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Ladder fuels
A shrub under a tree is a vertical path. CAL FIRE wants at least three times the shrub height between the shrub and the lowest branches, and also names limbing branches at least six feet off the ground (dspace). IBHS Base, in the 5–30 foot band, does not place shrubs under larger trees. Oregon R327 does not print this picture.
You are looking up through a juniper into the lowest pine limbs. Fire can climb that.
FA Exhibit ZON-012 is a pair. Left, touching fuels. Right, a 3× vertical break. Guidance language, not a permit. Illustrative, not a construction document. CC BY 4.0.
Why it matters
Surface fire becomes crown fire when the rungs exist. Horizontal spacing does not fix a shrub that is already under the canopy. See crown and shrub spacing. Zone 0 still wants no plants in the first five feet.
How the rules split
| Document | Vertical rule |
|---|---|
| CAL FIRE | Limb ≥ 6 feet. Also ≥ 3× shrub height to the lowest branches. Example: 5-foot shrub → 15 feet |
| IBHS Base 5–30 ft | Do not place shrubs under larger trees |
| Oregon R327 | Not a ladder-fuel section |
The exhibit’s drawn case is a 2-foot shrub and a 6-foot break (3×). CAL FIRE’s published example is a 5-foot shrub and 15 feet. Use the document you are scoring.
The paths
1. Remove the lower rung. Take the shrub out from under the canopy, or limb to the 3× number, or both.
2. IBHS. Do not put the shrub there. Horizontal 2× spacing is a different sentence.
3. Zone 0. Nothing in the five feet is a shrub-under-tree problem. It is a noncombustible band. See Zone 0.
What people mix up
- Limbing to 6 feet and leaving a 6-foot shrub.
- Treating R327 as this picture.
- Calling a hedge a spaced shrub. IBHS does not permit privacy hedges in that 5–30 foot rule.
- Planting under a tree because this site rates the species fire-resistant.
How to check
- Stand at the trunk. Is there a shrub or a pile under the drip line.
- Measure shrub height. Multiply by three if you are on the CAL FIRE vertical rule.
- Name IBHS vs CAL FIRE vs a local inspection.
- Walk Zone 0 separately.
Limits
ZON-012 is illustrative. This page does not spec a species. FireApproved is not a forestry inspector. Do not scrape a private planting plan.
Related records
Questions
What is a ladder fuel?
Fuels that can carry a fire vertically, typically a shrub or low branch under a tree canopy. CAL FIRE’s example is a 5-foot shrub needing 15 feet of clearance to the lowest branch. IBHS Base simply does not put shrubs under larger trees in the 5–30 foot band.
Does limbing to 6 feet finish it?
CAL FIRE also names removing branches at least six feet from the ground, and extra vertical space of three times shrub height to the canopy. Six feet is not a substitute for the 3× rule when a tall shrub is present.
Is this Oregon R327?
No. R327 is construction. This is defensible-space guidance or a designation standard.