Defensible Space Zones and Where You Set Equipment

Why this matters

In a wildfire zone, the strip of ground right against the house is where an ember shower turns into a house fire. Trades set things in exactly that strip: condensers, standby generators, pool equipment, propane cylinders, line sets, pads, and the gravel or bark around them. A placement that is perfect for service access and code clearance can still put a fuel bed, or a fuel source, against the wall. Knowing the zones lets a tech place equipment so it does not become the first thing that burns, and lets the office answer the customer whose insurer just sent a defensible-space notice that names the equipment the shop installed.

What defensible space is

Defensible space is the managed area around a structure where vegetation and other fuels are reduced so that a fire approaching or an ember landing has less to burn, and so firefighters have room to work. It is organized in rings measured outward from the walls, with the strictest management closest in.

Three terms used here:

• Fuel is anything that burns: grass, shrubs, mulch, firewood, lumber, plastic, stored items, and the house.
• Ladder fuel carries fire from the ground up into shrubs and trees.
• Ember-resistant zone is the innermost ring, usually the first 5 feet, where the goal is close to nothing that an ember can ignite.

The zones, with California as the worked layer

California has the most developed statewide rule, so it is the example here. Other states range from guidance only to county or municipal ordinances, and the same rings show up in most guidance with different legal force.

State law, California. Public Resources Code Section 4291 requires owners of buildings in and adjoining mountainous, forest, brush and grass-covered lands in state responsibility areas to maintain defensible space of 100 feet from each side of the structure, or to the property line if closer. Many local fire codes impose the same or stricter rings within cities. CAL FIRE's guidance divides the 100 feet into:

• Zone 0, 0 to 5 feet (ember-resistant). Created in law by a 2020 statute that directed the state Board of Forestry to write the specific regulations. Those regulations were still being developed and phased in at the time of writing, so check the current text before quoting a Zone 0 requirement to a customer as law. CAL FIRE's guidance for this zone already calls for hard surfaces or bare mineral soil, no bark mulch, no combustible fencing attached to the house, and nothing stored against the walls.
• Zone 1, 5 to 30 feet ("lean, clean and green"). Dead vegetation removed, plants spaced and irrigated, tree limbs kept clear of the roof and chimney, woodpiles moved out of this zone.
• Zone 2, 30 to 100 feet ("reduce fuel"). Grass cut low, horizontal and vertical spacing between shrubs and trees, ladder fuels removed.

Other states. Several western states publish the same ring structure as guidance, and some enforce it only through county or municipal ordinance or through the wildland-urban interface code where adopted. Oregon's statewide program has changed more than once since 2021. Treat any state rule you did not just read as unverified and send the customer to their state forestry agency or local fire marshal.

PLAN VIEW, one side of a house (not to scale)

 property line ...............................
 |   ZONE 2: 30 to 100 ft, reduce fuel       |
 |   (woodpile goes here, not closer)        |
 |   - - - - - - - - - - - - - - - - - - -   |
 |   ZONE 1: 5 to 30 ft, lean clean green    |
 |      [propane tank]  10 ft cleared ring   |
 |   - - - - - - - - - - - - - - - - - - -   |
 |   ZONE 0: 0 to 5 ft, gravel or concrete   |
 |   [condenser on concrete]  [generator]    |
 +=================WALL======================+
 |   window   meter   hose bib   dryer vent  |

The diagram shows the conflict this card is about: the equipment that has to be near the house (the condenser, the generator, the meter) sits in the zone that is supposed to hold nothing combustible, so everything around and under it has to be chosen for that zone.

Condensers and heat pumps

A condenser is mostly steel and aluminum, so it is not usually the ignition point. The ignition point is what surrounds it: bark mulch around the pad, a plastic or composite pad, weeds grown up through the coil, leaves packed against the base, foam line set insulation running along the wall, and a wooden lattice screen built around the unit for looks.

Set it on concrete or another noncombustible pad where the zone calls for it, and surround the pad with gravel or pavers rather than mulch. A composite pad that meets the manufacturer's requirements for support may still be a fuel; confirm what it is made of before setting it in Zone 0.

Keep the manufacturer's airflow clearances anyway. Defensible space does not relax the clearances the installation instructions require around the coil and above the fan, which are a condition of the unit's listing. Where a homeowner's screen or fence violates those clearances, it is both a performance problem and a fuel problem.

Protect the line set. Exterior line set insulation is a combustible foam. Covering it with a metal or noncombustible line set cover along the wall protects it from embers and from ultraviolet breakdown, and it is cheap compared to the consequence.

