Planning a Fuel Break on Acreage in Western Montana

What is the difference between a firebreak and a fuel break?

The Pacific Northwest Extension publication Reducing Fire Risk on Your Forest Property (PNW 618) draws the line clearly. A firebreak is an area where all vegetation and organic matter is removed down to mineral soil, so a surface fire has nothing to burn. Firebreaks are narrow, typically 2 to 15 feet, and the guide recommends making one two to three times as wide as the height of the nearest grass or shrubs. They are used to keep surface flames from touching outbuildings and other important things.

A shaded fuel break is a strip where the fuel is modified rather than removed: trees are thinned and lifted, brush and small trees are cut, heavy ground fuel and snags are reduced, and the canopy that remains keeps the ground cooler, moister and greener later into the season. PNW 618 lists the trade-offs. A shaded fuel break looks natural, can be partly paid for by selling merchantable trees, and improves the health of the trees left. But fire can still burn through it at reduced intensity, it has to be much wider than a firebreak, and it needs to be retreated roughly every 10 years depending on how fast the site grows back.

Will a fuel break stop a wildfire?

No, and nobody who builds them claims it will. PNW 618 describes what shaded fuel breaks do under moderate conditions: they can slow an advancing fire and reduce its intensity, and in fires that have burned into fuel breaks or other areas where fuels were reduced, the fire dropped to the ground where firefighters could suppress it more easily. They give crews safer access and a place to anchor fire line.

Under extreme wind, embers will cross any break on a western Montana property. That is why a fuel break is the outer layer of a plan, not the whole plan. The house itself and the first 100 feet around it are still where most homes are won or lost; our guides on the home ignition zone and Zone 0 cover that ground.

How wide should a fuel break be on a slope?

PNW 618 gives a minimum recommended width of approximately 200 feet for shaded fuel breaks in the drier forests of eastern Oregon and Washington and in Idaho, the forest types closest to ours, and it adapts a table from the Colorado guide Fuelbreak Guidelines for Forested Subdivisions (Dennis 1983). Breaks are usually built along a road, and the table shifts more of the width downhill as the slope steepens, because fire runs uphill. On flat ground the guide gives 100 feet above and 100 below the road.

At 20 percent slope the table gives 80 feet uphill and 130 feet downhill, 210 feet in total. At 40 percent it gives 60 feet uphill and 160 feet downhill, 220 in total. At 60 percent it gives 40 feet uphill and 190 feet downhill, 230 in total. Downhill distances are measured from the toe of the road fill and uphill distances from above the road cut, along the slope. The guide adds that very steep ground with heavy fuel may call for more than 200 feet, and that these are general guidelines to be checked with a state forester. Our guide to slope, aspect and wind explains why the downhill side matters so much.

Where should a fuel break go on your property?

Along roads, first. PNW 618 notes that fuel breaks are usually placed above and below existing roads to take advantage of the noncombustible road surface, or next to natural barriers such as wet meadows, streams and rock outcrops. On most acreages the driveway is both the obvious anchor and the thing you most need to protect, because it is the way out and the way in for an engine. Our driveway access guide covers clearance for the travel lane itself.

PNW 618 lists three things that set how much break you need and where: the risk of ignition, from roads, neighboring homes, railroads or lightning; the forest type, density and fuel arrangement; and slope and terrain. DNRC's structure assessment form adds the questions that matter on a bench above the valley floor: where the house sits on the slope, whether it is set back more than 150 feet from the edge of a slope, and whether chutes, chimneys or saddles nearby can funnel wind. A break across the bottom of a draw below a house does more than the same acreage thinned behind it.

What comes out of a shaded fuel break, and what stays?

PNW 618 describes thinning the overstory to reduce crown-to-crown overlap, especially between conifers, and spacing trees in the break wider than in the rest of the stand. Understory trees and combustible shrubs that act as ladder fuels, heavy ground fuels and snags are reduced or removed. The cutting creates slash, and the guide is blunt that for an effective break that slash has to be dealt with, because a fuel break full of cut limbs is not a fuel break.

For a local benchmark, the 2020 CPAW recommendations to Missoula County described defensible space along driveways and water supplies as lifting tree crowns to create 6 to 8 feet between surface fuels and crown bases, and creating at least 10 feet between tree crowns or groups of crowns where needed, prioritizing dead, suppressed and damaged trees. DNRC's West-Central Montana landowner guide notes that ponderosa pine and western larch forests here historically saw mixed severity fire every 10 to 30 years that kept them open, and that thinning to restore that open structure is part of good management. The healthiest, best-formed ponderosa and larch are the trees to keep.

How do you keep a fuel break working?

Plan the second treatment when you do the first. PNW 618 says shaded fuel breaks must be maintained periodically, that the interval depends on how fast fuel reaccumulates and how open you want the stand, and that maintenance can include cutting, piling, burning, grazing or herbicide. Its advice is to write a retreatment plan and do a little every year rather than let it slide for a decade.

Watch for weeds. The same publication warns that exposing mineral soil, which firebreaks always do and fuel breaks sometimes do along skid trails and landings, creates conditions for invasive weeds to establish. On western Montana ground that usually means spotted knapweed and cheatgrass, and cheatgrass in particular puts dry, fine fuel right back into the break. See noxious weeds as wildfire fuel and cheatgrass and dry grass.

Who can help plan a fuel break on private land?

Start with a DNRC service forester. DNRC says its 14 service foresters provide free one-on-one consultations to private landowners, including stewardship and management plans, identifying cost-share programs, hazard reduction and defensible space, and forest practice questions such as slash treatment and hazard reduction agreements. The page lists a Missoula unit and a Hamilton unit. DNRC also offers a free home wildfire risk assessment, with a representative contacting you within 7 business days of a request.

If you plan to sell logs from the break, talk to the service forester before the cutting starts, because harvest brings its own rules on slash and streamside zones. Cost-share programs change from year to year; our guide to fuel reduction cost-share in Montana tracks what we could verify. Neighbors matter too. A break that stops at a fence line where the next parcel is thick timber protects less than one planned with the people next door.

How do we approach fuel break work?

We walk the property with the owner and, when there is one, the forester's plan. We mark the trees to keep before anything is cut, because selective thinning is a judgment made tree by tree. Then the crew lifts and thins, removes ladder fuel and dead material, and either chips the slash on site or hauls it, whichever suits the ground; see chipping versus hauling slash. Justin Johnson is an ISA Certified Arborist, which matters when the trees being kept are as important as the ones coming out.

Fall and winter are usually the best time for this work in the valleys, for reasons covered in why fall and winter suit fuel reduction. Our fire mitigation page explains the rest of what we do, from the house outward, and the Montana Wildfire Guide collects every chapter of this series.

Call HJ Property Care & Tree Service at (406) 215-7319 or request a free estimate and we will walk the property with you.

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