Ventilating Steel Roof Decks

Article and photos by Gregory Havel

Firefighting textbooks and magazine articles have cautioned us for decades about ventilating a roof with a corrugated or ribbed steel roof deck, which is part of the load-bearing structural system of the roof and is usually supported by steel bar joists or another type of lightweight steel truss. If we make a ventilation opening in one of these types of roof, we may discover that we are now standing on an unstable surface that will bend and collapse under our weight and drop us into the fire building below. Ventilation openings in these steel roof decks frequently result in cantilevered sheets of roof decking, which were strong enough to support our weight when they were supported at their ends and at intermediate points. The cantilevered end of a sheet of steel roof decking is unable to support much weight. 

(1)

Photo 1 shows the underside of a typical ribbed steel roof deck supported on steel bar joists on six-foot (1.83 m) centers. The decking in this photo is of two types.

  • In the background, the decking is plain sheet steel.
  • In the foreground, the “acoustical decking” has thousands of small holes (1/8-inch or 3-mm diameter) punched in the vertical ribs that help absorb sound when the underside of the roof is also the ceiling of the room. Before the roof insulation and roof membrane are applied, the spaces between these perforated ribs are filled with a non-combustible mineral-wool acoustic insulation to absorb sound.          

Both types of roof deck in Photo1 are installed to the standards outlined in Manual of Construction with Steel Deck by the Steel Deck Institute www.sdi.org, which requires that the end laps of roof deck sheets occur on top of the bar joists. (See “Lightweight Steel Construction” in Fire Engineering, November 2005). Both types are welded, screwed, or otherwise fastened to each bar joist at the bottom of each rib; and both types have screws on the edge of the lap joints between the bar joists, called “stitching.”

(2) 

Photo 2 shows the underside of a pitched steel roof deck supported by lightweight steel triangular trusses on eight-foot (2.44 m) centers. This roof was constructed to the same standards as the roof in Photo 1. (See “Building Construction: Lightweight Steel Framing” in Fire Engineering, January 2008)

(3) 

Photo 3 shows the underside of a steel roof deck that was installed in the 1960s, before the steel roof deck standards were developed. The steel bar joists are on eight-foot (2.44-m) centers, and the end laps of the roof deck sheets are visible between the bar joists, with screws at each rib in an attempt to make the joint between the cantilevered sheets more stable. No screws or pins are visible attaching these sheets of roof decking to the bar joists, indicating that they are either attached by spot welds (concealed by paint) or unattached. Because of the lap joints between the bar joists, this roof deck has less strength than those in Photos 1 and 2 and is more likely to be unstable if there is a fire below it. There are thousands of buildings still in use with roofs installed before the steel roof deck standards were developed. It is likely that your response area includes several. 

Because of the potential for creating cantilevered roof deck panels, we must be very cautious when ventilating these types of roof. It is best to work from the platform on a tower ladder or from some other stable structure that is independent of the roof. Working from a ladder laid flat on the roof will probably provide little safety margin, since there is no assurance that we will be working from a spot on the ladder that is supported by at least two bar joists. A roof ladder will provide a false sense of security on a pitched roof, since there usually is no ridge board connecting the lightweight steel trusses that support the roof.

 

Gregory Havel is a member of the Burlington (WI) Fire Department; a retired deputy chief and training officer; and a 30-year veteran of the fire service. He is a Wisconsin-certified fire instructor II and fire officer II, 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. 

Subjects: Building construction for firefighters 

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