Standby generators

Generator placement is governed first by the unit's listing and installation instructions and the adopted codes, and those set distances from windows, doors, vents and combustible walls for carbon monoxide and fire reasons. NFPA 37, the consensus standard for stationary combustion engines, sets a default of 5 feet from openings in walls and from combustible walls for engines installed outdoors, and permits closer placement where the enclosure has been tested and listed for it; it binds where the fire code adopts it, and the manufacturer's instructions state the distances that unit is listed for.

In a fire zone, add two considerations the listing does not make for you:

• What is under and around the pad. A generator in Zone 0 needs the same noncombustible surround as a condenser, and its exhaust side should not face shrubs, a fence or the woodpile.
• The fuel line. A natural gas or propane supply run along the wall is a fuel source next to the house. Keep it on the noncombustible surround, sleeved where it crosses vegetation, and accessible to a shutoff that a firefighter or the homeowner can reach.

Propane tanks and cylinders

NFPA 58, the liquefied petroleum gas code, as adopted by your fire code, requires readily ignitable material such as weeds and long dry grass to be removed within 10 feet of a container. That ring overlaps the defensible space zones rather than replacing them, so a tank in Zone 1 sits inside its own 10 foot cleared circle. Separation of the tank from the building and from openings is set by the same code by container size; those distances belong to the propane supplier who sets the tank, not to a tech moving it.

Never set a portable cylinder (a grill or heater cylinder) against the house, under a deck or beside a wood fence. In an ember storm, a cylinder in a burning fuel bed vents through its relief valve, and that release feeds the fire.

The meter wall

Electric and gas meters are on the wall by necessity and the trades do not move them. What the trades can do is notice the fuel next to them: a woodpile stacked under the electric meter, a wooden trellis over the gas meter, a plastic storage bin against the regulator. CAL FIRE's guidance moves woodpiles out of the 30 foot zone entirely. Point it out and photograph it; you are not the defensible-space inspector, but you are often the only person who looks at that wall.

Worked example: a heat pump swap plus a new standby generator

A single-story house in a California state responsibility area, 100 feet of defensible space required, property line 70 feet from the back wall. The shop is replacing a condenser with a heat pump and adding a standby generator with a propane tank. The homeowner wants the generator under the primary bedroom window "where it's out of sight," the heat pump on its existing composite pad in a bark-mulched bed, and the tank beside the back fence, 40 feet out.

Defensible space extent. 100 feet is required but the property line is at 70, so on this side the owner's obligation ends at 70 feet. The tank at 40 feet is in Zone 2.

Generator. The manufacturer's instructions for the selected unit (illustrative figures here; the real ones come from that model's manual) list it for 18 inches from a noncombustible or combustible wall but 5 feet from any window, door or vent opening. The bedroom window is directly above the proposed spot, so the location fails at 0 feet from an opening, regardless of fire zone. The tech moves it along the wall to a spot 7 feet from the window edge and 6 feet from the dryer vent, which clears both 5 foot requirements, on a noncombustible pad with gravel surround. It is still in Zone 0, so the shrub that would sit 2 feet from the exhaust side comes out.

Heat pump. The pad is composite and the bed is bark. In Zone 0 that is a fuel bed under the unit. The shop sets the new unit on a concrete pad and replaces the bark with gravel for the full 5 feet in front of the wall section, which also keeps the coil clear of debris. Line set goes in a metal cover up the wall.

Propane tank. At 40 feet in Zone 2 beside a wood fence, the tank's own 10 foot ring includes the fence line, which has dry grass along it. The 10 foot clearance of readily ignitable material applies regardless of zone, so the grass is cut back in a 10 foot radius. The wood fence itself is not "readily ignitable material" in the NFPA 58 sense the way dry grass is, but it is a fuel the tank is now beside; the supplier, who sets the tank and is responsible for its separation distances, is told about the fence before the set.

What changes the answer. If the house were in a city with a local fire code stricter than the state minimum, the local rule governs and the rings may be larger or the Zone 0 rule already enforceable. If the house is outside any mapped zone, every placement above is still sound advice, but the customer's obligation is voluntary and the office should say so plainly rather than implying a legal requirement.

What to record

A placement photo of each piece of equipment showing the ground around it, a note of the distances to openings you measured (the generator's 7 feet and 6 feet above), what surround you installed, and any fuel you pointed out but did not remove. If the customer declines a recommendation (keeps the bark, keeps the woodpile under the meter), note it. When an insurer or a defensible-space inspector later asks what was installed and how, that note is the shop's answer.

References

• California Public Resources Code Section 4291, and CAL FIRE defensible space guidance
• NFPA 37, Standard for the Installation and Use of Stationary Combustion Engines and Gas Turbines, as adopted by your fire code
• NFPA 58, Liquefied Petroleum Gas Code, as adopted by your fire code
• See related: Ember-Resistant Vents and Why Embers Burn Houses
• See related: The Wildland-Urban Interface Code in Plain Terms