Reading A Building – The Bowstring Roof

By John W. Mittendorf

Reading A Building: More Roof Size-Up looked at various types of roofs and overviewed items that should be evaluated when confronted by a particular roof type. Now, let’s briefly look at a roof that should be recognized by every firefighter: the arch roof known as a bowstring roof. This popular roof design was employed during the late 1800s until the mid 1900s on small, moderate, and large commercial-type structures. Usually large-size wooden members with steel plates/bolts were used and covered with 1 x 6-inch sheathing. This roof has earned a reputation as “the most dangerous roof a firefighter will encounter”, and, “a roof that will fail in the early stages of a fire”. Although any roof is worthy of these remarks when exposed to sufficient fire, the bowstring roof has been a point of special recognition due to numerous firefighter deaths attributed to its failure during fire. However, two often cited examples do not support the bowstring’s reputation as an overly weak roof that is susceptible to early collapse:

  • Hackensack, New Jersey
    Three factors contributed to the collapse of this bowstring arch:
    • alterations that consisted of a heavy ceiling of cementitious material on wire lathe;
    • auto parts storage in the attic; and
    • fire that burned for a significant length of time and was well advanced prior to detection.

    This roof collapsed 35 minutes after the initial units arrived.
  • Waldbaum’s Supermarket, New York City
    Two factors contributed to the collapse of this bowstring arch:
    • a double roof (rain roof) alteration; and
    • the extent and severity of the fire.

    This roof collapsed 32 minutes after the initial units arrived.

Neither of these incident upholds the view that timber truss fail quickly, even when they have been significantly altered from their original design. Fires in these types of roofs suggest a difference between timber truss roofs located in the Western and Eastern states. Although these roofs are primarily constructed the same, the timber truss roofs on the East Coast are significantly older and have been subjected to harsh weather conditions, wood rot, termites, renovations, and other circumstances for a longer period of time than their West Coast counterparts. Additionally, older roofs will likely have been re-roofed more often, resulting in a higher “dead load” on the roof.

So, are timber truss bowstring arch roofs dangerous? Absolutely, and so are other types of roofs when exposed to fire. However, fireground statistics indicate that these roofs in Western states can last longer when exposed to fire than their Eastern counterparts. The major hazard attributable to timber truss construction (which includes all timber truss roofs, not just the bowstring arch) may not be the construction itself but a combination of the following factors:

  • alterations that exceed the design criteria of the original roof;
  • the inherent size and strength of the construction allows fire to burn for a period of time while personnel initiate an interior attack and/or roof ventilations; and
  • significant storage area in the attic space when the lower chords of the truss have been modified with flooring.

Timber truss roofs can be among the most hazardous types of roof construction encountered by suppression personnel, but they can also offer strength and time necessary to conduct successful operations in and around them. Don’t let the hazards of these roofs detract your focus from the hazards inherent in all roofs. Remember that any roof can be fatal if the proper ratio of fire, time, and type of construction is present. As a side note, what is the most dangerous timber truss roof? Go back to Reading A Building: More Roof Size-Up and look at the tied truss. It is an arch roof constructed with a top arch of heavy timber, no webbing or bottom chord, and a steel tie-rod underneath each arch that keeps the building walls from being pushed outward by the wooden arches.

John W. Mittendorf joined the Los Angeles City (CA) Fire Department (LAFD) in 1963, rising to the rank of captain II, task force commander. In 1981, he was promoted to battalion chief and in the year following became the commander of the In-Service Training Section. In 1993, he retired from LAFD after 30 years of service. Mittendorf has been a member of the National Fire Protection Research Foundation on Engineered Lightweight Construction Technical Advisory Committee. He has provided training programs for the National Fire Academy in Emmitsburg, Maryland; the University of California at Los Angeles; and the British Fire Academy at Morton-in-Marsh, England. He is a member of the editorial advisory board of Fire Engineering and author of the books Truck Company Operations (Fire Engineering, 1998) and Facing the Promotional Interview (Fire Engineering, 2003).

No posts to display