T1-11 siding panels are 4 x 8 sheets of plywood, and they are usually installed vertically, which creates horizontal T1-11 siding joints on walls higher than 8 feet. These horizontal joints where upper and lower sheets butt together have to be sealed to prevent water from seeping behind the siding. That's what T1-11 flashing is for.
T1-11 flashing is a type of Z-flashing, which means the cross-section is shaped like the letter "Z" on its side. The lower part of the Z is just wide enough to wrap around the edge of the siding, which can be 3/8, 5/8, or 3/4 inch thick. The upper part of the Z is longer than the lower part. It goes underneath the upper siding panel and gets attached to the sheathing or framing. Manufacturers have specific instructions for installing this flashing.
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The Need for T1-11 Flashing
If builders don't make an effort to prevent moisture from seeping behind siding, the results can be disastrous. Mold and rot are inevitable results of trapped moisture, and they undermine the integrity of the siding and shorten its life. That's why section 1405.4 of the International Building Code specifically requires flashing.
T1-11 siding panels have a tongue on one side and an overlap on the other, and when properly installed, the overlap covers the tongue and makes the vertical seals watertight. There's nothing on the ends of the panels, however, so when you stack sheets vertically, water can seep freely into the butt joints. Caulk is insufficient to prevent this because a caulk joint isn't permanent; the caulk can separate when the sheets naturally expand and contract, and caulk has a limited service life.
Common Practice When Installing T1-11 Flashing
The installation of T1-11 usually proceeds from the bottom up. Installers line up the bottom end of the sheets an inch or two below the sill plate and install them side by side to cover the lower portion of the wall. They then install the Z-flashing on the top edge of those sheets before installing the next course.
A common practice is to place the flashing directly on the top edge of the lower course of siding and nail the flashing to the wall sheathing or framing. Installers then place the upper sheets directly on the flashing and nail or screw those sheets in place. In some cases, they may put a bead of caulk on the top edge of the lower course of siding before installing the flashing, and they may put more caulk on the flashing before installing the upper course of siding. The recommended procedure is a little different.
The Right Way to Install T1-11 Flashing
Z-bar for siding (which, by the way, can be cut to length with tin snips) should be installed in such a way as to allow any moisture that does collect behind the siding to escape. Since caulk prevents this, you shouldn't apply any between the flashing and the top panel. In addition, you should leave a 1/4-inch gap between the top panel and the flashing. That way, any condensation that does form behind the top panels can drip down and flow out.
Z-flashing should always be installed in such a way that it slopes away from the wall, and the horizontal portion is manufactured with a slight tilt to make this possible. Some builders advise against nailing through the flashing when attaching it to the sheathing because that creates holes through which moisture can seep. The safest way to attach it to the sheathing is to drive 1-inch roofing nails along the top edge with the heads overlapping the flashing.