Back to Basics: December 2011
By Mark van der Feyst
By Mark van der Feyst
We have been looking at ventilation in relation to truck-company operations. Going to the roof to conduct vertical ventilation is not always an option as sometimes rooftop obstructions prevent firefighters from accessing the roof.
We have been looking at ventilation in relation to truck-company operations. Going to the roof to conduct vertical ventilation is not always an option as sometimes rooftop obstructions prevent firefighters from accessing the roof. Obstructions used to be found mainly on commercial and industrial buildings. Now we see similar and new types of obstructions on homes, as people become more environmentally aware.
Adding solar panels to the roof of residential structures adds to the
dead load on the roof. This, combined with a live energy source,
presents a risk to firefighters performing vertical ventilation.
The weight of these new types of obstructions prevents firefighters from conducting vertical ventilation. As the fire develops and grows, it weakens the roof support members, which can lead to a roof collapse. Incident commanders and front-line officers reading the conditions will recognize the signs of collapse and will prohibit any roof work.
HVAC systems and water tanks are common obstructions found on the roofs of commercial, industrial and business structures. These obstructions are factored into the construction of the building for structural stability. However, when there is fire present, the structural members will be weakened and the collapse of the roof will be accelerated because of the added weight of HVAC units and water tanks.
Solar panels and rooftop gardens are becoming more popular to produce energy and reduce carbon footprints as people become more environmentally conscious. These types of obstructions are common on the rooftops of industrial, commercial and business buildings, but they are becoming more prevalent on residential roofs too. Government programs that pay customers for energy production into the hydro grid are an incentive for homeowners to install solar-panel systems on their rooftops.
|Photo 2: The
nursing home shown here is not a likely candidate for vertical
ventilation as the entire roof is covered in solar panels.
A variation of the solar panel is the solar thermal panel. This type of panel is used to heat water using the sun’s heat. Homeowners install these types of systems to heat pool water, and commercial buildings, such as aquatic centres, also use them.
Solar grids vary in size depending on the size of the roof and the amount of energy a homeowner wants to produce. The roof of the house in photo 1 has two sides covered with 21 solar panels, and there are plans to cover the other two sides with 21 more. The number of sides covered is dictated by the available space and the path of travel of the sun on the house. In photo 2, the roof of a nursing home is covered in solar panels. In both cases, the solar panels limit the space available for vertical ventilation.
There are two dangers to firefighters attempting to ventilate from a rooftop: the added weight of the solar panels and a live energy source. In residential structures that use lightweight truss construction, adding solar panels to the roof increases the dead load on the roof. This extra weight is probably not accounted for in the specs for lightweight roof construction. Typically, solar panels weigh about 50 pounds each, not including the weight of the hardware and installation systems used. If a thermal panel system is being used, there is even more weight due to the water that travels through the grid. The timeframe for a lightweight truss to fail is between five and 15 minutes. With a solar-panel system on the roof of a residential structure, there will be less time to failure, and other methods of ventilation may have to be used.
|Photo 3: Rooftop gardens, which are becoming more prevalent, add considerable weight to rooftops; adding fire will cause the roof to collapse more quickly.|
The live energy source is the second hazard that firefighters may face. A solar-grid tie system used for power generation works with all of the panels strung together to produce the maximum amount of power. Each panel produces 24 to 48 volts. When tied together, the voltage can range from 120 to 400 volts. If a firefighter were to cut a vent hole through the solar panel with an axe or chainsaw, that firefighter will feel the full voltage being produced within that grid system. If vertical ventilation is required, it is best to relocate to a spot where the roof can be cut.
Rooftop gardens (see photo 3) are popular in many inner-city dwellings and apartment buildings. These gardens come in a variety of layouts and are mostly found on flat roofs, but can be installed on sloped rooftops. In Switzerland, builders are required to replace the green space taken by the building with a green space of equal size on the roof. This law includes existing buildings and all types of rooftops. A school in France is covered by a tent-type structure with a layer of grass on top. It performs double duty, providing a green footprint and holding the tent in place. Sod, water and plants to grow food add considerable weight to the roof. Most installation manuals for rooftop gardens advise homeowners to seek a structural engineer to determine if their roof can handle the added weight. Adding fire to the scenario will certainly lead to a quicker collapse of the roof.
When sizing up and looking at the outside of a residential or commercial building, look up to see what rooftop obstructions are present and, if necessary, reconsider what rooftop operations will be conducted (if any at all) and consider how this may affect interior operations.
Mark van der Feyst is a 12-year veteran of the fire service. He currently works for the City of Woodstock Fire Department in Ontario. Mark is an international instructor teaching in Canada, the United States and India. He is a local-level suppression instructor for the Pennsylvania State Fire Academy, an instructor for the Justice Institute of British Columbia and a professor of fire science at Lambton College. E-mail him at Mark@FireStarTraining.com