Article and photos by Gregory Havel
We have all been warned about the dangers of roofs that feel “spongy” when we are sent up to ventilate above a fire. This warning comes from long experience with roofs constructed with boards supported by wood joists or rafters on 16-, 18-, or 24-inch centers. If the roof felt spongy when we stepped onto it, it was either in danger of burning through from below, or too rotten to support our weight.
In today’s construction, this is no longer a reliable standard, since a brand-new roof on a brand-new building often feels spongy even without fire below. These roofs are designed using computer models that tell the architect and engineer exactly what weight of roof and supporting system (dead load) will support the anticipated live (snow or standing water) load, or is required to comply with the building code. Although these models and formulas include a safety factor, this is less than the safety factor estimated in the designing and constructing a roof of wood joists and deck boards.
Photo 1 shows a roof under construction at a big-box” store in southeastern Wisconsin. The steel roof deck is visible at the upper left, with lines of spot-weld burn marks showing the location of the steel bar joists below-spaced on 96-inch (8-foot) (2.44-meter) centers. Two layers of 3-inch (7.6 cm) thick polystyrene insulation board faced with roofing felt will be installed on top of the steel deck. This will be covered with a rubber roof membrane, and held in place by loose gravel ballast (photo 2).
The building’s blueprints state that the design for this roof is “30 pounds per square foot live (snow) load.” The combined weight of bar joists, steel deck, insulation board, roof membrane, and gravel ballast (dead load) is calculated at 17 pounds per square foot. When walking across this roof, it feels spongy underfoot. When standing still, the roof can be felt moving up and down as others walk across it two bar-joist bays away. This roof is not a floor, and we must never assume that it will support us like a floor.
A service person with a toolbox on this roof to repair heating and air-conditioning units is probably not a problem, since the roof deck and bar joists are interconnected as a system, and can accommodate short-term local overloads.
However, a team of firefighters on this roof will be a problem. Average-sized firefighters in full personal protective equipment weigh between 200 and 250 pounds (91 to114 kg) each, not including the tools they carry. Firefighters also tend to stay close together, which makes them a significant local overload on a span of roof deck between widely-spaced bar joists, or on top of a single bar joist.
According to the old teaching, when we are on a roof for vertical ventilation and it feels spongy underfoot, we need to get off before it collapses under us.
Today, our building prefire plans should show the type of roof and its supports. If the roof feels spongy underfoot with no fire in the building, the prefire plan should tell us to keep off the roof for ventilation.
Subjects: Building construction for firefigher operations, construction hazards, roof loads