Quick Facts...
- Wildfire will find the weakest links in the defense measures you have
taken on your property.
- The primary determinants of a home’s ability to survive wildfire
are its roofing material and the quality of the “defensible space”
surrounding it.
- Even small steps to protect your home and property will make them
more able to withstand fire.
- Consider these measures for all areas of your property, not just the
immediate vicinity of the house.
Fire is capricious. It can find the weak link in your home’s fire
protection scheme and gain the upper hand because of a small, overlooked
or seemingly inconsequential factor. While you may not be able to accomplish
all measures below (and there are no guarantees), each will increase your
home’s, and possibly your family’s, safety and survival during
a wildfire.
Start with the easiest and least expensive actions. Begin your work closest
to your house and move outward. Keep working on the more difficult items
until you have completed your entire project.
Defensible Space
Two factors have emerged as the primary determinants of a home’s
ability to survive wildfire. These are the home’s roofing material
and the quality of the “defensible space” surrounding it.
Use fire-resistive materials (Class C or better rating), not wood or
shake shingles, to roof homes in or near forests and grasslands. When
your roof needs significant repairs or replacement, do so with a fire-resistant
roofing material. Check with your county building department. Some counties
now restrict wood roofs or require specific classifications of roofing
material.
Defensible space is an area around a structure where fuels and vegetation
are treated, cleared or reduced to slow the spread of wildfire towards
the structure. It also reduces the chance of a structure fire moving from
the building to the surrounding forest. Defensible space provides room
for firefighters to do their jobs. Your house is more likely to withstand
a wildfire if grasses, brush, trees and other common forest fuels are
managed to reduce a fire’s intensity.
The measure of fuel hazard refers to its continuity, both horizontal
(across the ground) and vertical (from the ground up into the vegetation
crown). Fuels with a high degree of both vertical and horizontal continuity
are the most hazardous, particularly when they occur on slopes. Heavier
fuels (brush and trees) are more hazardous (i.e. produce a more intense
fire) than light fuels such as grass.
Mitigation of wildfire hazards focuses on breaking up the continuity
of horizontal and vertical fuels. Additional distance between fuels is
required on slopes.
Creating an effective defensible space involves developing a series of
management zones in which different treatment techniques are used. See
Figure 1 for a general view of the relationships among these management
zones. Develop defensible space around each building on your property.
Include detached garages, storage buildings, barns and other structures
in your plan.
The actual design and development of your defensible space depends on
several factors: size and shape of buildings, materials used in their
construction, the slope of the ground on which the structures are built,
surrounding topography, and sizes and types of vegetation on your property.
These factors all affect your design. You may want to request additional
guidance from your local Colorado State Forest Service (CSFS) forester
or fire department. (See the Special Recommendations section of this fact
sheet for shrubs, lodgepole pine, Engelmann spruce, and aspen.)
Defensible Space Management Zones
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| Figure 1: Forested property showing the three fire-defensible
zones around a home or other structure. |
Zone 1 is the area of maximum modification and treatment. It consists
of an area of 15 feet around the structure in which all flammable vegetation
is removed. This 15 feet is measured from the outside edge of the home’s
eaves and any attached structures, such as decks.
Zone 2 is an area of fuel reduction. It is a transitional area
between Zones 1 and 3. The size of Zone 2 depends on the slope of the
ground where the structure is built. Typically, the defensible space should
extend at least 75 to 125 feet from the structure. See Figure 2 for the
appropriate distance for your home’s defensible space. Within this
zone, the continuity and arrangement of vegetation is modified. Remove
stressed, diseased, dead or dying trees and shrubs. Thin and prune the
remaining larger trees and shrubs. Be sure to extend thinning along either
side of your driveway all the way to your main access road. These actions
help eliminate the continuous fuel surrounding a structure while enhancing
homesite safety and the aesthetics of the property.
 |
Figure 2: This chart indicates the minimum recommended
dimensions for defensible space from the home to the outer edge of
Zone 2. For example, if your home is situated on a 20 percent
slope, the minimum defensible space dimensions would be 90 feet uphill
and to the sides of the home and 104 feet downhill from the home. |
Zone 3 is an area of traditional forest management and is of no
particular size. It extends from the edge of your defensible space to
your property boundaries.
