BAL Fire Risk Roof Ventilation

Roof Ventilation in high fire risk areas

Roof ventilation in high fire risk areas is always a problem not because of what is known, but more about what isn’t.

Anyone can induce fear of the unknown by making various claims to suit their agenda and most of them work on the ignorance of the mark.

The greatest cause of house fires in rural areas is the proximity of the fire to the house and airborne embers.

Embers do not enter against the flow of a roof ventilator but they can and do enter via poorly sealed flashings and plastic eaves vents that cannot resist the onslaught of heat and pressure.

Eaves vents used in high fire risk locations should be of perforated stainless steel material with nominal aperture size of 2mm and not woven wire which tends to clog up with dust.

At this point one prudently errs on the side of caution and as roof ventilation is essential in maintaining a pressure equilibrium between internal and ambient, a barrier is employed to offset any adverse phenomena by way of encapsulation and filtering any negative flow.

Before you go running off to stuff mesh into the throat of your roof ventilator ask yourself 3 questions…

1/ is this going to stop present performance?….
2/ by how much?….
3/ what are other consequences?

If the mesh, or screen, employed has a resistance value of 50% then you’ve just wiped out 50% of the ventilator performance.

The surface area of any screen device used has to be equal to the open area of the roof ventilator throat therefore the ember screen should look something like as below.

Nominal hole size should be no larger or smaller than 2mm with material being stainless steel preferably perforated and not woven wire
Do not use flywire or smaller apertures as these will clog up with dust and require continual maintenance

If the supplier cannot certify that the ‘free air flow” of the screen supplied is equal to the free air value of your ventilator throat do not install

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