Defensible space vital to fire safety |

Defensible space vital to fire safety

Mike Terwilliger, chief of the Truckee Fire District

Seems like Fire departments spend a bunch of time telling people to do things to make their lives safer. The messages being seasonal in nature, the big story is wild fire and we tell you to clear around your house so we can save it. We must sound like the teacher in the Peanuts cartoon – blah, blah, blah.

Must be, because I don’t see much happening on a grand scale to accept accountability for your desire to live in the woods. I got to thinking that maybe if you understood why fires burn, it might help. With that thought in mind I am going to tell you about wildfire from a different angle. Let me welcome you to the Fire Environment. Keep in mind this is a thumbnail sketch of a complex process, but I am going to isolate the key components for you. The Fire Environment is made up of three components: Fuels, weather and topography. Stay with me, it will make sense.


This is the stuff that burns in a fire, not the stuff you put in your car. It is the organic material like pine needles and brushes. Its ability to burn is based on many things but for this discussion, primarily water is the driving force being its availability to be consumed by fire. You see, water absorbs heat so the more water in fuel, the more heat it takes to drive it off before the fuel will burn.

Water shows up in fuel three ways: rain, humidity and the growing process. The water related keys to fuels burning are the time since the last rain, how dry the air is (humidity) and if the plant is growing. What does this mean to you? If it has not rained for 30 to 45 days, the forest might be available to burn. If the humidity is below 20 percent and it has not rained, the fuels might be available to burn. Simply, that is why fire in May and June here normally does not burn well, but fuels burn well in August and September, excluding drought situations like last spring. Because this is Fire Environment, I stated ‘might be’ three times as it takes more than fuels to get a fire going. Confused, well hold on.


Weather is a significant part of our lives here. Certain components make fire burn better. Wind is good for fire because it introduces oxygen, which is a key component in the chemical reaction. More importantly, wind bends the flames closer to the fuel, which fries out the unburned fuel faster by pre-heating – it then burns sooner. The wind effect on flames increases the driving off of the water the same as blowing on a smoldering campfire to get it going. Obviously weather is the process that places water into the fuel complex in the methods I listed above and dictates how well fuel will burn. The last part is temperature. It does two things. It preheats the fuel so it takes less heat to reach 430 degrees, which is where normal combustibles burn, and more importantly it has negative effects on firefighters, slowing their fire-fighting efforts. In fact it kills them, we call it heat stroke.


This is the lay of the land. The keys are aspect and slopes. If the hill faces west and south it gets more sun, drying the fuels out faster.

The same reason you buy a house with good sun exposure for snow removal also dries the fuels out faster as well. The steepness of the hill does two things. It acts just like wind in that it allows the flames to get closer to the fuel, dries it out faster and it burns quicker. Secondly, it makes it hard to put the fire out, so a fire gets bigger simply because we can’t build fire lines as fast up a hill.

There you have it, the fire environment. The danger is I have created many questions, one being “what is the point?” Well let’s take your newfound knowledge and start a fire on the hill below Glenshire behind Martis Lake, in theory. The date is August 10, it is 2 p.m. and it is 92 degrees. The humidity is 12 percent, and it has not rained since June 10.

What do we know? Well, it has not rained in more than 45 days, so the fuels are dry and the prevailing winds here are out of the southwest at 10 to 15 miles per hour. The weather is conducive for a fire, right? The fuels are very dry this time of year and the fuel type is conducive to carrying fire. What about “topography?” Well, it is a western facing slope and very steep. The fire environment is aligned and ready to burn. This potential fire is bore-sighted on Glenshire and it does not look good. Based on your knowledge now, this has the potential of being an extremely dangerous wild fire.

“But Mike, you have spent tons of money on all these new fire engines. Put it out!”

We would love to and 97 percent of the time we will. I am taking this story a full circle now back to defensible space. This fire may burn into Glenshire on a large front, maybe 1/4-mile wide. With mutual aid we have the ability to place about 25 fire engines in the commitment. Once an engine is assigned to a house and active fire passes, they will have to stay at the house at least 25 minutes and if the fuels are heavy, like timber, they will have to stay more than an hour.

Why? If they save the house and leave too soon, it will burn down. One in the hand is better than two in the bush. Math is not my strong point, but I know this fire will burn aggressively through the community and there are more than 35 homes in the way.

Let me tell you how homes catch fire in wild fires. Direct flame impingement, radiant heat, or airborne brands. Good defensible space work is intended to minimize the effect of the three methods of ignition. This allows the fire resources to become much more effective because they do not spend so much time protecting homes. It benefits you directly because your house is less vulnerable but more importantly if you have a vulnerable house by choice, you are hogging all the resources, or worse, you will be ignored. Keep in mind that the more fire-fighting resources parked at homes translates to fewer resources putting out the problem in the first place, the wild fire. There, you are now privy to the secrets of this business.

I have just touched the tip of the iceberg concerning fire environment. Let me finish with one more secret in this business, and I hope it drives home the point. When you hear in the news that fire fighters are determining which house to protect and which house to let burn during a major wildfire you probably think to yourself that must be a herd decision for them to make. During a major firefight in a community like I created for you in this article, the decision is not hard.

The reason is simple: You the homeowner made the decision for us long before the fire. We are just carrying out your wishes. If you want to know how to ensure your home can survive peacefully in the “fire environment” call us at 530-582-7580 or e-mail me at We love to hear from you.

Mike Terwilliger is Fire Chief of the Truckee Fire District.

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