Skip to main content
U.S. flag

An official website of the United States government

Official websites use .gov
A .gov website belongs to an official government organization in the United States.

Secure .gov websites use HTTPS
A lock ( ) or https:// means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.

Structure Separation Experiments

Campfire Shed House
A residential structure moments before catching fire due to exposure from the fully involved [3.6 m by 8 m (12 ft by 26 ft)] shed/garage located 2.4 m (8 ft) from the eaves. Photo courtesy of CAL FIRE, Camp Fire 2018.

A major contribution to the severity and devastating outcomes of many wildland-urban interface (WUI) fires stems from the continuation of fire spread from the wildlands to deeper within the built community due to structure-to-structure fire spread. In the case of most destructive WUI fires, this fire spread is also enhanced by high winds and structures burning unmitigated by suppression actions.

The focus of the structure separation experiments is to investigate the relationship between the burning of one structure (i.e., source) and the exposure to a neighboring residential structure (i.e., target), and to identify safe structure separation distances. The goal of these experiments is to improve the understanding of the exposures produced by burning structures, characterize the burning behavior of these common structure types, and to provide a dataset that is compatible with the advancement of computer fire modeling tools. Ultimately, the findings are intended to provide scientific basis for recommendations and best practices for building code, spacing, and zoning requirements in communities at risk of WUI fires.

Three phases are proposed, progressively increasing the size of the burning source structure, to produce a range of exposure magnitudes that can be expected in a residential community.

  • Phase 1: sheds [1.5 m2 to 25 m2 (16 ft2 to 267 ft2)]
  • Phase 2: in-law suites / accessory dwelling units (ADUs) [41 m2 (~440 ft2)]
  • Phase 3: single family home [93 m2 (~1000 ft2)]

The Phase 1 Preliminary Test Plan has been published as NIST Technical Note 2161 (

The Structure Separation Experiments are a collaboration between the NIST Fire Research Division, the California Department of Forestry and Fire Protection (CAL FIRE), the U.S. Forest Service (USFS), the Insurance Institute of Building and Home Safety (IBHS), the California Building Industry Association (CBIA), Western Fire Chiefs Association (WFCA), and the U.S. Fire Administration (USFA).


Picture from video of SSE Experiments on
Watch on YouTube
Credit: NIST






Maranghides, A., et al. (2021) Structure Separation Experiments Phase 1 Preliminary Test Plan. NIST Technical Note 2161. National Institute of Standards and Technology. Gaithersburg, MD.


Maranghides, A., Johnsson, E. (2008) Residential Structure Separation Fire Experiments. NIST Technical Note 1600. National Institute of Standards and Technology. Gaithersburg, MD.


For more information, contact:

Created September 1, 2021, Updated October 6, 2023