Decisions, Decisions: Your School's Roof
- By Ken Richardson
- October 1st, 1999
It was no surprise when a recent Request for Proposal (RFP) for the design of a new middle school asked prospective architects to address questions about roofing systems. Roofing integrity is a concern for most educational clients. Here's what we would have liked our proposal to convey, if there had been room.
The life expectancy of a roofing system depends upon many factors, including proper design, construction, and maintenance. Administrators may think nothing of replacing the carpeting in a building every 10 years, but they balk at replacing the roofing system every 20 or 30 years. Carpets are cleaned and shampooed regularly, but roofs go unmaintained for years at a time. Lack of maintenance and reluctance to replace at the end of a life cycle increase the importance of roof design and construction.
The Scoop on Slope
The primary factor in roof design is roof slope. At one end of the spectrum is a "flat roof" with minimal pitch; at the other end is a roof with "ski slope" pitch. Most educational facility roofing systems fall between these two extremes. The amount of roof pitch, (measured in inches of rise per foot of run (in./ft.), will affect a number of building conditions.
The roofing pitch can affect the interior volume of a building. Start with a hypothetical 80-ft.-wide gymnasium or classroom wing, and assume that the roof rises to a peak at the center; then visualize what variations in interior volume the roof pitch causes. At a minimal 1/4-inch per foot slope, the roof will rise from a "zero point" at the eaves to a 10-inch height at the center. Compare this to a roof that has a 4-in-12 pitch (4-inch rise in 12-inch run), which will have a 13-foot rise at the ridge. In a classroom space with a 10-foot ceiling, this rise will increase the interior volume of that classroom by over 50 percent. If this additional volume is to be heated or cooled, the roof rise will add to long-term utility costs.
Which Materials Are Best?
Roof slope helps determine the appropriate materials for the roof. A roof with a 4-in-12 pitch will allow products such as shingles or tiles that will not do on a low-slope roof. Conversely, a single-ply membrane or a built-up roof will not be appropriate for a high-slope roof.
Building form also influences appropriate roof materials. A complex building plan with many level changes, penetrations, and parapets will make a shingle, tile, or metal roof more complicated to install and difficult to maintain in a watertight condition. These materials are more appropriate for simple forms. Membranes such as plastic-based single-ply that can be formed are more appropriate for complex building types.
Exterior environmental conditions will have a significant influence on the type of roofing system. In cold climates, the dangers of ice dams and water back-up through shingle systems must be addressed. High wind areas may require systems and applications that prevent blow-offs.
Methods of handling water or snow runoff are a major consideration in schools. Care should be taken in directing runoff away from pedestrians and walkways. If the water is to be collected in a downspout or gutter system in a cold climate, consider how to keep these secondary dispersal channels open and free of ice. Metal roofing systems in areas with lightning storms may need lightning protection. Hail can severely damage the coatings on a polyurethane foam roof or an asphalt shingle roof, but not damage a single-ply membrane.
Interior usages and conditions may influence selection of roof systems as well. A roof system suitable for a classroom wing may not be suitable for a swimming pool because of humidity. If the facility has a large freezer or cooler space without box containment, the roofing system over these areas may be entirely different that over the kitchen. If the building is to be pressurized by air conditioning systems or will have large openings and a perforated acoustical deck, special considerations must be given to roofing securement. Airborne materials such as cooking grease from the kitchen or fumes from the chemistry lab may create additional complications for certain types of roofing systems.
Fire codes will affect roofing material choices. Most model codes for educational facilities require at least a minimal resistance to surface flame spread. Although most roofing systems provide this resistance, wood shingles or shakes must be treated with flame-retardant materials to bring them into compliance. Materials that the roofing system is applied over may also be affected by code considerations. Using certain foam plastics directly on metal decking can result in a serious melting and flowing of the insulation if there is an internal fire. Certain roofing materials are most efficiently applied over a plywood or wood board deck; the use of the wood may result in lower code classifications and perhaps higher insurance rates.
What Else Should I Consider?
Aesthetic considerations may also play a role in selection of roofing systems. Materials such as asphalt shingles, wood shakes, metal, or tiles will generally result in a more residential scale and will allow more color, texture, and interest in the roof.
Maintenance, repair, and ease of determining specific leak locations (heaven forbid!) may be secondary considerations in selecting a roof system. If the only qualified personnel are three states and seven days away, a roof leak can cause a considerable amount of interior damage. Conversely, if the district's maintenance personnel can perform emergency repairs with a minimum of expertise and tools, then that system may be a better choice for a remote site. Considerations such as energy conservation, roof overhangs for shading, skylighting, and heating and cooling needs will also affect the selection of roof systems and the materials within the systems such as insulation or vapor barriers.
If the roof surface will be home to HVAC equipment, consider how that equipment will be serviced and repaired. It may be physically impossible to access a rooftop heating unit in the dead of winter if the roof is covered with ice or snow. Roof slopes above 2-in-12 can be particularly treacherous under conditions of frost, ice, rain, or snow. Certain single-ply membranes can be very slippery down to 1-inch per foot slope.
Access for Service
Consider placing walkways and walkpads for rooftop equipment service or relocating that equipment off the roof. Unless the roof surface is tough, (steel, clay tile, heavy wood shakes), maintenance personnel should be cautioned about dropping sharp, pointed tools. Many otherwise leak-free roof systems have been perforated by tools or carelessly dropped sheet metal scraps; this is particularly true of unballasted single-ply membrane systems.
When recommending specific manufacturers of roofing products, the architect may rely upon personal experiences and ask:
- Has a manufacturer altered the chemistry of their product to save money?
- What attitude does the manufacturer have concerning adjustment of the cost of repairs under warranty?
- Does the manufacturer have factory-trained technicians, inspectors, or training programs for their applicators?
- How well do the manufacturers "police" their authorized installers (if they have them), or will they sell products to anyone with a check?
- What type of support does the manufacturer offer the designer? Are there skilled technicians, chemists, or engineers to consult with the designer on special applications? Does the manufacturer offer to review and critique specific details?
- Will the company be able to stand behind replacement of an entire roof surface should it prove to be defective?
There is no ideal roofing system for every condition and application; each system will offer advantages and disadvantages. It is important for school boards and administrators to be aware of the choices available and be able to discuss those choices with their building designers. Most architects will welcome an opportunity to discuss any aspect of school construction.