Reading A Building: More Roof Size-Up

By John W. Mittendorf

When reading a building, do you include the roof in your size-up, and if so, what are you thinking about? To assist with this question, let’s consider some important factors that are worthy of your consideration. Obviously, some factors will be dependent on the type of roof construction in your particular area, however, West Coast roofs and East Coast roofs have a lot in common in both construction methods and styles.

Open Web Bar Joist
Open web bar joist (or metal deck) roofs are the commercial roof of choice in the Midwest and Eastern portions of the country and are primarily steel truss construction underneath a metal decking (Q decking). The metal decking is covered by built-up layers of insulation material, tar, and composition. As steel loses it’s strength around 1,000 degrees, such roofs have a quick failure rate with minimal warning, and suppression personnel should be aware of these hazards. However, another more subtle hazard is that fire can propagate between the metal base and the composition covering, enhancing the spread of fire with minimal visible warning signs.

Older Truss Roofs
These roofs are found anywhere in the country and on various types and sizes of commercial buildings primarily constructed during the 1800s until the 1950s – until the introduction of the flat roof with its numerous variations. The older truss roofs were normally constructed with a “large” size of wooden truss members, 1 x 6-inch sheathing as a roof base/covering, and can be found in numerous styles as follows:

Bridge Truss: This type is recognizable by it’s characteristic sloping sides, ends, and flat top.

Gable Truss: This type is also identified by it’s gable or peaked roof design.
Parallel Chord Truss: This roof looks similar to other types of flat roofs but can be found on older buildings and is constructed from a “large” size of truss members (compared to newer lightweight truss members).

Lamella: Although this roof can be similar in external appearance to other types of arch roofs, it is significantly different as it was constructed in an egg crate – geometric or diamond-patterned – design. This roof can be found on gymnasiums, recreational buildings, large supermarkets, etc.

Tied Truss: This arched roof uses metal tie rods to give lateral support to the walls of the building. Tie rods with turnbuckles are used below each arch member (as there is no bottom chord) to ensure that the arches do not push the exterior walls outward. With this mind, it is easy to see if fire exposes metal tie rods in this type of roof, a collapse of the building is more than a possibility. Hint: If you are ever inside a building and observe this type of roof construction, make a mental note for future reference as it may save your life!

Bowstring Truss: Most firefighters are familiar with the “bowstring truss” roof as numerous fire service writers have appropriately written on the hazards of this common roof. Interestingly, whether you are a firefighter on the East or West coast (or anywhere in between), you will likely have this roof in your municipality. It is constructed of “large-size” wooden members (Note: most wooden members used in these older truss roofs were “rough-cut” or full size lumber and used steel plates and bolts for connectors) with 1 x 6-inch sheathing roof decking. Multiple firefighter deaths attributed to this specific roof have cautioned firefighters to assume a defensive position if a working fire is encountered.

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).

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