Wildfire Preparedness: How to Create Defensible Space and Know When to Go

Wildfire preparedness comes down to two separate jobs, and most people only do the first one halfway. The first job is the house: what you clear, what you move, and what you screen off, measured in feet from your siding. The second job is the decision: how you know when to leave, and how you avoid being the family still loading the car when the road closes. September is peak season across most of the West, and it is also National Preparedness Month, so this is the right week to walk your own property with a tape measure.

I am going to give you the actual distances, the actual mesh size, and the actual numbers that fire researchers use. Then I am going to walk through the go decision, because that is the part where all the yard work either pays off or does not matter.

What is defensible space, and how far does it extend?

Defensible space is the managed area around your home, and it extends 100 feet from your walls, split into three zones. CAL FIRE defines them this way:

• Zone 0, the ember resistant zone: 0 to 5 feet from buildings, structures and decks.

• Zone 1, the lean, clean and green zone: out to 30 feet, or to your property line, whichever is closer.

• Zone 2, the reduce fuel zone: from 30 feet out to 100 feet, or to your property line.

The word "defensible" is doing something specific there. It does not mean fireproof. It means a firefighter can stand near your house and work without being cooked, and it means the fire arriving at your property has nothing easy to climb. Colorado State University Extension is blunt about the limit here: there are no truly fireproof plant species. Conifers like juniper, pine, fir and spruce carry oils and pitch that make them burn hard no matter how well you water them.

Why does the first 5 feet around your house matter more than anything else?

Because that strip decides whether an ember that lands near your house turns into a fire against your house. The Insurance Institute for Business and Home Safety (IBHS) surveyed more than 250 properties after the January 2025 Palisades and Eaton fires in Los Angeles County, backed by CAL FIRE damage data covering more than 30,000 structures. Their finding is the single most useful number in this whole article: when more than 25 percent of Zone 0 was covered in fuel, the risk of damage or destruction rose to nearly 90 percent.

They also found that stacking mitigations beats doing one thing well. Homes with four hardening features (a Class A roof, noncombustible siding, double pane windows and enclosed eaves) had a 54 percent chance of avoiding damage. Homes with only one of those had a 36 percent chance.

So here is the Zone 0 rule, and it is stricter than most people expect. Nothing in the first 5 feet should be able to burn. Not "fire resistant." Not burnable. IBHS puts it plainly in their Wildfire Prepared Home research: embers exploit the weakest link, so all combustible items have to go, even the ones labeled fire resistant. Their lab work showed that well watered plants can dry out in minutes during a real conflagration.

What to pull out of that 5 foot strip:

• Bark and wood mulch. IBHS calls it highly combustible and says it can carry fire straight to the structure. Replace it with gravel, river rock, pavers or bare soil.

• Plants. All of them, including the ones sold as fire resistant. Shrubs against the foundation are the classic wick.

• Trash and recycling bins. Plastic bins full of paper are a fuel pile sitting against your siding.

• Doormats, patio furniture cushions, stacked cardboard, brooms.

• Firewood. CAL FIRE says move it all the way out to Zone 2.

What can stay: concrete, stone, metal furniture, a masonry bird bath. Solid, heavy, and not going to light.

What goes in Zone 1, from 5 to 30 feet?

Zone 1 is about breaking the connection between fuels so fire cannot walk toward the house. CAL FIRE's rules for this band:

• Remove all dead plants, dry grass and weeds.

• Clear dead leaves and pine needles out of the yard, off the roof, and out of the gutters.

• Trim trees so branches keep a 10 foot gap from other trees.

• Keep tree branches 10 feet away from your chimney or stovepipe.

• Keep 10 feet of clear space, down to bare soil, around propane tanks and outbuildings.

• Move woodpiles out to Zone 2.

The part people skip is vertical spacing. A low shrub under a tree is a ladder. Fire climbs from the grass to the shrub to the low branches to the crown, and once it is in the crown it throws embers for a mile. Cutting that ladder means either removing the shrub or pruning the low branches off the tree, and it is more useful than any single thing you can buy.

What do you actually do in Zone 2, from 30 to 100 feet?

