Top Job

Dec. 1, 1997
Many things will affect the choice of a roof for a new or remodeled school. Some are obvious, such as the initial cost of the roof; some are obscure,

Many things will affect the choice of a roof for a new or remodeled school. Some are obvious, such as the initial cost of the roof; some are obscure, such as the insurance requirements of the building. Although roofing is only about 3 percent of the cost of a new facility, water penetration into buildings represents more than half of the damage claims filed in the construction industry, so a roofing choice must be made with care.

Making the choice The slope of a roof affects the type of roofing to be used. Low-slope roofs (nearly flat) generally are of built-up- or single-ply-membrane construction. High-slope roofs typically are of metal, tile or shingle construction. There is an in-between zone that is too steep for built-up roofs and too flat for metal, tile or shingle roofs. For these intermediate-slope roofs, single-ply-membrane roofs are a suitable solution. Single-ply roofing membranes can be used on all roof slopes, as demonstrated by domed stadiums around the country. Other types of roofing used include liquid- and spray-applied-coating systems.

The most common roof type for education institutions is low-slope roofs. This is due to the nature of institutional buildings, which generally have large floor areas and long wall dimensions. A sloped roof adds height to a building, so the flatter roofs help reduce construction costs by reducing the height of a building. High-sloped roofs are used on special-purpose or feature buildings, such as athletic centers.

There are different kinds of low-slope roofing systems. The traditional built-up roof consists of several layers of felts and asphalt or coal-tar bitumen, which together form a reinforced, waterproof membrane. In recent years, a variety of single-ply membranes have entered the market. These include vulcanized elastomeric sheets (EPDM), non-vulcanized elastomeric sheets (CSPE) and thermoplastic sheets (PVC).

Single-ply membranes rely on carefully adhered or welded edge seams for waterproofing. A variation of a single-ply roofing system adapts built-up roofing technology, using rubber-modified asphalt with felts, hot-mopped or torch-applied. Hot-mopped systems use styrene-butadiene-styrene (SBS) rubber-modified asphalt and flashing sheets. Torch-applied systems (which use atatic polypropylene (APP) modified asphalt) are not recommended for institutional buildings due to the risk of fire during installation. Another variation of a single-ply membrane is an all-metal system, riveted and sealed into a single sheet that is attached to hat channels, as well as the edges of the roof.

Additional considerations The selection of one of these roofing systems is dependent upon additional considerations. The roofing membrane must be compatible with the roof deck and roof-deck insulation used. If the roof is a fire-rated assembly, the materials must pass a fire test.

The roofing membrane and insulation must withstand wind-uplift pressures. Certain roofing types are more suited for roofs where rooftop mechanical units are planned, or where contamination from cooking grease or oils may be present. A higher initial-cost roofing system may be desirable if the institution's insurance rates will be reduced over the long run. Finally, do not overlook the institution's capability to maintain the roof. Some types of roofing systems require special training, or only can be patched or repaired by an approved applicator.

Keep in mind that different regions of the country tend to favor different roofing types. High-slope roofs are more likely to be used in heavy snow climates. In the desert Southwest, high temperatures on the roof surface mean that asphalts with higher-level softening points should be used. Reflective roofing surfaces also are appropriate in dry or humid hot climates. In areas that experience wide temperature ranges from summer to winter, the roofing system and flashing must be detailed to accommodate thermal movement.

The choice of roof insulation also is critical to the success of a roof. The insulation can be on the roof deck, directly below the membrane or in the ceiling space below the roof, or all three. Some factors influencing the choice of roof insulation include: *Its compatibility with the roofing membrane. *The fire-protection requirements for the roof. *The amount of insulation capacity needed. *The wind-uplift requirements. *Humidity. *The construction of the roof deck.

A roof for the Gulf Coast will be constructed very differently from a roof in the desert Southwest. The function of the building also may be a factor--buildings that are humid, such as natatoriums and food-service facilities, usually require a vapor retarder in addition to the roof membrane. The placement of the vapor retarder will depend on the roof insulation and the climate.

Roof inspections A well-maintained roof can prolong the life expectancy of any roof and prevent damage to the building equipment, inventory and furnishings. An important part of the maintenance program is periodic roof inspections. A history of the roof--type of roof deck, insulation material, roof membrane and flashing--should be maintained for easy reference. The records should include the name of the contractor and the roofing subcontractor, the roof warranty, a copy of the last roof inspection and the remediation repairs performed.

