Low Slope Standing Seam Metal Roofing:Rooftop Penetrations and Ancillaries

Standing seam metal roofing represents the state-of-the-art when it comes to durable,sustainable, eco-friendly roofing alternatives, providing service life of three or four decades.Sadly, this high tech roofing option and the maintenance freedom it offers is often sabotagedwhen it comes to the mounting of essential rooftop equipment and ancillary mechanicals.

Any roofing consultant or professional will agree that the best way to prevent rooftopproblems is to clear the rooftop of everything possible, and this is sage advice for any roofmaterial, metal included. At times, however the perfect roof eludes us! It becomes eithernecessary or convenient to mount HVAC equipment -- then screens to hide it, piping to fuelit, scuttles to access it, and walkways to service it; not to mention satellite dishes, lightningprotection, solar panels, advertising signage, fall protection systems -- and the list goes on.With some basic understanding of the “do’s and don’ts” however, when rooftop equipmentmounting becomes unavoidable, it can be made relatively trouble-free.

Penetration-free AttachmentA “first rule” about any rooftop mounting is to do it without penetrating the roof membranewhenever possible. While this may seem obvious, on metal roofs it is often violated. Thenorm for attaching things seems to be, “anchor to structure through the roof”. When this isdone, it not only threatens weather integrity, but also violates thethermal cycling behavior of the roof membrane. Fortunately,scores of things can be securely mounted to metal rooftopswithout any roof penetration whatsoever.

Standing seam metal actually offers clear advantages toother roof types when mounting of ancillaries becomesnecessary. The metal roofing industry has developedspecial seam clamping hardware that grips the standingseam without puncturing the membrane. Unlike other typesof roofing, metal is a rigid, high-tensile material. The seamarea creates a beam-like structure that can provide con-venient anchorage for things like walkways, solar arrays,

Illustration shows profile of clamping hardware affixed to standing seam.

condensing units, gas piping, and thelike without harming the weatheringcharacteristics of the roof.Mechanicals can be safely and cost-effectively secured to these seamclamps leaving the roof membranepenetration-free. They provideincredible holding strength, last thelife of the roof, and preserve thermalcycling characteristics.

A few things to keep in mind:

• Clamps should be made only ofnon-corrosive metals -- typically,aluminum with stainless mountinghardware. These metals are compatible with anythingfound on a metal roof except copper. (If the roof iscopper, use brass clamps with stainless hardware.)

• Clamps should be attached to seams with round-pointsetscrews to prevent galling or other damage thatcould lead to corrosion.

• Remember that any loads introduced into the clampwill be transferred to the panels and their anchorageto the structure. That anchorage must be capable ofwithstanding the added load.

When Penetration is Unavoidable…In the case of HVAC and plumbing vents, the roofmembrane must often be penetrated, so the “first rule”doesn’t apply. The soil stack must carry gasses frominside out and the HVAC unit must either transferinside air out, outside air in, or both. In these situa-tions, holes in the roof are unavoidable and the challenge is to waterproof the hole, yetmaintain the thermal cycling integrity of the roof system. There are a few rules abouthandling these kinds of rooftop penetrations in low slope standing seam metal that willensure a trouble-free installation.

Seam clamps have made rooftop mounting so simple and cost effectivethat metal roofing is now the preferred roof type for mounting photovoltaicsolar arrays.

Closeup view

Seam clamps allow even cumbersomeancillary items to be attached to metalroofs without penetrating the rooftop.

Mounting HVAC With Structural CurbsMost small, bottom-ducted HVAC units are “curb mounted”, utilizing a pre-formed structuralequipment curb specially manufactured to integrate with the specific profile of the roof. Thiscurb type carries the weight of the unit, seals to the roof, and maintains the thermal cyclingintegrity of the system. It is important to engage a company that specializes in manufacturingcurbs for the metal roofing industry; they can usually be identified by the metal roofingmanufacturer.

The best curb is an all-welded design using sheet aluminum that is at least .080" thick (notcoated carbon steel). Sheet steel does not weld well in thin gauges, and it heat-warps whenwelded. Also, the protective coatings must be burned off at welds and cannot be suitablyrestored. Aluminum welds exceptionally well and does not heat-warp because of its lowmelting temperature. It is very compatible with sheet steels used for roofing and can providedecades of trouble-free service when designed, fabricated and installed correctly.

The curb design should provide that the curb flange underlays the roof panels at the upslopeend, and overlays them at the downslope end (no “back-water” laps). This is normallyaccomplished by terminating the curb’s side flanges by marrying them into a panel seam ateither side of the curb. The curb walls are built up to a minimum height of 6", and flangedat the top to provide an adequate structural mounting surface for the equipment. They arealso 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 areweight constraints. These curbs can accommodate units weighing up to about 1,000 poundsplaced anywhere on the roof -- even heavier units if located near the roof’s point of fixitywhere movement is minimal. They are ordered from a manufacturer for a pre-determined

roof location, specific roof type,and by equipment model number --or in lieu thereof -- exact equipmentdimensions.

