Article and photos by Gregory Havel
Unless you live in a part of the world where heavy snowfalls are frequent, the roofs on your buildings are not designed with the live load-bearing ability of floors.
Floors in residential buildings are often designed for a live load of 50 to 60 pounds per square foot (244 to 293 kg/m
2). Floors in public buildings are often designed for a live load of 100 or more pounds per square foot (psf) (488 or more kg/m
2). Floors in factories and industrial buildings are often designed for a live load of 200 or more pounds psf (976 or more kg/m
2), to support manufacturing machinery, raw materials, and finished product. A
safety factor is included to account for weak or deteriorated construction components.
Roofs are usually designed to support their own weight (dead load) and the weight of any permanently attached heating, ventilating, and air
-conditioning machinery (also dead load), plus a live load that includes accumulated snow or rain and pressure from the wind, plus a safety factor to account for weak or deteriorated construction components. Both the dead- and live-load figures are usually included on building blueprints. On several recent sets of building plans in southeastern Wisconsin, this was stated as “Dead load 8 pounds psf [39 kg/m2],live (snow) load 30 pounds psf [136 kg/m2].” The live (snow) load requirements are calculated for snowfalls that are heavier than the norm but not for the exceptionally heavy snowfalls that occur only occasionally. Such extreme winter weather results in roof collapses that become part of the national news.
(1)
Photo 1 shows a team of firefighters practicing roof ventilation skills on a house built in the 1960s that was scheduled for demolition. This house was built of 2 × 4 wood studs, with a roof of plywood sheets supported by 2 × 8 wood rafters with a ridge board. This roof probably was designed for more than a live load of 30 pounds psf [136 kg/m2] but less than the designed live load of the floors.
The kneeling position used by the firefighters in photo 1 is common when making openings in roofs or floors. Measurements taken during classroom and field training show that the points of contact of the boot toe and the knee on the same leg will be within 24 inches (61 cm) and that the knees or toes will usually be 12-16 inches (30-41 cm) apart. This is true even of tall firefighters.
This kneeling position distributes the firefighter’s weight between four contact points within an area of about four square feet (0.37 m2). An average-sized firefighter would be six feet tall (1.829 m), weigh 200 pounds (91 kg), be wearing personal protective equipment and self-contained breathing apparatus weighing 50 pounds (23 kg), and carrying tools weighing 20 pounds (9 kg) for a total weight of 270 pounds (123 kg).
This average-sized firefighter, when kneeling, will impose a live load on the roof at that point of 67.5 pounds psf. When standing, this firefighter will impose a live load on the roof at that point of 135 pounds psf. The roof shown in photo 1 does not collapse under this overload, since it is local and not evenly distributed and because the plywood sheets of the roof deck and their connections to the rafters distribute this overload so that it is carried across a larger area of the roof.
If the firefighters in photo 1 were not working from a roof ladder and had fire in the attic space below them, their combined weight would not be distributed over as large an area, and the roof would be likely to collapse locally, possibly taking a firefighter through a small hole.
(2)
The roof shown in photo 2 is of 21st-century design: “dead load 8 pounds psf [39 kg/m2], live (snow) load 30 pounds psf [136 kg/m2].” The lightweight steel roof trusses are on four-foot centers and are fabricated from cold-formed galvanized sheet steel (like steel studs) that depend on their shape to remain rigid. The roof decking is 19/32-inch (15 mm) thick oriented strand board (OSB) attached with screws. What appears to be a galvanized steel angle at the ridge is simply a piece of sheet metal placed there to keep the plastic ridge vent from sagging.
The roof shown in photo 2 is not part of a fire-rated (one-hour or two-hour) roof-ceiling assembly. A fire-rated roof-ceiling assembly will have more mass and more closely spaced rafters, joists, or trusses to achieve its rating. The construction methods and manufactured materials used would be similar to those in the non-rated assembly shown in photo 2. A fire-rated roof-ceiling assembly will not be labeled except on the building plans and will appear the same as a non-rated assembly when viewed from above or below.
If firefighters are on the roof shown in photo 2, they will be working under unacceptably high-risk conditions because of the wide spacing of the roof trusses, the lack of a structural ridge for use with a roof ladder, and the record of this type of roof collapsing in a large area (globally) from a local overload (like a team of firefighters) or from heating and burning by fire below.
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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, 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.
