Wind-Driven Fire Spread to a Structure from Landscape Timbers

Published

April 16, 2025

Author(s)

Erik Johnsson, Kathryn Butler, Marco Fernandez, Wei Tang, Shonali Nazare, Philip Deardorff, Sebastian Arana, Alexander Maranghides

Abstract

NIST is studying how combustible landscape features around a home burn to better understand their levels of hazard and potential roles in spreading wildland-urban interface (WUI) fires. A series of field experiments was conducted to examine the effects of landscape timbers 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. Fifteen 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 use as a retaining wall. 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. The experiments in this study demonstrated that landscape timbers can be sources of spot fire ignitions and can spread fire to nearby structures. Fire behavior was classified as medium hazard for moderate fire spread rates along timbers and igniting spot fires under all wind conditions. 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. This study of the fire hazard of landscape timbers is part of a series designed to better inform standards and codes regarding placement of landscape features around homes that are at risk of exposure to wildland-urban interface fires.

Citation

Technical Note (NIST TN) - 2307

Report Number

2307

NIST Pub Series

Technical Note (NIST TN)

Pub Type

NIST Pubs

Download Paper

https://doi.org/10.6028/NIST.TN.2307

Local Download

Keywords

Embers, landscape timbers, retaining walls, firebrands, fire spread, structural ignition, structure vulnerability, wildland urban interface fires, wind-driven fires, WUI fires

Citation

Johnsson, E. , Butler, K. , Fernandez, M. , Tang, W. , Nazare, S. , Deardorff, P. , Arana, S. and Maranghides, A. (2025), Wind-Driven Fire Spread to a Structure from Landscape Timbers, Technical Note (NIST TN), National Institute of Standards and Technology, Gaithersburg, MD, [online], https://doi.org/10.6028/NIST.TN.2307, https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=957994 (Accessed April 19, 2025)