Prescriptions
Zone 1
The size of Zone 1 is 15 feet, measured from the edges of the structure.
Within this zone, several specific treatments are recommended.
Plant nothing within 3 to 5 feet of the structure, particularly if the
building is sided with wood, logs or other flammable materials. Decorative
rock, for example, creates an attractive, easily maintained, nonflammable
ground cover.
If the house has noncombustible siding, widely spaced foundation plantings
of low growing shrubs or other “fire wise” plants are acceptable.
Do not plant directly beneath windows or next to foundation vents. Be
sure there are no areas of continuous grass adjacent to plantings in this
area.
Frequently prune and maintain plants in this zone to ensure vigorous
growth and a low growth habit. Remove dead branches, stems and leaves.
Do not store firewood or other combustible materials in this area. Enclose
or screen decks with metal screening. Extend the gravel coverage under
the decks. Do not use areas under decks for storage.
Ideally, remove all trees from Zone 1 to reduce fire hazards. If you
do keep a tree, consider it part of the structure and extend the distance
of the entire defensible space accordingly. Isolate the tree from any
other surrounding trees. Prune it to at least 10 feet above the ground.
Remove any branches that interfere with the roof or are within 10 feet
of the chimney. Remove all “ladder fuels” from beneath the tree.
Ladder fuels are vegetation with vertical continuity that allows fire
to burn from ground level up into the branches and crowns of trees. Ladder
fuels are potentially very hazardous but are easy to mitigate. No ladder
fuels can be allowed under tree canopies. In all other areas, prune all
branches of shrubs or trees up to a height of 10 feet above ground (or
1/2 the height, whichever is the least).
 |
| Figure 3: X = crown spacing; Y = stem spacing. Do not measure between stems for crown -- measure between the edges of tree crowns. |
Zone 2
Zone 2 is an area of fuel reduction designed to reduce the intensity
of any fire approaching your home. Follow these recommended management
steps.
Thin trees and large shrubs so there is at least 10 feet between crowns.
Crown separation is measured from the furthest branch of one tree to the
nearest branch on the next tree (Figure 3). On steep slopes, allow more
space between tree crowns. (See Figure 4 for minimum recommended spacing
for trees on steep slopes.) Remove all ladder fuels from under these remaining
trees. Carefully prune trees to a height of at least 10 feet.
Small clumps of 2 to 3 trees may be occasionally left in Zone 2. Leave
more space between the crowns of these clumps and surrounding trees.
Because Zone 2 forms an aesthetic buffer and provides a transition between
zones, it is necessary to blend the requirements for Zones 1 and 3. Thin
the portions of Zone 3 adjacent to Zone 2 more heavily than the outer
portions.
Isolated shrubs may remain, provided they are not under tree crowns.
Prune and maintain these plants periodically to maintain vigorous growth.
Remove dead stems from trees and shrubs annually. Where shrubs are the
primary fuel in Zone 2, refer to the Special Recommendations section of
this fact sheet.
Limit the number of dead trees (snags) retained in this area. Wildlife
needs only one or two snags per acre. Be sure any snags left for wildlife
cannot fall onto the house or block access roads or driveways.
Mow grasses (or remove them with a weed trimmer) as needed through the
growing season to keep them low, a maximum of 6 to 8 inches. This is extremely
critical in the fall when grasses dry out and cure or in the spring after
the snow is gone but before the plants green up.
Stack firewood and woodpiles uphill or on the same elevation as the structure
but at least 30 feet away. Clear and keep away flammable vegetation within
10 feet of these woodpiles. Do not stack wood against your house or on
or under your deck, even in winter. Many homes have burned from a woodpile
that ignited as the fire passed. Wildfires can burn at almost any time
in Colorado.
Locate propane tanks at least 30 feet from any structures, preferably
on the same elevation as the house. You don’t want the LP container
below your house — if it ignites, the fire would tend to burn uphill.