Review the records and history of the roof before inspection. Then, interview maintenance personnel and building users to discuss any problems experienced since the last inspection. Next, inspect the perimeter of the building and the interior for building settlement and cracks in the walls. Look for evidence of water stains or rusting at penetrations and the underside of the deck. Look for water damage in the ceiling tile, spalling of the plaster and blistering of the paint.Once on the roof, the field of the r oof should be inspected for blisters, splitting, fishmouths, slippage and alligatoring. On single-ply membranes, check for punctures and inspect seams for proper bonding. Check the roof penetrations to make sure the membrane properly overlaps the top of the curb and that the counterflashing is positioned to prevent capillary action. Check roof drains and overflow scuppers, and make sure drains are not clogged with debris. Check that the metal flashing and roof edging is fastened securely and the joints are sealed properly with caulk. Look for splits in the base membrane at the joints of the metal flashing. Look at the condition of the base membrane flashing for degradation of the felt material.

If any of the above problems are evident, then further investigation will be necessary, such as cutting into the roof to determine if the insulation is wet. Another way to determine if the insulation is wet is by thermal imaging. For single-ply roofs under warranty, notify the roofing manufacturer so as not to void the warranty.

After the extent of damage is determined, a plan of action can be formulated, such as patching the damaged area, reroofing or total tearing-off, including the insulation and possibly any damaged deck. To maintain a good, watertight roof for many years, plan and implement a maintenance program, keep good records, maintain good drainage and control roof traffic.

Special considerations A different kind of trouble spot is a vapor retarder. When used in a humid building in a cold climate, the vapor retarder may collect condensation at an alarming rate, which can degrade the roofing system and damage the building interior. The design of a roof assembly with a vapor retarder requires study of the dew point within an insulated assembly. The installation of the vapor retarder must be accurate and complete, with all seams taped carefully. Expert advice is helpful for special facilities.

Reroofing also requires special consideration. A vapor retarder can be created inadvertently when a new roof membrane is installed over an existing roof. Building codes typically limit the number of times a roof can be reroofed before the entire roof must be removed, to prevent overloading the roof structure of a building. The existing roof must be inspected before reroofing--trapped moisture in an existing roof also will ruin a reroof. On older roofs, the felts and mastic may contain asbestos. If asbestos is suspected, the roofing materials should be tested before being patched or removed.

Roof warranties Warranties must be reviewed carefully; the useful information is in what is excluded from the warranty, which can be most everything.

Warranties usually are limited to replacement of roofing materials only, for a limited number of causes. Some warranties even may reduce the institution's right of recovery for damages under statutes of limitations. Most building owners do not realize that a careless repair of a roof under warranty may void a roof warranty.

In recent years, 10-, 15- or 20-year "no dollar limit" warranties have become popular. Roofing manufacturers are willing to offer these, as they include the costs of these premium warranties in the initial cost of the roofing material (as much as 1 percent of the cost). Even these warranties are of no value if the original roofing manufacturer or installer goes out of business during the warranty period. The backing of an established manufacturer, coupled with installation by a contractor trained and certified by the manufacturer, is the best way to a good warranty.

There are ways besides the use of warranties to assure a quality roof installation. The institution should consider specifying that the manufacturer have a representative present during the roofing installation, to assure that the manufacturer's instructions are followed. This is particularly important with the more sophisticated single-ply-membrane roofing systems. The institution also should consider hiring an architect or a certified roofing consultant to observe the installation of the roofing system. The independent observer is not supervised by the manufacturer, and can provide an objective assessment of the quality of the installation.

The new Desert Ridge Middle School, Albuquerque, N.M., features three roofing systems.

The 107,000-square-foot facility, constructed for Albuquerque Public Schools, is situated in a residential neighborhood of single-family, Mediterranean-style homes with red tile roofs. The school is a one-story building, which uses the colors of the adjacent homes to tie it into the neighborhood.

To echo the tile roof colors, a metal roof system was used at the main entries and lobby of the school. A second metal roof system, a sheet system designed for low-slope roofs, was used over the gymnasium, where the lack of roof penetrations or rooftop mechanical units made this system a suitable choice.

The majority of the new school's roofing is a built-up roofing system, with SBS-modified asphalt, and a mineral cap sheet. This system is used widely by the school district due to its suitability for Albuquerque's extreme winter-to-summer temperature variations, the ability to install the roof in conjunction with rooftop mechanical units, the durability of the roof under foot traffic, and the ease with which the roof can be patched or repaired.

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