T1-11 siding is a specialized form of plywood that was extensively used in the 1970s through the 1980s. It has a vertical seam every 8 inches or so, providing a built-in visual pattern. It is still manufactured but is now made from engineered wood products instead of wood.
T1-11 is made in stained or painted sheets that are 4 feet wide and 8 feet high. Unless a home is two stories, the sheet is able to cover the side of the home without requiring a horizontal seam. It is often nailed directly to the studs with only a waterproof plastic sheath in between. The construction goes much faster and is less expensive without a secondary plywood sheath underneath.
An experienced carpenter can put up a sheet of T1-11 almost faster than the eye can follow. The traditional way to attach the sheets is with nails and a framing hammer. One or two blows to the nail head by a carpenter who has done this task repeatedly, and the the nail is solidly into the stud below. The first few nails require an assistant to hold the board in place while the corners are tacked down.
Screws can be used to attach the panels, but the materials cost is greater. Screw heads need to be sunk in enough so that they can be covered with a protective compound and painted over, making them disappear. A professional-grade screw gun for decks work well in applying the T1-11 to the studs. Whether nails or screws, it is important to use galvanized or stainless steel fasteners and then caulk them afterward. Otherwise, the open metal is going to attract rust.
T1-11 siding often needs replacing because it is not weatherproof and sustains damage with time, exposure and insect infestation. The older style of siding was just bare plywood that was stained or painted. If the exterior is all that is damaged, it is easy to remove that panel and replace it. If the water damage extends to the interior of the home, it might be a larger problem. Replace the damaged area before it becomes a greater problem.