On the other hand, if the tank is above your house and it develops a leak,
LP gas will flow downhill into your home. Clear and keep away flammable
vegetation within 10 feet of these tanks. Do not screen propane tanks
with shrubs or vegetation.
| Figure 4: Minimum tree crown and shrub clump spacing. |
| % slope |
Tree Crown Spacing |
Brush and Shrub Clump Spacing |
| 0 -10 % |
10´ |
2 1/2 x shrub height |
| 11 - 20% |
15´ |
3 x shrub height |
| 21 - 40% |
20´ |
4 x shrub height |
| > 40% |
30´ |
6 x shrub height |
Dispose of slash (limbs, branches and other woody debris) from your trees
and shrubs through chipping or by piling and burning. Contact your local
CSFS office or county sheriff’s office for information about burning
slash piles. If neither of these alternatives is possible, lop and scatter
slash by cutting it into very small pieces and distributing it over the
ground. Avoid heavy accumulations of slash. Lay it close to the ground
to speed decomposition. If desired, no more than two or three small, widely
spaced brush piles may be left for wildlife purposes. Locate these towards
the outer portions of your defensible space.
Zone 3
This zone is of no specified size. It extends from the edge of your defensible
space to your property lines. A gradual transition into this zone from
defensible space standards to other management objectives you may have
is suggested. Typical management objectives for areas surrounding homesites
or subdivisions are: provide optimum recreational opportunities; enhance
aesthetics; maintain tree health and vigor; provide barriers for wind,
noise, dust and visual intrusions; support limited production of firewood,
fence posts and other forest commodities; or grow Christmas trees or trees
for transplanting.
| Figure 5: Minimum tree spacing for Zone
3. |
| Tree Diameter (in inches) |
Average Stem Spacing Between Trees (in feet) |
| 3 |
10 |
| 4 |
11 |
| 5 |
12 |
| 6 |
13 |
| 7 |
14 |
| 8 |
15 |
| 9 |
16 |
| 10 |
17 |
| 11 |
19 |
| 12 |
21 |
| 13 |
23 |
| 14 |
24 |
| 15 |
26 |
| 16 |
28 |
| 17 |
29 |
| 18 |
31 |
| 19 |
33 |
| 20 |
35 |
| 21 |
36 |
| 22 |
38 |
| 23 |
40 |
| 24 |
42 |
Specific requirements will be dictated by your objectives for your land
and the kinds of trees present. See Figure 5 for the minimum suggested
spacing between “leave” trees. Forest management in Zone 3 is
an opportunity for you to increase the health and growth rate of the forest
in this zone. Keep in mind that root competition for available moisture
limits tree growth and ultimately the health of the forest.
A high canopy forest reduces the chance of a surface fire climbing into
the tops of the trees and might be a priority for you if this zone slopes
steeply. The healthiest forest is one that has multiple ages, sizes, and
species of trees where adequate growing room is maintained over time.
Remember to consider the hazards of ladder fuels. Multiple sizes and ages
of trees might increase the fire hazard from Zone 3 into Zone 2, particularly
on steep slopes.
A greater number of wildlife trees can remain in Zone 3. Make sure that
dead trees pose no threat to power lines or fire access roads.
While pruning generally is not necessary in Zone 3, it may be a good
idea from the standpoint of personal safety to prune trees along trails
and fire access roads. Or, if you prefer the aesthetics of a well-manicured
forest, you might prune the entire area. In any case, pruning helps reduce
ladder fuels within the tree stand, thus enhancing wildfire safety.
Mowing is not necessary in Zone 3.
Any approved method of slash treatment is acceptable for this zone, including
piling and burning, chipping or lop-and-scatter.
Special Recommendations
Tree spacing guidelines do not apply to mature stands of aspen trees where
the recommendations for ladder fuels have been complied with. In areas
of aspen regeneration and young trees, the spacing guidelines should be
followed.
Brush and shrubs
Brush and shrubs are woody plants, smaller than trees, often formed by
a number of vertical or semi-upright branches arising close to the ground.
Brush is smaller than shrubs and can be either woody or herbaceous vegetation.
