Images courtesy of Roof Products, Inc. (RPI)

When it comes to roofing, selecting a sustainable strategy means designing a roof that will endure with minimal repair - and that means a watertight roof. Roofs can be sabotaged by leaks which can result from many factors including accumulated water or snow, punctures caused by flying debris, deterioration of the roofing material, and faults in the flashing around chimneys, vents, fans and the skylights. For those roofs that do need flashings and curbs, another cause of leaky roofs are improperly manufactured or installed roof curbs, the square metal boxes that surround penetrations to assure that the roof remains watertight.

A big question here is who is responsible for roof curbs. Equipment manufacturers do fabricate roof curbs, but they don't design them with leak prevention in mind. They concentrate on supporting the unit they manufacture, and often they don't have the knowledge of proper roofing practices or the requirements of a particular roof. Usually, their standard roof curbs do not mate with the many kinds of roof deck situations being specified. Improper curb design for rooftop HVAC units, exhaust fans and other equipment can all mean roof leaks down the line.

Leaving the job of building curbs to on-the-job contractors is also problematic. Contractors typically build curbs to roof-opening size, not to the size of the actual equipment . Many times scrap material available on the job site is used to build the curbs. When this happens, the equipment being mounted on the job-built curb can be larger than the opening, leaving a gap between curb and equipment cap - and an opportunity for water to seep in. Using tar to meld it all together can result in further problems down the line as tar can crack in winter and melt in summer, opening the way for leakage.

In many instances, custom prefabricated roof curbs offer a more effective alternative. These curbs are fabricated to fit the exact units selected after bid, rather than an approximate roof-opening size plugged in during design. They're also designed for the specific type of roofing condition involved, including standing seam metal roofs, membrane roofing, different roof slopes and difficult roofing conditions.

Prefabricated curbs are installed before the roofer is required and therefore become part of the roofing system. If they are mounted properly beneath the insulation, most roof leaks will be eliminated - they should never be installed on top of the roofing or insulation on new or existing construction. Prefabricated curbs are also available with raised cants to allow the roofer's insulation to mate, that is, the roof cant starts at the finished roof and not below the insulation thickness. Prefabricated curbs are also available in all sizes and custom configurations.

Image courtesy of Roof Products, Inc. (RPI)
Prefabricated curbs insure a proper fit for HVAC equipment

Proper Specification
In the fast paced design process, roof curbs often get lost in the shuffle. In some cases, they're shown in three parts of the spec for the same project - 07720-Roof Accessories and 15000 HVAC and 08000 skylights--evidence of lack of coordination between the architect, specification writer, and mechanical engineer. As roof specification and design is a responsibility of the architect, it should be noted that the proper way to specify a roof curb is the one recommended by the Construction Specifiers Institute under section 07720-Roof Accessories. In the HVAC and skylight sections, a footnote should be included that curb specifications will appear in 07720. So specifying will ensure that the roof curb will mate with all roof applications and will coordinate with all equipment-mechanically, architecturally, and structurally.

Beyond that, choosing one manufacturer to provide curbs for all roof penetrations, including expansion joints, HVAC, fans, skylights, etc., offers the advantages of cost effectiveness and a single responsible source to ensure roof curbs are on site when the steel is delivered. "Asking a mechanical contractor to be responsible for a watertight roof, and providing curbs for work other than HVAC work is folly, says Robert Banicki, President of Roof Products, Inc. "With proper specification, you guarantee all roof curbs to be designed and coordinated for the project at hand."

All roof curbs should be a minimum of 8 inches above the finished roof and should be welded or sheet-screwed to a metal, wood, or tectum deck or to the structural steel. On a concrete roof deck, ram setting, or power-charged screw, is the most popular. The curbs are insulated with 1.5-inch, 3-pound density of rigid fiberglass insulation to reduce condensation. If the insulation is not applied at the factory, the roofer must cut in insulation at the job, which is difficult and more expensive, especially if curbs are canted and pitched-and there is little margin for error. If the insulation is installed on the outside instead of within the curb, condensation occurs and, later, dripping water into the building.

Even though mechanical contractors throughout the United States still use wood timbers for roof-mounted condensing units, gas pipes, and rooftop units, wood may not the best material to use. Any wood member larger than 2 x 4 will start to warp, especially when used as an expansion joint. When cracks form, water seeps in. Wood installed on top of insulation also causes movement of gas pipes, scraping away roofing stones and eventually digging into the insulation. The result is clear-water will find its way into the building.

Easing Retrofits
In rooftop equipment retrofits or in situations where tenants' HVAC needs change, as in shopping centers or office buildings, a curb adapter, with a sloping top to ensure proper drainage, can facilitate the situation. A universal base curb is set down, and an adapter installed until the unit itself can be is installed based on the building tenants' needs. Adapter curbs enable the building owner to upgrade old, inefficient HVAC units to new units without disturbing the roofing. They can be manufactured with integral supply and return transitions to match existing configurations. Standard features include sloped covered areas for water drainage, lifting points, glued and pinned insulation, and heavy gauge galvanized metal. Manufacturers can provide custom configurations, on-site evaluations, and expedited shipping for emergency change-outs. Rooftop equipment is removed, and rather than tear up the roof, a roof curb adapter can be installed over the existing roof curb to accommodate the new equipment - often at one fourth the cost of digging out the old curb and disturbing the roof. Likewise, for new speculative space, a pitched base curb is roofed in and temporarily covered. When tenants' needs are assessed, an insulated adapter is installed over the base curb to accommodate the HVAC equipment, without having to disturb the roof and open it up to potential leaks.