Construction Concerns: Bar Joist Roofs

By Gregory Havel

Often, when we hear of a roof supported by steel bar joists, we assume that the bar joists support a steel roof deck (photo 1). The steel roof deck is attached to the bar joists by spot welds, self-drilling screws, or steel pins, although decades ago the steel roof panels were sometimes laid on top of the bar joists without any attachment.

1. (Photos by author unless otherwise noted.)

 

Photo 2 shows a variation of the steel roof deck, called an “acoustical deck.” The vertical planes of the steel panels are perforated with thousands of small holes. The space between these holes on top of the panels is filled with noncombustible mineral wool insulation before the roof insulation and roof membrane are applied. This system provides better acoustics (less echo) in finished spaces where the underside of the steel roof is the ceiling.

2.

 

Steel bar joists often support roof decks comprised of materials other than steel. Photo 3 shows a gypsum roof supported by bar joists. This roof used lightweight corrugated steel as a form. Gypsum was poured on top of it for rigidity and strength. Foam insulation board was applied and topped with a built-up roof of multiple layers of roofing felt and hot asphalt. Other variations of a gypsum roof include paper-faced gypsum board as a form with poured gypsum on top and thick interlocking gypsum roof planks set in place on the bar joists with insulation board and roof membrane applied on top. Search the Internet for “gypsum roof deck” and “gypsum roof panels” for more information on this roof system.

3. [Photo by Stacy Richard, Miami-Dade (FL) Fire Rescue.]

 

Photo 4 shows a cementitious roof deck of wood-fiber cement board supported by bar joists. This material is made of a mat of Excelsior (coarse wood fiber) soaked in a mixture of chemicals, which may include gypsum or Portland cement and is cured under pressure with heat. This type of cement board is impact resistant and sound absorbing, which makes it popular for spaces where sound control is needed; where the underside of the roof is also the ceiling. It is also advertised as having a low fire spread rating. Search the Internet for “cementitious roof deck” for more information on this roof system.

4.

 

This type of cement board is also available as structural insulating panels (SIPs). The block of polystyrene insulating foam has a sheet of oriented strand board (OSB) adhered to the top side and a sheet of the cement board adhered to the bottom side for acoustic purposes.

Bar joists may also support other types of roof deck materials including roof boards (photo 5), plywood, and concrete on corrugated steel sheets.

5.

 

A key point to remember when working on or below a roof supported by bar joists is that if the fire is hot enough to char or ignite a combustible roof deck or to weaken a steel roof deck it is also hot enough to weaken the steel bar joists. Under these conditions, we should not attempt vertical ventilation while working from the roof, because of the danger of roof collapse.

Our preincident plans must include notes on the type of roof deck and roof supports if it is to be of real value to us during an incident response.

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Gregory 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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