Perhaps the most obvious and essential function of the building envelope is to keep water out. The roof is the first line of defense in that effort.
Today, roofs do more than simply keep the rain out. Roofing systems can be, or in some cases must be, white, green, or blue. The color code? White means solar reflective; green means vegetated; and blue intentionally supports retained water.
Properly designed, a roofing system can be an environmental asset supporting energy conservation, clean air, and groundwater management. Improperly designed or installed, a roof can fail at its most basic mission.
Generally, in roof construction, a waterproof layer (e.g. membrane, slate, sheet metal) resides over the underlying supporting structure of the roof.
In NYC, we’re typically talking about re-roofing: removing an existing roofing surface that’s outlived its function and replacing it with a new system.
Over the past 50 years, numerous “low-slope” (flat) roofing systems and materials have come to (and in some cases disappeared from) the market.
Today’s systems can exclude water so reliably that manufacturers offer 20-year, no-dollar-limit guarantees. They include:
Varieties of polymer-modified asphalt (APP, SBS)
Liquid-applied resins saturating a polyester or fiberglass scrim
Roof membrane systems can be single, double, or triple ply, or a hybrid of materials. There are multiple methods (e.g. torch, solvent) to adhere the membrane to the underlying substrate, and to seal the seams between membrane sheets.
Which system is best? There's no simple answer. The decision depends on a multitude of factors—which is why you need an E/A firm to help you sort through and decide. Among them:
The slope of the roof. Generally speaking, the more slope, the better. For low-slope roofs, the recommended minimum pitch is 1/8” of height per foot of length. Membrane systems designed for low-slope roofs don’t work for more steeply pitched roofs. Steeply sloped roofs are typically clad in shingles or metal panels.
Presence of roof “discontinuities.” Roofs rarely fail in the “field” of the membrane. Failures typically occur at discontinuities—where the membrane turns a corner or is interrupted by a penetration (e.g. a chimney or a pedestal supporting an air conditioning unit).
Roofs receive special treatment at these discontinuities. “Flashing” is an impervious sheet material (e.g. sheet metal) that reinforces the membrane where the roof changes material or direction—for example, where the membrane terminates at the face of a parapet wall.
Certain liquid-applied membranes can also function as flashing–a major advantage on complex roofs with numerous penetrations. But on large, simple roof expanses, these advantages can be offset by the higher cost of the material.
Roof size. Depending upon conditions and complexity (e.g., number of penetrations), on average, a “new” roof can cost between $30 to $150 per square foot. A small footprint with many penetrations (e.g. a hospital or laboratory) can be a more challenging installation than a straightforward roof over a two-acre warehouse.
Environmental exposure. Certain materials are damaged by exposure to UV radiation or cyclical temperatures. In some geographic locations, wind-induced uplift can separate a membrane from its substrate. How will it be fastened or adhered?
Installation logistics. Can the rooftop be easily accessed? Will it be difficult to hoist materials and equipment to the work zone? Will the installation occur in very cold or very hot weather?
Sensitivity to odor and VOCs. Certain solvents, materials, and processes can create objectionable, though not necessarily toxic, odors.
Substrate characteristics. Roofs are best installed over a “clean” substrate, not over existing layers of roofing. How many existing roofing layers need to be removed and discarded? Will the substrate itself require reinforcement once removal of existing roofing exposes it to view? Are there existing pavers in the “sandwich” to be removed? Does the material to be removed contain asbestos?
Insulation requirements. Roof systems insulate the building, reducing heat loss during winter or heat gain in summer. It’s typically accomplished via a layer of rigid insulation installed beneath the membrane.
Insulation requirements have become increasingly stringent over the past decade. Increased insulation value means thicker insulation. Thicker insulation raises the top surface of the finished roof, often reducing effective parapet height, requiring costly extension of the parapets.
Luckily, the innovative vacuum-insulated panel (VIP) has been transformational. A VIP comprises a vacuum (an excellent insulator) trapped between layers of metal foil, with a spacer to maintain separation between foil layers. A VIP is much thinner than conventional rigid board with the equivalent insulation value. (Incidentally, Superstructures was the first E/A firm to employ VIPs on a NYC project).
Traffic and use. Will the roof be covered with a paver system–either placed directly on the membrane, or pedestal supported? Will maintenance personnel need a well-traveled path between the doorway and rooftop equipment?
“Color” selection: A green roof will cost more to install and maintain than a white or blue roof.
Reliability and longevity. What’s in the space directly beneath the roof? Leaks are never desirable, but if the upper floor houses a bio lab, priceless art, or the office of the University president, roof reliability becomes the dominant consideration. (Incidentally, Superstructures has designed roof replacements for the Museum of Modern Art, the president of Yale, and numerous laboratories).
Budget is always a concern. Realistically, once the foregoing issues are considered, there might only be a single suitable system or approach. On occasion, multiple systems may be used on a single building to capitalize on the strengths of each. In all cases, the best avenue for cost savings is a tight set of contract drawings and specifications that facilitates competitive bidding among qualified contractors and manufacturers.
A roof is not an “off the shelf” commodity. There’s a wide array of roofing systems and materials to fit their building’s needs and budget; that’s where an E/A firm experienced in designing the optimal roof for each application—along with an experienced contractor—is essential to success and longevity.