Zone 2 is fuel reduction, not removal. You are trying to slow the fire down and drop its flame length before it reaches Zone 1. CAL FIRE's specifics here are refreshingly concrete: cut annual grass to a maximum height of 4 inches. Space vegetation horizontally and vertically. You can leave up to 3 inches of fallen needles and small branches on the ground, which matters for erosion control on a slope. Keep exposed woodpiles clear 10 feet down to the soil in every direction.

If your ground slopes, the numbers get less forgiving. Fire moves uphill faster because the flames preheat the fuel above them, so the uphill side of a house on a slope needs more attention than the flat side. Many county fire codes stretch the requirement past 100 feet on steep lots. Check yours.

How do embers actually get into a house?

Through openings, and the openings are smaller than you think. An ember does not need a door. It needs a gap.

Here are the fixes, with the numbers CAL FIRE publishes in its home hardening guidance:

• Vents. Cover attic, crawlspace, soffit and gable vents with noncombustible, corrosion resistant metal mesh between 1/16 inch and 1/8 inch. Plastic or fiberglass screen melts. Standard window screen is too coarse. This is a cheap Saturday job and it closes the most common ember path there is.

• Chimney. Fit a spark arrester with metal screen openings no smaller than 3/8 inch and no larger than 1/2 inch. That is for embers going out, and it also blocks embers coming in.

• Roof. Composite, asphalt, metal, clay or tile. Wood shake is the worst possible roof in fire country. If yours is wood, that is the biggest ticket item on your list.

• Roof valleys and gutters. Where two roof planes meet, needles pile up. That pile is an ember catcher sitting on your roof deck. Clean it, and clean the gutters, every fall and again before fire season.

• Windows. Dual pane with at least one tempered layer. Single pane glass breaks from radiant heat before the fire ever touches the house, and once the glass is gone the fire is inside.

• Fences. A wood fence attached to the house is a fuse with a handle. The last 5 feet before the wall should be metal or masonry, or there should be a gap.

• Driveways. Keep 10 feet of vegetation clearance on both sides of driveways and access roads. That is not for your house. That is so an engine can get in and you can get out.

What do wildfire evacuation levels actually mean?

Most of the West uses a three level system, and the words are chosen so a stranger can understand them in one second. Oregon's statewide wildfire program defines them like this:

• Level 1, BE READY. There is danger in the area. Stay informed, pack, plan your route, sign up for county alerts. If anyone in the household moves slowly, leave now.

• Level 2, BE SET. Significant danger. Be ready to leave at a moment's notice. Do not spend time packing. If you cannot move quickly, go.

• Level 3, GO NOW. Extreme danger. Leave immediately. Do not gather belongings. Do not try to protect the house. If you stay, do not expect anyone to come get you.

California and much of the Southwest uses Ready, Set, Go with the same three tiers. Some counties use colors or the words "evacuation warning" and "evacuation order" instead. Learn the words your county uses this week, not on the day. And if anyone in the house moves slowly, evacuating with limited mobility needs its own plan, built well before a Level 1.

Why does "wait and see" fail so often?

Because the window between "it looks distant" and "the road is gone" can be under an hour, and it closes without warning you can act on. Oregon State University Extension puts the honest version in writing: sometimes wildfires move so quickly that officials cannot issue evacuation orders early enough, so if you feel unsafe you should consider leaving before you get a notice.

Read that again. The people whose job is to warn you are telling you not to wait for the warning.

Three things make "wait and see" worse than it sounds. Traffic, because everyone waits and then everyone leaves at once on the same two lane road. Visibility, because heavy smoke can drop you to near zero at midday. And time cost, because loading a car takes 40 minutes if you are calm and it takes 40 minutes if you are rushed, but you only have those 40 minutes in one of those cases.

The fix is a trigger you set in advance, in plain words, and share with everyone in the house. Something like: "If we hit Level 2, we go, whether or not I think it looks bad." Write it down, along with the routes and meeting points in your family evacuation plan. A decision made on a calm Tuesday is a much better decision than one made at 2 a.m. with an orange sky.

What belongs in a wildfire go-bag that is not in a normal one?

Your standard 72 hour bug-out bag covers most of it. Fire adds a short list of specific items, and they matter because the hazard is heat, particles and broken glass rather than wind or water.

