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Effective slope (AS 3959 BAL input)

Effective slope is the slope under the classified vegetation, one of the three AS 3959 BAL inputs. Downslope raises the BAL; upslope and flat are treated the same.

By Chalkline. Last updated 26 May 2026. Verified 26 May 2026.

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Effective slope is the slope of the land under the classified vegetation, and it is one of the three inputs that set a site’s Bushfire Attack Level (BAL) under AS 3959, alongside the vegetation group and the separation distance. It is measured from the edge of the classified vegetation nearest the building, not across the whole site.

Why it matters

Slope drives how fast a fire spreads and how much radiant heat reaches the building. Fire accelerates uphill, so the slope under the vegetation changes the heat load at the site:

  • Downslope (the vegetation is on ground that falls away below the site): the fire runs uphill toward the building, increasing the rate of spread and the radiant heat. Downslope raises the BAL, and the standard bands the downslope into steps, each steeper step pushing the BAL higher.
  • Upslope and flat (the vegetation is on level or rising ground): treated as a single, lower-risk category. There is no slope penalty; you do not get a worse rating for upslope, and you do not get a discount for it either.

So the same vegetation at the same distance produces a higher BAL on a downslope than on flat or upslope ground.

The method limits

Effective slope also decides which assessment method applies:

  • Method 1 (the simplified procedure) is not valid beyond 20 degrees of downslope. Steeper downslopes fall outside the simple tables.
  • Method 2 (the detailed procedure) handles steeper situations (broadly up to 30 degrees downslope), and beyond that the site needs specialist engineering input.

A steep downhill block below bushland is exactly where the BAL (and the build cost) climbs.

For a builder

  • It is the assessor’s measurement, not an eyeball. A BPAD assessor measures the effective slope under the vegetation as part of the BAL assessment; do not estimate it off the site fall.
  • Downhill-of-bushland blocks cost more. If the classified vegetation sits below the building on falling ground, expect a higher BAL than a flat site at the same distance.
  • Get the BAL early. The slope is baked into the BAL, which drives the construction requirements; have it determined before design and contract.

Also known as: effective slope bushfire, slope under classified vegetation.

See also

Last updated: 2026-05-26. Verified: 2026-05-26. Quarterly review for currency.

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