On nearly level ground, minimum spacing recommendations between clumps
of brush and/or shrubs is 2 1/2 times the height of the vegetation. Maximum
diameter of clumps should be 2 times the height of the vegetation. As
with tree crown spacing, all measurements are made from the edges of vegetation
crowns (Figure 3).
For example: For shrubs 6 feet high, spacing between shrub clumps should
be 15 feet or more apart (measured from the edges of the crowns of vegetation
clumps). The diameter of shrub clumps should not exceed 12 feet (measured
from the edges of the crowns). Branches should be pruned to a height of
3 feet.
Grasses
Keep dead, dry or curing grasses mowed to less than 6 inches. Defensible
space size where grass is the predominant fuel can be reduced (Figure
5) when applying this practice.
Windthrow
In Colorado, certain locations and tree species, including lodgepole
pine and Engelmann spruce, are especially susceptible to damage and uprooting
by high winds (windthrow). If you see evidence of this problem in or near
your forest, or have these tree species, consider the following adjustments
to the defensible space guidelines. It is highly recommended that you
contact a professional forester to help design your defensible space.
Adjustments: If your trees or homesite are susceptible to windthrow
and the trees have never been thinned, use a stem spacing of diameter
plus five instead of the guides listed in the Zone 3 section. Over time
(every 3 to 5 years) gradually remove additional trees. The time between
cutting cycles allows trees to “firm up” by expanding their
root systems. Continue this periodic thinning until the desired spacing
is reached.
Also consider leaving small clumps of trees and creating small openings
on their lee side (opposite of the predominant wind direction). Again,
a professional forester can help you design the best situation for your
specific homesite and tree species. Remember, with species such as lodgepole
pine and Engelmann spruce, the likelihood of a wildfire running through
the tree tops or crowns (crowning) is closely related to the overabundance
of fuels on the forest floor. Be sure to remove downed logs, branches
and excess brush and needle buildup.
| Figure 6: Minimum defensible space size for grass
fuels. |
| % slope |
D-space size (uphill, downhill, sidehill) |
| 0 - 20 % |
30’ |
| 21 - 40% |
50’ |
| > 40% |
70’ |
Maintaining Your Defensible Space
Your home is located in a forest that is dynamic, always changing. Trees and
shrubs continue to grow, plants die or are damaged, new plants begin to
grow, and plants drop their leaves and needles. Like other parts of your
home, defensible space requires maintenance. Use the following checklist
each year to determine if additional work or maintenance is necessary.
Defensible Space and FireWise Annual Checklist
- Trees and shrubs are properly thinned and pruned within the defensible space. Slash from the thinning is disposed of.
- Roof and gutters are clear of debris.
- Branches overhanging the roof and chimney are removed.
- Chimney screens are in place and in good condition.
- Grass and weeds are mowed to a low height.
- An outdoor water supply is available, complete with a hose and nozzle that can reach all parts of the house.
- Fire extinguishers are checked and in working condition.
- The driveway is wide enough. The clearance of trees and branches is adequate for fire and emergency equipment. (Check with your local fire department.)
- Road signs and your name and house number are posted and easily visible.
- There is an easily accessible tool storage area with rakes, hoes, axes and shovels for use in case of fire.
- You have practiced family fire drills and your fire evacuation plan.
- Your escape routes, meeting points and other details are known and understood by all family members.
- Attic, roof, eaves and foundation vents are screened and in good condition. Stilt foundations and decks are enclosed, screened or walled up.
- Trash and debris accumulations are removed from the defensible space.
- A checklist for fire safety needs inside the home also has been completed. This is available from your local fire department.
References
Colorado State Forest Service, Colorado State University, Fort Collins, CO 80523-5060; (970) 491-6303:
- FireWise Construction -- Design and Materials
- Home Fire Protection in the Wildland Urban Interface
- Wildfire Protection in the Wildland Urban Interface
- Landowner Guide to Thinning
Colorado State University Extension, 115 General Services
Bldg., Fort Collins, CO 80523-4061; (970) 491-6198; E-mail: resourcecenter@ucm.colostate.edu.
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