• N95 or P100 respirators, not cloth masks. The CDC recommends NIOSH Approved respirators if you must be outside in smoke, and names N95 and P100 as the most protective. NIOSH notes that cloth and disposable masks are not made to any standard and provide unknown protection, while an N95 filters at least 95 percent of particles. It only works with a seal, and facial hair is a common cause of gaps.

• Closed leather or sturdy work shoes. Not sandals, not canvas slip-ons. You may be walking on hot debris and broken glass.

• Natural fiber clothing that covers skin. CAL FIRE recommends long pants and long sleeves, and says 100 percent cotton is best. Synthetics like polyester and fleece melt onto skin under radiant heat. Cotton and wool char instead.

• Goggles. Smoke and grit in your eyes is a driving hazard, and driving is the whole plan.

• A paper map of your county. Cell service and navigation apps fail in fire areas, and the route you know may be the route that is closed.

• Pet carriers staged by the door. Chasing a cat under a bed is how people lose their last 20 minutes.

Keep it in the car during fire season, not in a closet. A go-bag in the house is a go-bag you might not get back to.

What should you do to the house on your way out?

Five minutes of the right actions makes your house easier to defend and easier to find. CAL FIRE's Go Evacuation Guide lists these, and a few of them surprise people:

Inside:

• Close all windows and doors, but leave them unlocked so firefighters can get in.

• Take down flammable window coverings like shades and curtains.

• Turn off the gas at the meter and the pilot lights.

• Leave your lights on. In heavy smoke, a lit house is a house a crew can find.

• Switch off the air conditioning so it stops pulling smoke inside.

Outside:

• Bring in patio furniture, toys and doormats, or move them well away from the walls.

• Shut off propane tanks and move portable grills away from the house.

• Leave exterior lights on for the same visibility reason.

• Park in the driveway facing out, loaded and ready.

• Leave a ladder out where a crew can see it, for roof access.

• Do not leave sprinklers or hoses running. This one runs against instinct. CAL FIRE says running water lowers the water pressure in the system, which is the pressure firefighters need.

And leave the gate open. A locked gate at the end of a driveway costs a crew minutes they do not have.

How bad does the smoke have to be before you change your day?

Use the Air Quality Index, and know the two numbers that matter for you. The AirNow guide for particle pollution sets the categories:

• 0 to 50, Good. Go outside.

• 51 to 100, Moderate. People with asthma, heart or lung disease should ease off hard outdoor effort and watch for symptoms.

• 101 to 150, Unhealthy for Sensitive Groups. If you have asthma, heart disease, are pregnant, are over 65, or have kids, shorten outdoor activity and take breaks.

• 151 to 200, Unhealthy. Everyone reduces strenuous outdoor activity. Sensitive groups move it indoors.

• 201 to 300, Very Unhealthy. Sensitive groups avoid all outdoor physical activity. Everyone else moves indoors.

• 301 to 500, Hazardous. Everyone avoids all outdoor physical activity. High risk people stay in and keep activity minimal.

The practical rule most families end up with: 100 is the number where you check on the vulnerable people in the household, and 150 is the number where outdoor plans get canceled for everyone.

How do you make one room in the house breathable?

Pick one room, seal it, and filter it. The EPA calls this a clean room, and the steps are specific:

1. Choose a room big enough for everyone, ideally a bedroom with its own bathroom so you can stay put.

2. Close the windows and doors, and if your heating and cooling system has a fresh air option, turn it off, close the intake, or set the system to recirculate.

3. On central HVAC, install a filter rated MERV 13 or higher and set the fan to On instead of Auto, so it keeps filtering instead of only running during a heating or cooling cycle.

4. On a window air conditioner, close the fresh air setting. On an evaporative cooler, EPA says to cover the outside air intakes completely with 4 inch thick MERV 13 furnace filters.

5. Run a portable air cleaner sized for the room continuously, on the highest setting you can stand, and pick one that does not produce ozone.

And stop adding particles indoors. EPA's list of things not to do during a smoke event: no smoking or vaping, no gas, propane or wood burning appliances, no aerosol sprays, no frying or broiling, no candles or incense, and no vacuuming unless the vacuum has a HEPA filter.

