years ago) that the house was painted white if timber, had all sub-floor areas fully boxed in, and eaves/roofing was gap filled. Even then there is a good chance of embers getting in, and this is where a water spray can help. Its function is to damp down all surfaces and cracks to extinguish embers that contact the structure, and it must do that in winds that easily exceed 50Kph. This means that any sprays must generate large droplet sizes, and direct the flow parallel and close to the surface to be protected. Sprinklers on the roof are largely a waste of water - if you must have them, then use a supply pipe along the ridge with the sprays under it so the spray pattern is close to the roofing. Roof spray water can be collected, filtered and recirculated to extend the running time IF you can design a system and filter that won't get clogged by all the flying debris. Its main use is filling the gutters to extinguish said flying debris since the gutters were spotless before the fire started, weren't they?
I forgot one use of roof sprays - tile roof. Most houses around here are steel roofed, so it slipped by me. Tiles without a fire blanket are a bad idea - on Ash Wednesday I saw a two story place with 30m of green grass and a 2m brick front fence between it and the fire front catch fire in the roof an hour after the front had passed either side. Just a small trickle of smoke from the end of the ridge, would have been an easy save except no ladder and there were other houses under threat from the flank of the fire. When we came back past there was just a heap of rubble.