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Factor 1: Forest Vegetation (Overstory)
Crown fire in the forest vegetation presents a significant hazard to adjacent buildings. Buildings may ignite by radiant heat transfer when the fire is burning all around it, or when firebrands land on the building before the wildfire arrives.
Crown fire is most likely to occur and spread rapidly in dense coniferous forest. Mixedwood forests are less likely to sustain crown fire, although firebrand transport from pockets of coniferous trees can threaten buildings. Deciduous forests are unlikely to sustain crown fire, especially after leaf flush.
The photos here illustrate the various types of forest overstory. Using them as a guide, please select that which is best representative of the forest overstory within 10m of your structure.
Deciduous forest >75% deciduous and <25% coniferous
Mixed wood forest 25-75% deciduous and 25-75% coniferous
Coniferous forest >75% coniferous and <25% deciduous
Separated coniferous forest
Continuous coniferous forest
No overstory
Deciduous
>90% deciduous
Mixed wood
>50% deciduous and
<50% coniferous
Coniferous
>50% coniferous and
<50% deciduous
Separated
Continuous
0
0
15
15
30
Factor 2: Surface Vegetation
Surface vegetation includes grasses, herbs, shrubs, dead and down debris (logs, branches, and twigs), and immature trees up to 2.5 metres in height. Concentrations of surface fuels will sustain high-intensity surface fires and can initiate crown fires.
A surface fire can ignite interface buildings by direct contact with the building exterior or nearby flammable materials. Untreated surface fuels can also support and spread small accidental ignitions from the site to the surrounding continuous forest.
Treatment of surface vegetation is most critical within 10 metres of buildings. Aggressive removal of all surface fuels and replacement with non-combustible or trimmed lawn is recommended.
The photos below illustrate various types of surface vegetation. Using these as a guide, please select that which best represents what lies within 10m of your structure.
Lawn or non-combustible material
Wild grass or shrubs
Scattered dead or down woody material
Abundant dead or down woody material
Lawn or non-combustible material
Wild grass or shrubs
Dead or down woody material
Scattered
Abundant
0
5
5
15
Factor 3: Ladder Fuels
Ladder fuels are shrubs, immature trees, and branches extending near the ground (e.g. within 2 metres) that give surface fires a pathway to the upper canopies of the trees. Trees with branches extending near the ground (within 2 metres) have ladder fuels. Removal of ladder fuels reduces the likelihood of crown fire development.
Absent: ladder fuels are considered absent if fewer than 25 percent of tress on site have ladder fuels closer than two metres to the ground.
Scattered: ladder fuels are scattered if 25-75 percent of trees on site have ladder fuels closer than 2 metres to the ground.
Abundant: ladder fuels are abundant if more than 75 percent of trees on site have ladder fuels closer than 2 metres to the ground.
Absent
Scattered
Abundant
Absent
Scattered
Abundant
0
5
10
Factor 4: Slope
Slope has a direct effect on fire's rate of spread: the steeper the slope, the faster the rate of spread. In other words, fire will burn more rapidly uphill than on a flat or level surface.
Slope is the angle of the ground measured from the horizontal. Percent slope is the change in elevation divided by the horizontal ground distance and multiplied by 100. A 50-percent slope means 50 metres of rise over 100 metres of horizontal distance. Make at least three measurements on the site and record the average value.
Even slopes have a smooth or rolling texture. No abrupt variations are noted.
Gullied slopes have cuts running up the slope, usually from water erosion, that provide funnels for upslope wind-driven fire spread.
0-10%
10-25%
>25%
Even
Gullied
Even
Gullied
0
4
5
8
10
Factor 5: Position on Slope
The location of the zone or site on the slope will affect the fire hazard levels. In general, locations higher up on slopes with fuels below face a significantly higher fire hazard from rapid, high-intensity fire spread up the slope.
Valley bottom or lower slope Areas of development on flat ground or valley bottoms, extending as high as one-third of the way of the slope.
Mid slope Areas of development on slopes with forested areas or grasslands below, extending as high as mid-way up the slope.
Upper slope Areas of development on the top half or crest of slopes with forested areas or grasslands below them.
