Proper execution of field applied sealants is vital to the finished weather integrity of any low slope, hydrostatic roof system. In principal, each panel is gasketed 360 degrees around its perimeter. Along the sideseams, sealant is mechanically applied by the roll forming machine. At all other panel termination points, it is the duty of the installer to apply sealant beads that marry into and establish the continuity of this gasketed seal between factory applied sealant beads. These sealant locations and procedures are described in the manufacture’s installation manual. Each is critical, and must be followed. Some of the usual areas for critical sealant applications include:
Eave: Panel to eave flashing or gutter. Panel rib closure to eave flashing or gutter. Panel to rib closure. Panel seam end.
Endlap: Panel flat to panel flat. Panel rib area to panel rib area (and into seam)
Ridge: Panel to closure components. Closure component to ridge flashing.
Terminations: Panel to flashing at rakes or gables. Panel to flashing at longitudinal expansion joints. Panel to flashing at parapet conditions.
Penetrations: Panel and ribs to preformed curbs. Panel and ribs to other roof penetrations.
Flashings: Laps of adjacent flashing segments. Flashings to panels.
In addition to the above, in some cases, panel attachment clips require wetting with sealant. This is because the clip tab interrupts the seam sealant, creating a dry joint between clip and male seam component. Some clips do not interrupt the seam sealant, and some manufactures do not deem clip wetting necessary. The manufacturer will note such a requirement in the installation manual, if clip wetting is necessary. If so specified, it is another critical aspect of installation.
Sealants are used within low slope, hydrostatic roof systems for several reasons, among them, to protect joints should they become periodically submersed (pressure); and to prevent capillary action from pulling moisture through the system. These are demanding circumstances for any joint. In order to perform and not leak, sealant continuity is the primary concern. The installer must be sure to “marry” (or meld) adjacent sealant beads one to the next.
For instance, when using vertical rib panels, the panel is normally sealed at the eave, through its flat area to the eave flashing, gutter leg or some other flashing component. This is usually done with a butyl tape. Of course, sealant seals the top of the seam. Where the seam terminates (at the panel end) there is a discontinuity of sealant in the vertical portion of the seam between the seal and the tape seal at the panel flat. It is not only vital that the installer seals the two rib surfaces together, but that he marry this vertical bead of sealant to both the eave tape, and the seam sealant. When properly done, this will create a continuous gasketed seal at the eave area.
One of the most frequent mistakes made by installers of low slope roofing, is the misplacement of sealant relative to related mechanical fasteners. In lap joints that do not involve cinching hardware, it is important that the fastener is on the “dry” side of the sealant, or alternatively through the sealant. The screw hole in the upper lapping component is sealed by the rubber washer on the screw. The hole in the lower joint component is sealed only by the joint sealant. If that sealant is upslope of the hole, then moisture drawn into the joint will migrate down the screw shank and drip into the building interior.
On joints where cinching hardware is used, the fastener must be through the sealant bead. Such a joint relies upon the sealant to swell into the threads of the screw and the upper joint component as it is tightened. The washer beneath the screw head is really of no effect, since the joint between cinch hardware and upper joint component is dry. Once installed, removal of a screw in such a joint is risky as the sealant may not “re-bed” upon reinstallation of the screw. Installing a small patch of sealant tape between the cinch hardware and the upper joint component can restore weathertightness in such a case.
It is rarely acceptable practice to use externally applied sealants on low slope, hydrostatic roofing. No sealant will have the same life expectancy as the roof material when exposed to ultra-violet light from the sun. Additionally, precipitation and particulate matter will continually abrade, wear away and deteriorate exposed sealants. In freezing climates, ice and snow will pull and tug at external sealants until their adhesion is lost and they peel from the roof.
Because of the numerous types of sealants on the market and the different types of materials they are used upon, its always best to contact a professional who’s knowledgeable in sealant application. To learn more about sealants and panel installation call 888-321-9630 or visit http://www.drexmet.com
By: Bill Dooley ASCE, CSI.
Director of Architectural Sales Drexel Metals Inc.