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
Thermal movement, freeze-thaw cycling,
and exposure to ultraviolet light will degrade even the best
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
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).
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
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.
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.