Standing-seam metal roofing represents the state of the art when it comes to durable, sustainable, eco-friendly roofing alternatives, providing 3 or 4 decades of reliable service life. Unfortunately, this high-tech roofing option, and the maintenance freedom it offers, is often sabotaged when it comes to mounting essential rooftop equipment and ancillary mechanicals.
Any roofing consultant or professional will agree that the best way to prevent rooftop problems is to clear the rooftop of everything possible (sage advice for any roof material, metal included). At times, however, the perfect roof eludes us. It becomes either necessary or convenient to mount HVAC equipment and then screens to hide it, piping to fuel it, scuttles to access it, and walkways to service it - not to mention satellite dishes, lightning protection, solar panels, advertising signage, fall protection systems, etc. With some basic understanding of the dos and don'ts when rooftop-equipment mounting becomes unavoidable, it can be made relatively trouble-free.
A first rule about any rooftop mounting is to do it without penetrating the roof membrane whenever possible. While this may seem obvious, on metal roofs, it is often violated. The norm for attaching things seems to be: "anchor to structure through the roof." When this is done, it not only threatens weather integrity, but also violates the thermal-cycling behavior of the roof membrane. Fortunately, scores of things can be securely mounted to metal rooftops without any roof penetration.
Standing-seam metal offers clear advantages over other roof types when mounting ancillary fixtures. The metal-roofing industry has developed special seam-clamping hardware that grips the standing seam without puncturing the membrane. Unlike other types of roofing, metal is a rigid, high-tensile material. The seam area creates a beam-like structure that can provide convenient anchorage for things like walkways, solar arrays, condensing units, gas piping, etc. without harming the weathering characteristics of the roof. Mechanicals can be safely and cost-effectively secured to these seam clamps, leaving the roof membrane penetration-free. They provide incredible holding strength, last the life of the roof, and preserve thermal-cycling characteristics.
A few things to keep in mind:
- Clamps should be made of non-corrosive metals (typically, aluminum with stainless mounting hardware). These metals are compatible with anything found on a metal roof except copper. (If the roof is copper, use brass clamps with stainless hardware.)
- Clamps should be attached to seams with round-point setscrews to prevent galling or other damage that could lead to corrosion.
- Remember that any loads introduced into the clamp will be transferred to the panels and their anchorage to the structure. That anchorage must be capable of withstanding the added load.
When Penetration is Unavoidable
In the case of HVAC and plumbing vents, the roof membrane is penetrated, so the first rule doesn't apply. The soil stack must carry gases from inside out, and the HVAC unit must either transfer inside air out or outside air in, or both. In these situations, holes in the roof are unavoidable - the challenge is to waterproof the holes while maintaining the thermal-cycling integrity of the roof. There are a few rules about handling these kinds of rooftop penetrations in low-slope, standing-seam metal that will ensure a trouble-free installation.
Mounting HVAC with Structural Curbs
Most small, bottom-ducted HVAC units are curb mounted, utilizing a preformed, structural-equipment curb specially manufactured to integrate with the specific profile of the roof. This curb type carries the weight of the unit, seals to the roof, and maintains the thermal-cycling integrity of the system. It is important to engage a company that specializes in manufacturing curbs for the metal-roofing industry; they can usually be identified by the metal-roofing manufacturer.
The best curb is an all-welded design using sheet aluminum that is at least 0.08-inches thick (not coated carbon steel). Sheet steel does not weld well in thin gauges, and it heat warps when welded. Also, the protective coatings must be burned off at welds and cannot be suitably restored. Aluminum welds exceptionally well and does not heat warp because of its low melting temperature. It is very compatible with sheet steels used for roofing and can provide decades of trouble-free service when designed, fabricated, and installed correctly.
The curb design should ensure that the curb flange underlays the roof panels at the upslope end and overlays them at the downslope end (no "back-water" laps). This is normally accomplished by terminating the curb's side flanges by marrying them into a panel seam at either side of the curb. The curb walls are built up to a minimum height of 6 inches and flanged at the top to provide an adequate structural mounting surface for the equipment. They are also beveled to compensate for the roof slope and provide for level mounting of the unit. Because this type of structural curb is "floating" (moves thermally with the roof), there are weight constraints. These curbs can accommodate units weighing up to about 1,000 pounds placed anywhere on the roof - even heavier units if located near the roof's point of fixity where movement is minimal. They are ordered from a manufacturer for a predetermined roof location, specific roof type, and by equipment model number (or, in lieu of a model number, exact equipment dimensions).
Installation details that seal the panels to the curb at its upslope end are similar to the details used to seal at the eave end of the panels, involving tape and/or tube grade butyl concealed within the joints and metal closure components depending upon the rib geometry of the panel. Panel ribs are terminated well upslope of the curb wall to allow easy drainage to the sides of the curb. At the downslope joint, the curb flange mates over the flat plane of the roof panels. Rib caps furnished loose or welded integrally into the curb flange serve to terminate the panel seams. This is again accomplished with butyl tape and tube seals concealed within the joints. The downslope joint thus created is normally reinforced beneath the assembly with a "back-up" plate or channel. The side flanges are likewise sealed to the roof panels with butyl seals inside the mating components.
