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Landscape Timbers

A photo of a stack of two railroad ties burning in front of a shed. The ties are 2.59 meters or 8.5 feet long and are 36 cm or 14 inches tall at a distance of 1.83 meters or 6 feet from the shed. The ties are oriented perpendicular to the face of the shed.
A photo of a stack of 2 railroad ties burning in front of a shed. The ties are 2.59 m (8.5 ft) long and are 36 cm (14 in) tall at a distance of 1.83 m (6 ft) from the shed. The ties are oriented perpendicular to the face of the shed.
Credit: Rik Johnsson/NIST

A series of field experiments was conducted to examine the effects of landscape timbers, often used as retaining walls or garden borders, on fire spread toward a structure under conditions that may be encountered during a WUI fire. The fire behavior of landscape timbers in multiple configurations was studied under various wind conditions. Sixteen experiments were conducted. Landscape timber types included pressure-treated rounded 7.0 cm x 8.9 cm (2.75 in x 3.5 in) pine timbers, pressure treated 14.0 cm x 14.0 cm (5.5 in x 5.5 in) pine lumber, and creosote-treated 17.8 cm (7 in) by 22.9 cm (9 in) railroad ties. Configuration variations included the number of timbers vertically stacked, the presence of mulch, orientation, and exposure on one or both sides. A wind machine provided a mean wind speed of nominally 6 m/s (13 mi/h) for most experiments. Timbers were ignited by a propane burner on the ground at the end farthest from a small structure located 1.83 m (6 ft) downwind of the trailing end of the timber. For most experiments, a target mulch bed at the base of the structure evaluated the ability of firebrands produced by the burning landscape timbers to ignite spot fires that could threaten the structure. A final experiment compared burning behavior of a stack of timbers in the open compared to one used as a retaining wall.

A photo of a close-up view of burning landscape timbers. The timbers are a stack of two six-by-six dimensional treated pine lumber. Fire is progressing mostly between and under the two timbers.
A photo of a close-up view of burning landscape timbers. The timbers are a stack of 2 6x6 dimensional treated pine lumber. Fire is progressing mostly between and under the two timbers.
Credit: Rik Johnsson/NIST

The experiments in this study demonstrated that landscape timbers can be sources of spot fire ignitions, can spread fire to nearby structures, and may exhibit deep-seated fires embedded within and between timbers. Fire behavior was classified as high hazard for moderate fire spread rates along timbers, igniting spot fires under all wind conditions, and the propensity of fire to embed in landscape timbers and resist extinguishment efforts.

Spot fire generation was affected by the wind field; the structure created both upward flow (enhanced by buoyancy) and a vortex that deposited firebrands next to the structure. During most but not all experiments, the burning landscape timbers, sometimes with a burning mulch bed, produced firebrands that ignited spot fires in the target mulch bed.

 

A photo of burning landscape timbers. The timbers are a stack of four rounded nominal three-by-four dimensional treated pine landscape timbers. The photo shows fire progressing along the outside and between the timbers.
A photo of burning landscape timbers. The timbers are a stack of 4 rounded nominal 3x4 dimensional treated pine landscape timbers. The photo shows fire progressing along the outside and between the timbers.
Credit: Rik Johnsson/NIST


PUBLICATIONS

Johnsson, E., et al. (2025) Wind-Driven Fire Spread to a Structure from Landscape Timbers NIST Technical Note 2307.
https://doi.org/10.6028/NIST.TN.2307

Fire and Wildland urban interface fire

Contacts

Created September 29, 2026, Updated September 30, 2026
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