Construction Concerns: The 20-Minute Rule

Article and photo by Gregory Havel

In his book Fireground Tactics, Emmanuel Fried stated in 1972: “When the fire has substantially involved more than one floor and is out of control after 20 minutes of inside operation, back all companies out and resort to the use of exterior streams. You may lose the building, but you may save your men.”1

This “20-minute rule” is still included in many of today’s firefighting textbooks. It is the basis of structural firefighting tactics and strategy of many of today’s fire departments.  It matches the periodic time check we receive from the dispatcher during fire operations, as recommended by the Incident Command System.

The “20-minute rule” was a valid and proven concept when it was written in the early 1970s, when almost all combustible buildings were constructed using the “legacy” methods shown on the left in Photo 1:

  • Joists in older buildings were of full-dimension lumber (a full 2 inches by 8, 10, or 12 inches). In newer construction in that era, joists were of planed lumber, with 3/8 to ½ inch of dimension lost to planing at the lumber mill
  • Subfloors were tongue-and-groove boards fastened to the joists in a 45-degree diagonal position in relation to the joists
  • Finished floors were tongue-and-groove hardwood boards fastened to the subfloor and joists, running perpendicular to the joists.

In the early 1970s there was already some use of construction methods using lightweight and manufactured wood materials. However, these materials were new. They were not yet in common use, and had not been in place long enough for them to have been involved in significant fires.

The “20-minute rule” was a valid and proven concept when it was written, in an era when most rooms and offices were equipped with furniture and furnishings of wood and cellulose-based materials, rather than with the plastics and synthetic fabrics that are in common use today. These modern furnishings contain more than twice the energy than those of wood and natural fibers, and burn faster with a greater rate of heat release.

Today, most new combustible buildings (and most remodeling done in older combustible buildings) use lightweight components and manufactured wood materials, as shown on the right in Photo 1:

  • I-joists or wood trusses attached to girders with sheet-metal joist hangers
  • I-joists of wood and plywood, or laminated veneer lumber top and bottom flanges, with a web of oriented strand board (OSB)
  • Wood trusses made of short pieces of lumber connected by gusset plates, steel pins, or glue
  • A subfloor of plywood or OSB
  • A finished floor of carpet and padding, or of thin hardwood sheets attached by adhesive

The manufactured lumber components (I-joists, trusses, plywood, OSB, laminated veneer lumber) do not have the mass of the sawn lumber and boards that they replace, and therefore do not have the inherent fire resistance of these earlier materials.

Since the use of lightweight and manufactured building components became common, news articles have been commenting on structural failure “earlier than expected” that has injured or killed firefighters. National Institute for Occupational Safety and Health (NIOSH) firefighter fatality reports have documented these. Tests conducted by the National Institute of Standards and Technology (NIST) and Underwriters Laboratories (UL) in the laboratory and in acquired structures with recordings of instrument readings, photographic and ultraviolet camera photos and videos, and notes from technicians have shown that assemblies of these lightweight and manufactured components can be expected to fail within five to seven minutes of ignition. Under these conditions, a time check from the dispatcher 20 minutes after our arrival is not adequate.

It is time for the fire service to adopt a new rule for operations in buildings of combustible construction. How about this: When a fire has extended from room contents to the structure of the building and is not under control within five minutes of our receipt of the fire alarm, back all companies out and resort to the use of exterior streams.

Time checks from the dispatcher at five minute intervals from the time the alarm was received until the fire is declared under control can remind us that the risk to firefighters (and to other building occupants) increases quickly as the time since ignition increases.

Many fire departments are unable to arrive at most incidents and have water on the fire within five minutes of the alarm. For our own safety, this suggests that we may have to knock down the fire from the exterior with a straight hose stream, conduct a structural evaluation, and conduct our interior searches and overhaul only if the structure is still stable enough to support this activity. Tests conducted by NIST and UL show that an exterior attack on a room fire with a straight stream will not upset the thermal balance in the structure and will not push the fire into uninvolved areas.

For more information on the behavior of lightweight and manufactured wood products in fires, visit the NIOSH Web site and under “Trauma” search for “Structure Fires – Collapse” and “Training – Live Fire”.

For more information on the recent testing done by the NIST and UL, visit these sites:


  1. Emmanuel Fried, Fireground Tactics, (Chicago: H. Marvin Ginn Corp., 1972),  Page 19

Download this article as a PDF HERE.


Building construction for firefighters: Greg HavelGregory Havel is a member of the Town of Burlington (WI) Fire Department; retired deputy chief and training officer; and a 30-year veteran of the fire service. He is a Wisconsin-certified fire instructor II, fire officer II, and fire inspector; an adjunct instructor in fire service programs at Gateway Technical College; and safety director for Scherrer Construction Co., Inc. Havel has a bachelor’s degree from St. Norbert College; has more than 30 years of experience in facilities management and building construction; and has presented classes at FDIC.



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