Posted on: November 18th, 2009
This Web Exclusive comes from Rob Haddock, director of the Metal Roof Advisory Group, Ltd. of Colorado Springs, CO. He is a consultant to The Metal Initiative, the educational arm of the metal roofing and wall industry in North America.
Standing seam metal roofing can represent the state of the art when it comes to a durable, sustainable, eco-friendly approach, providing three or four decades of reliable service life. Unfortunately, this roofing option and the maintenance freedom it affords is often sabotaged when it comes to mounting essential rooftop equipment and ancillary mechanicals.
Regardless of the roof type, the best way to prevent rooftop problems is to clear the roof of any unnecessary equipment. And while facility managers would prefer an uncluttered roof, it is sometimes necessary or convenient to mount HVAC equipment—as well as screens to hide it, piping to fuel it, scuttles to access it, and walkways to service it.
There may also be a need for satellite dishes, lightning protection, solar panels, advertising signage, fall protection systems—and the list goes on. However, with some basic understanding of the “dos and don’ts,” rooftop equipment mounting, while unavoidable, can be made simple and trouble free on low-slope metal roofing.
Standing seam metal actually offers advantages over other roof types when mounting of ancillary fixtures does become necessary. These roofs are particularly well suited to accept special seam clamping hardware that grips the standing seam systems without puncturing their membranes (see example at right).
Unlike other roof materials, metal is rigid. The standing seam area creates a beamlike structure that can provide an anchor for things like walkways, solar arrays, condensing units, and gas piping without harming the weathering characteristics of the roof. Mechanicals can be secured safely and cost effectively to these seam clamps leaving the roof membrane free of penetrations. The clamps provide great holding strength, last the life of the roof, and preserve thermal cycling characteristics of the roof system.
If roof attachments are required, here are some tips that could prevent problems over time:
- Use penetration-free attachments whenever possible.
- Never use adhesives to secure attachments to metal roofing.
- Use only attachment clamps made of non-corrosive metals such as aluminum along with stainless steel mounting hardware. These metals are compatible with anything that may be found on a metal roof.
- Be sure that round-point setscrews are used to secure the clamp to the seam. This will prevent galling or other damage that could lead to corrosion.
- Any loads placed on the clamp will be transferred to the panels and their anchorage, and subsequently 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 often penetrated. The soil stack must carry gases from inside out, and the HVAC unit must bring either inside air out, outside air in, or both (see example below). Consequently, holes in the roof are inescapable. The challenge is to waterproof the holes, while also maintaining the thermal cycling integrity of the roof system.
There are a few rules for handling these kinds of rooftop penetrations in low slope standing seam that will help ensure trouble-free service. HVAC units and/or related ductwork penetrations should use pre-formed equipment curbs specifically designed to integrate with the roof profile being used. The curb is sealed to the roof and maintains the thermal cycling integrity of the system.
The best curbs are made of all-welded aluminum construction. This material is very compatible with sheet steel (or aluminum) used for roofing and should provide decades of service if designed, fabricated, and installed correctly. Often these curbs are load bearing “structural” varieties that simultaneously provide support and waterproofing. Roof curb suppliers are located throughout the U.S. and can be readily identified by most metal roofing manufacturers.
When unusual HVAC equipment sizes and weights are involved, often the support and weatherproofing functions are divided as the unit is mounted on a structural curb, which is integral to the building’s structural framing system. When such a design is used, a second “flashing curb” must be employed to satisfy the specific waterproofing challenges of a metal roof. The first curb (or frame) supports the weight of the unit, while the second does the waterproofing and is integrated into the roof system. The outer curb features the same design and material as previously described.
When equipment curbs are used, it is imperative that:
- Welded, aluminum curb construction be used
- Curbs be equipped with diverters on the upslope flange
- Curbs be shingled into the roof so as to avoid “back-water” laps
- Curb walls are at least 6″ high
- Curb and installation be “floating” and not pinned to the building structure
- All seals be made with butyl tape/tube grade within the joints (not exposed sealants), with careful attention paid to “marrying” seals at the panel seams
- Curb sidewalls be located at least 6″ from the nearest adjacent seam to allow sufficient drainage to the sides of curbs
Round shapes, such as plumbing vents or pipe supports for rooftop equipment, should be flashed through the roof using EPDM (Ethylene Propylene Diene Monomer) rubber pipe flashings. The cone-shaped rubber is field cut to size and stretch-fitted to the pipe. It is recommended that a stainless steel draw band be used at the top of the flashing to ensure that the flashing never inverts itself. The part has an integral aluminum compression ring that is laminated to the rubber base.
The pipe flashing must be anchored to the roof panel only, and not to the building structure or deck. To do the latter would create an inadvertent “pinning” of the roof panel, compromising its freedom of thermal movement. Ideally these flashings should be centrally located to ensure free drainage.
In any event, interrupting a seam should be avoided. This flashing assembly, which is sealed to the roof with butyl copolymer tape sealants, should offer 20 or more years of service life. In summation, when using these rubber pipe flashings, it is important to remember the following:
- Use unitized EPDM rubber pipe flashings (black preferred) with stainless steel draw band.
- Locate round penetrations centrally in the panel
- Seal with butyl tape beneath base; then fillet with one part polyurethane
- Do not pin flashings to the structure or deck
Rooftop mountings and penetrations are a challenge for any roof type or material. But following these guidelines will help to ensure trouble-free and enduring performance for low-slope metal roof systems.
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