Sealants are rarely used on barn roofs because the roof slopes are typically steep enough that rain and snow drain off in true watershed fashion. But house roofs, with dormers, hips, valleys, chimneys, skylights, and plumbing vent penetrations, put more demands on metal roofs. On houses, building a "watertight" roof by sealing all joints, side seams, and endlaps in a metal roof is critical for preventing leaks.

Tape vs. Gun-Grade Sealants
Thermal movement, freeze-thaw cycling, and exposure to ultraviolet light will degrade even the best quality sealants.

There are two types of sealants used for metal roofs - tapes and gun-grade. Tapes are preferred to gun-grade sealants because they have a high solids content (about 97%) and don't shrink. Most tapes for metal panels are butyl polymers, which will last 30 years or more if protected from exposure to ultraviolet light. The gun-grade types are usually about 70% solids, which means that when the 30% solvents evaporate, the sealant shrinks.

No sealant should ever be applied to the outside of a seam. But when external sealant applications are unavoidable, do not use silicone. Many silicone sealants contain acids, which can damage metallic coatings. Silicone also has poor adhesive strength on metal. Urethanes are generally much better. Still, any external sealant requires maintenance even under the best of circumstances. The best option by far is to rethink the detail and try to find a way to use a hidden butyl sealant.

Sealing Side Seams and Endlaps
Butyl tape is available in a variety of sizes. I recommend using a minimum tape width of 1/2 inch. The tape typically comes in a roll, and should be cut to length with a sharp knife, rather than torn, so that the bead maintains its intended dimension and is not stretched.

To seal the seams between adjacent panels, apply the tape to the top shoulder of the panel rib, making sure that the majority of the sealant winds up on the weather side of any sidelap fasteners (see Figure 1).

Figure 1. To seal the lap between adjacent panels, apply butyl tape to the weather side of the rib shoulder.

Don't peel away the release paper until you're ready to install the overlapping panel. Take care to ensure the exact alignment of the panel before it contacts the sealant. Once a panel touches the tape, even a small adjustment is extremely difficult.

Side seam fasteners should pierce through the sealant or be placed on the "dry" side from the sealant. The ideal fastener for sheet-to-sheet joining is a 3/4-inch 1/4x14 hex-head galvanized screw with a #1 drill point.

Seal the laps between panel ends with tape sealant, as shown in Figure 2.

Figure 2. Use butyl tape to seal panel endlaps, making sure the screws that fasten the panel to the deck top) or to purlins and sleepers (bottom) are placed above the sealant, as shown.

by Rob Haddock

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