Installation details that seal thepanels to the curb at its upslopeend are similar to the details usedto seal at the eave end of thepanels, involving tape and/or tubegrade butyl concealed within thejoints and metal closure compo-nents depending upon the ribPre-formed structural curbs support weight and seal tightly to the roof.

geometry of the panel. Panel ribs are terminated well upslope of the curb wall to allow easydrainage to the sides of the curb. At the downslope joint, the curb flange mates over the flatplane of the roof panels. Rib caps furnished loose or welded integrally into the curbflange serve to terminate the panel seams. This is again accomplished with butyl tapeand tube seals concealed within the joints. The downslope joint thus created is normallyreinforced beneath the assembly with a “back-up” plate or channel. The side flanges arelikewise sealed to the roof panels with butyl seals inside the mating components.

Other substructural components may be employed to facilitate the installation, and this typeof curb is often furnished with board stock insulation mounted to the curb walls. Installationof all critical seals (especially those at seam interfaces) is of paramount importance and fas-teners must be to the “dry” side of sealant beads. It is also important that such a curb and itscomponents are fastened together without pinning to the building structure. The resultingassembly 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 unitshould be oriented so that the smallest dimension opposes the flow of water, e.g. if a unit is3' x 5', the 5' dimension should be parallel to the slope of the roof.

Frame-mounted HVACWhen it is necessary to mount heavier equipment, the unit is sometimes mounted abovethe roof on a galvanized steel frame. The frame is constructed using round pipe legs, so thatthey can be flashed with rubber pipe flashings. These legs extend through the roof tosupporting structural membersbelow. Such a mounting is sta-tionary -- that is to say, there willbe differential movement betweenthe frame/unit and the roof.Depending upon the weight of theunit, the support frame can also bemounted on seam clampsdescribed earlier to avoid pipepenetrations through the roof. Theribs of structural metal panels willnormally support point loads of atleast 150 pounds; hence, a unit thatweighs 1,500 pounds and spansacross five panel seams can bemounted this way, resulting in 10bearing points on the five seams. Frame mounted HVAC units using seam clamps that avoid roof penetration.

Closeup view

When ducting a frame-mounted unitthrough the roof, it is always advisableto utilize the smallest hole possible:e.g. a very large unit may onlyrequire a very small duct penetration.A small curb of the same typedescribed earlier is used to waterproofthe ducted hole(s) in the roof. In thiscase, the curb need not be structural,as it supports no weight, but acts as aflashing only around the duct thatpasses through the roof. The curbstyle is the same in all other respects.

If the unit is mounted on a stationary frame, then the curb size must be slightly larger thanthe actual duct size to allow differential thermal movement between duct and curb. If theunit is mounted to seam clamps, then this is unnecessary as the unit and curb move intandem with one another.

Double-curbsWhen larger HVAC equipment size and weight is involved, often the unit is mounted on astructural curb, which is integral to the building’s structural framing system. When such adesign is employed, a second “flashing curb” is used for waterproofing reasons. The concepthere is that the first curb (or frame) supports the weight of the unit and the second does thewaterproofing and integrates into the roof system. In this case, there is differential movementbetween the two, so the outer “flashing curb” is oversized to the first, and a counter-flashingof 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 exceptionthat it need not be such a heavy gauge, since it supports no weight.

The following points apply to all the curbs described above:

• Use all-welded, aluminum curb construction

• Equip curbs with diverters on the upslope flange

• Upslope curb flange should underlay roof panels

• Lower curb flange should overlay roof panels

• Curb walls should be a minimum of 6" in height

Frame mounted HVAC unit using pipe supports extendingdown to the building structure and flashed through the roofusing rubber pipe flashings.

• Curb and installation should be “floating” (not pinned to building structure)

• All seals should be accomplished with butyl tape/tube grade within the joints (not exposedsealants), with careful attention to “marry” seals at panel seams

• Curb sidewalls should occur 6" minimum from the nearest adjacent seam location to allowsufficient drainage to the sides of curbs

Round PenetrationsRound shapes, such as plumbing vents, should be flashed through the roof using EPDM(ethylene propylene diene monomer) rubber pipe flashings. Although these parts arewidely available in a variety of colors, black has the greatest UV resistance, and hence thelongest life. Standard installation is to simply stretch-fit the rubber to the pipe. Utilizing astainless steel draw band at the top of the flashing to further secure it will ensure that theflashing never inverts itself, and will typically add about five years of life to the assembly.The part has an integral aluminum compression ring laminated to the rubber base, whichshould be sealed to the roof panel using butyl copolymer tape. It should then be securedusing #14 x 7/8" tek screws with #1 drill point at 2" centers through the compression ring,rubber and butyl and into the metal panel. Finally, excess butyl tape should be trimmedaway, and a bead of one-part polyurethane sealant filleted around the joint thus 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, and not into thebuilding 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 centrallylocated on the roof panel so that there is free drainage to both sides with no interruption ofthe seams. If the location of the pipe interrupts a seam, and it cannot be relocated, then apreformed adapter plate can be fabricated to span both panels adjacent to the seam and thepipe flashed per the above to the adapter plate. Companies that pre-manufacture curbs willmake such adapter plates upon request.

When installing pipe flashing, remember:

• Use unitized EPDM rubber pipe flashings (black preferred)

• Locate centrally in panel

• Use stainless 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 andenduring performance for a state-of-the-art low-slope metal roof system.

Soil stacks and other round penetrations are flashed withunitized rubber pipe flashings.

All photos courtesy of Metal Roof Advisory Group, Ltd.