There is no single roofing material or installation system that is best in all situations. What is most important is to select the most appropriate whole roof assembly (substrate, insulation, membrane, ballast, etc.) as approved by the manufacturer of the roof membrane, and to have it installed by a contractor approved or licensed by the manufacturer.
SUBSTRATES : Wood and steel are easy to fasten to, yet most subject to damage from leaks. Concrete is very stable but more difficult to fasten to.
DRAINAGE : Roof system must have at least 1/4″ per foot slope. If internal drains rather than gutters are used, each roof panel must have an overflow drain. It is important to direct the overflow water to a place that will be noticed by maintenance staff, who will then take action to clear the primary drain. Perhaps the best place for an overflow drain to spill is just over the window of the principal’s office. Make sure that all metal components of a roof system (flashing, gutters, drains, pipes, etc.) are of compatible metals (no combination of galvanized steel with copper, for instance), to avoid galvanic corrosion. It is not adequate to simply isolate these materials, as water passing from one to the other is sufficient to cause corrosion in our wet climate.
INSULATION : Rigid and batt insulation come in many types with various R-ratings (thermal resistance). When re-roofing, it is always wise to consider improving the insulation level of the roof. It is even more important to select an insulation material that has the necessary compressive strength if installed over the roof deck (to protect against damage from walking, etc.), durability (when exposed to high temperatures, and so forth), and fire resistance (if appropriate) to work in your roof assembly. Not only is it important to select an insulation that matches the roofing material and installation system, it is equally important to ensure that it is installed according to the roofing manufacturer’s requirements (such as joint staggering, material layering, etc.). Remember that slope can be added to a roof by using tapered rigid insulation. If designing a new roof, you can save money by designing most of the slope into the structure and only using tapered insulation or other means to direct the roof runoff to the drains.
ROOF MEMBRANE MATERIALS:
Built up roof (BUR), the traditional “hot tar” roof, still works well if properly detailed and installed. Installation details such as temperature of asphalt are of critical importance, so you may want a manufacturer’s warranty on the entire installation (thereby requiring a manufacturer-licensed installer). Pick a cap sheet of light color in order to keep the temperature of the roofing assembly lower on hot days.
Modified bitumen (mod bit) is similar in material to BUR, but is applied in a single pre-manufactured sheet, often through a “torch-down” application process. It avoids the problem of asphalt temperature, but other installation details are very important, so contractor selection is still of utmost importance.
Rubber roofs (EPDM, Hypalon, and other synthetic rubber materials) are applied in large flexible sheets that are usually seamed with a solvent cement. They can be glued to the substrate (fully adhered), held by special fasteners, or covered with a layer of gravel or concrete pavers to prevent wind uplift. The installation system should be designed for the structural substrate, the strongest anticipated winds in your area, and other factors. Avoid using small gravel for ballast as strong winds can pick it up and blow it through the windows of the buildings down wind.
Poly Vinyl Chloride (PVC or plastic roofs) come in rigid sheets that are cemented together. PVC roofs and flashings must be carefully detailed to handle thermal expansion and contraction. Early PVC roofs failed from degradation from ultraviolet light, but manufacturers have changed the formula in the plastic to increase UV resistance.
Foam – and flow-on roof systems are fairly easy to install but generally difficult to repair. They have not attracted a wide following.
Metal roofs can be manufactured in many materials, including aluminum, copper, stainless steel, and coated steel. They are usually installed in batten-seam or standing seam configurations, as shingles, or else as preformed rolled sheets held by fasteners drilled through the sheet. Properly installed with the most durable materials (such as a copper standing seam roof), they can last for generations. On the other hand, if not properly installed (such as mixing incompatible metals to create galvanic corrosion) or if of inferior materials, they will be neither reliable nor durable. Metal roofs are usually installed at steeper pitches than the other roof systems listed above. Attention to expansion and contraction is essential.
Clay tile and glazed concrete tile roofs are durable and attractive. If properly installed, clay tile can last a lifetime. Less is known about concrete tile, as it is newer. Tile roofs are heavy, and structures must be adequate to hold them. They also must be detailed to resist the winds anticipated in your area.
Composition roofing: This roof material, which is used on most homes in this country, is simple and economical to install onto a wood substrate. Although not as long lived as some other roofs (15 to 20 years), it is fairly easy to inspect and repair and inexpensive to replace. In the maritime Northwest, consider installing zinc strips at the ridge to prevent moss growth or plan for the need for chemical treatment to remove moss in shady areas.
Wood shingle and shake: Cedar and redwood shingle and shake roofs are seldom used in schools. Unless treated with a fire retardant they present a significant fire hazard. Recent materials appear to not provide an effective life cycle cost. If used, they should be well ventilated underneath with an attic.
It is vitally important to design flashing to be appropriate to the various substrates and roofing materials and to easily allow re-roofing and repair as well as keeping the water out. This usually means using a two-part system of flashing and counter flashing that can be taken apart. Avoid reliance on caulking compounds for waterproofing. The flashing and roofing should have integrity against water (including wind-blown water), with the caulking seal, if used, providing a second layer of protection. Make sure that all long runs (over 20 feet) of flashing have provisions for thermal expansion and contractions.
Warranties should be from the manufacturer for materials and installation on all but the simplest roof systems. Roofing contractors come and go, and, given public bidding requirements, you will probably have little or no choice in contractors.
5. Maintenance and Inspection:
Walk only on roof areas designed for walking. The walkways should have a surface that protects the roofing material and spreads out the load of the foot over a larger area. Concrete pavers are sometimes used for this purpose on relatively flat roofs.
Keep a record of all roof repairs (which will help tell you that a roof needs major help).
Don’t do anything except in emergencies (repairs, penetrations, remodels) on a roof that is still on warranty without the written permission of the manufacturer or you will probably void the warranty.
Inspect each roof at least once per year.
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