Does a box fan and a furnace filter actually work?

Yes, and EPA has measured it. Their research on DIY air cleaners tested several builds and reported Clean Air Delivery Rate (CADR) for each. Higher is better:

• One 1 inch filter taped to a box fan: CADR of 111.

• One 1 inch filter plus a cardboard shroud: 156.

• One 4 inch filter with a shroud: 248.

• Two 1 inch filters with a shroud: 263.

• The four filter Corsi-Rosenthal cube: 401.

Three things decide whether yours works. Use a MERV 13 filter, because EPA found the MERV rating strongly drives CADR. Use a thicker filter if you can, since the 4 inch beat the 1 inch by 123 percent. And change the filter often, because EPA found DIY air cleaners were almost completely ineffective with dirty filters. That last one is the failure mode nobody warns you about.

On safety: EPA says use only box fans made in 2012 or later, which have fused plugs and thermal cutoffs, and check for a UL or ETL mark. Older fans have a known fire risk, which is a bad irony to run into during a wildfire. Keep a rated fire extinguisher in the room where you run one.

When should you get medical help?

Call 911 for trouble breathing that does not ease when you rest, chest pain or pressure, confusion, fainting, or any burn that is larger than the palm of a hand, on the face or hands, or blistering badly. Call 911 immediately if anyone was in a structure fire or has soot around the nose or mouth with a hoarse voice, because airway swelling from smoke can worsen fast.

Short of an emergency, the CDC's guidance for people with asthma, COPD, heart disease or diabetes is simple: get medical help for any new or worsening symptoms. If you have an asthma action plan, this is what it is for. Refill rescue inhalers before fire season, not during it, because pharmacies close when neighborhoods evacuate.

Frequently asked questions about wildfire preparedness

How far should defensible space extend from my house?

100 feet, or to your property line, whichever comes first. CAL FIRE splits it into Zone 0 from 0 to 5 feet, Zone 1 out to 30 feet, and Zone 2 from 30 to 100 feet. Some counties require more on steep slopes, because fire climbs faster than it spreads on flat ground, so check your local fire code.

What size mesh should I use on vents to stop embers?

Noncombustible, corrosion resistant metal mesh between 1/16 inch and 1/8 inch, per CAL FIRE. Standard window screening is too coarse to stop embers, and plastic or fiberglass screening melts. Attic, crawlspace, soffit and gable vents are the openings that matter most.

Is bark mulch really that dangerous near a house?

Yes. IBHS classifies wood mulch as highly combustible and found it can carry fire quickly to a structure. In the first 5 feet, replace it with gravel, river rock, pavers, or leave bare soil. Mulch farther out in Zone 1 and Zone 2 is a much smaller concern.

Should I leave sprinklers running when I evacuate?

No. CAL FIRE specifically says not to leave sprinklers or water running, because it lowers the water pressure that firefighters need in your neighborhood. Wet the roof and landscaping before you go if you have time, then shut the water off.

Will an N95 mask protect me from wildfire smoke?

A properly fitted N95 or P100 respirator filters at least 95 percent of the fine particles in smoke, and the CDC recommends NIOSH Approved respirators when you have to be outside. It has to seal against your face, so facial hair breaks it. No respirator filters out gases or vapors, so it reduces your exposure rather than eliminating it. Staying indoors with filtered air is still better.

What is the difference between an evacuation warning and an evacuation order?

A warning means significant danger and you should be ready to leave immediately, which matches Level 2 Be Set. An order means extreme danger and you leave now, which matches Level 3 Go Now. If anyone in your household needs extra time, treat the warning as the order.

You never know, but you can always be ready.

Sources: CAL FIRE defensible space, CAL FIRE home hardening, CAL FIRE Go Evacuation Guide, IBHS Palisades and Eaton fire findings, IBHS Wildfire Prepared Home, Oregon Wildfire Response evacuation levels, Oregon State University Extension, Colorado State University Extension, AirNow air quality guide for particle pollution, EPA clean room guidance, EPA DIY air cleaner research, CDC wildfire safety, NIOSH respirators and masks.