Other sub-structural components may be employed to facilitate the installation, and this type of curb is often furnished with board stock insulation mounted to the curb walls. Installation of all critical seals (especially those at seam interfaces) is of paramount importance and fasteners must be to the "dry" side of sealant beads. It is also important that such a curb and its components are fastened together without pinning to the building structure. The resulting assembly is free to move thermally with roof panels while sealing completely into the roof "bathtub" style, in layman's terms (or in accordance with ASTM E2140, in technical terms). Diverters should be used on the upslope flange of the curb and, whenever possible, the unit should be oriented so that the smallest dimension opposes the flow of water (e.g. if a unit is 3 feet by 5 feet, the 5-foot dimension should be parallel to the slope of the roof).
When it is necessary to mount heavier equipment, the unit is sometimes mounted above the roof on a galvanized steel frame. The frame is constructed using round pipe legs so that they can be flashed with rubber pipe flashings. These legs extend through the roof to supporting structural members below. Such a mounting is stationary (there will be differential movement between the frame/unit and the roof). Depending upon the weight of the unit, the support frame can also be mounted on seam clamps described earlier to avoid pipe penetrations through the roof. The ribs of structural metal panels will normally support point loads of at least 150 pounds; hence, a unit that weighs 1,500 pounds and spans across five panel seams can be mounted this way, resulting in 10 bearing points on the five seams.
When ducting a frame-mounted unit through the roof, it is always advisable to utilize the smallest hole possible (e.g. a very large unit may only require a very small duct penetration). A small curb of the same type described earlier is used to waterproof the ducted hole(s) in the roof. In this case, the curb need not be structural (as it supports no weight, but acts as a flashing only around the duct that passes through the roof). The curb style is the same in all other respects. If the unit is mounted on a stationary frame, then the curb size must be slightly larger than the actual duct size to allow differential thermal movement between duct and curb. If the unit is mounted to seam clamps, then this is unnecessary as the unit and curb move in tandem with one another.
When larger HVAC equipment size and weight is involved, the unit is often mounted on a structural curb, which is integral to the building's structural framing system. When such a design is employed, a second "flashing curb" is used for waterproofing reasons. The concept here is that the first curb (or frame) supports the weight of the unit and the second does the waterproofing and integrates into the roof system. In this case, there is differential movement between the two, so the outer "flashing curb" is oversized to the first, and a counter-flashing of either metal or flexible membrane is married to the unit to shed water over the outer curb. The outer curb is of the same design and material as previously described, with the exception that it need not be such a heavy gauge since it supports no weight.
It is imperative that:
- Welded, aluminum curb construction is used.
- Curbs are equipped with diverters on the upslope flange.
- Upslope curb flange should underlay roof panels.
- Lower curb flange should overlay roof panels.
- Curb walls are a minimum of 6-inches high.
- Curb and installation are "floating," not pinned to building structure.
- All seals are made with butyl tape/tube grade within the joints (not exposed sealants), with careful attention to marrying seals at panel seams.
- Curb sidewalls are located at least 6 inches from the nearest adjacent seam to allow sufficient drainage to the sides of curbs.
Round shapes, such as plumbing vents, should be flashed through the roof using EPDM (ethylene propylene diene monomer) rubber pipe flashings. Although these parts are widely available in a variety of colors, black has the greatest UV resistance and, hence, the longest life. Standard installation is to simply stretch-fit the rubber to the pipe. Utilizing a stainless steel draw band at the top of the flashing to further secure it will ensure that the flashing never inverts itself, and will typically add about 5 years of life to the assembly. The part has an integral aluminum compression ring laminated to the rubber base, which should be sealed to the roof panel using butyl copolymer tape. It should then be secured using #14 x 7/8-inch tek screws with #1 drill point at 2-inch centers through the compression ring, rubber, and butyl, and into the metal panel. Finally, excess butyl tape should be trimmed away, and a bead of one-part polyurethane sealant filleted around the joint created (base-to-roof). This bead will hide and protect the butyl from direct exposure to sunlight, ensuring a longer life. After a service life of 25 years or so, this assembly is easily replaced.
When attaching the pipe flashing, it must be anchored to the roof panel only - not into the building structure or deck. To do so would create an inadvertent "pinning" of the roof panel, violating its freedom of thermal movement. Ideally, these flashings should be centrally located on the roof panel so that there is free drainage to both sides with no interruption of the seams. If the location of the pipe interrupts a seam and it cannot be relocated, then a preformed adapter plate can be fabricated to span both panels adjacent to the seam and the pipe flashed per the above to the adapter plate. Companies that pre-manufacture curbs will make such adapter plates upon request.
When installing pipe flashing, remember:
- Use unitized EPDM rubber pipe flashings (black preferred).
- Locate centrally in panel.
- Use stainless steel draw band.
- Butyl tape beneath base, then fillet with one-part polyurethane.
- Do not pin to structure or deck.
Rooftop mountings and penetrations are a challenge for any roof type or material. Following these simple guidelines will ensure trouble-free and enduring performance for a state-of-the-art, low-slope metal-roof system.
Toy Henson is director of market development and education at The Metal Initiative, Glenview, IL.