Reading A Building - Sizing Up the Roof

By John W. Mittendorf

When reading a building, do you include the roof in your size-up. If so, what are you thinking about? Some factors depend 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.

The six most common roof styles in the United States are: gable, hip, flat, bridge truss, arch, and sawtooth.

Hip Roof
This roof is primarily found on residential structures and some moderate commercials. Look at the age of the building to determine if the construction is older (conventional) or newer (lightweight). Also look at the pitch of the roof (attic space and roof ladder considerations). If the rafter tails are exposed, this is an excellent indicator of the size and spacing of the rafter construction.

Gable Roof
Primarily found on residential structures, this roof is similar to the hip roof. Look at the age of the structure, pitch of the roof, and the presence of exposed rafter tails.

Sawtooth Roof
Sawtooth roofs are easily identified by their characteristic shape, and are found on commercial structures that normally need easy light and ventilation for the structure and manufacturing processes. These roofs are normally of conventional construction with 1 x 6-inch sheathing on rafter members. Normally, 1 x 6-inch sheathing is associated with older conventional construction, not lightweight truss construction. This sheathing will last significantly longer when exposed to fire than plywood, OSB, and other similar materials. Both of these considerations are a plus from the perspective of fireground time.

Flat Roofs
Cost is a prime reason why most commercial and some residential roofs are flat. It is faster and cheaper to construct a flat roof instead of a sawtooth, arch, gable, or other types of roofs. Flat roofs become even more cost-effective when the builder uses lightweight materials. From the perspective of suppression personnel, this means be careful.

  • Common Flat Roof
    These roofs can be constructed using a variety of methods - conventional, lightweight, wood, metal, concrete, etc. Try to identify the type of flat roof based on a prior knowledge (i.e., fire prevention, etc) and/or the age of the building.
  • Lightweight Concrete
    This roof is constructed of a "lightweight - non structural concrete" mixture that results in a very strong roof. Lightweight concrete is not easily recognizable and demands specific tools (rotary saw equipped with a carbide tipped wood blade) for roof ventilation operations.
  • Panelized
    This roof is a West Coast favorite and is so named because it is finished with 4 x 8-foot plywood panels. Though this roof is used on large spans, it is still primarily constructed of 1/2-inch 4 x 8-foot plywood panels supported by 2 x 4-inch joists. Upon completion, the roof's underneath portion is covered with an insulation paper that will readily separate and fail when exposed to high levels of heat and/or fire. This roof is a companion of concrete tilt up commercial buildings.

The next installment will cover other types of flat roofs and the older truss roofs.

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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