Lumber siding should be sound and free of knotholes, loose knots, checks or splits. Cedar and pine are the most commonly used species. It has also become more common to use pressure-treated lumber for siding or wood that has been factory-finished with stain or paint. The finish should cover the front and back surfaces of the siding to reduce water absorption. At the time of application, the siding should have moisture content similar to what it will be exposed to, that is, between 12 and 18 per cent, depending on a region’s humidity and climate.

Rainscreen wall assemblies (Figure 110, for example) are required in wet, humid climates such as the coastal regions of Canada. A vertical space is created behind the siding to facilitate drainage and drying. This is accomplished by mounting the siding on furring strips nailed on top of the sheathing membrane to the studs behind.

Corrosion-resistant nails such as hot-dipped galvanized nails will hold the siding permanently and will not disfigure the paint surface. Casing or siding nails are normally used for this purpose. Drive the heads flush with the face of the siding. If finishing nails are used, set the heads below the surface and fill the holes with putty. The length of the nails required depends on the thickness of the siding and the type of sheathing used and must be long enough to penetrate at least 25 mm (1 in.) into the nailing support.

Horizontal Application

Bevel or feather-edge siding (see Figure 109) is installed from the bottom up with the bottom edge of the lowest course mounted on a 6 mm (1⁄4 in.) thick furring strip. Overlap each succeeding course at least 25 mm (1 in.) over the lower course. Spacing for the siding should be planned before the installation starts. Deduct the minimum lap from the overall width of the siding to determine the maximum board exposure. The number of boards needed to clad a wall must respect the maximum allowed exposure. This may lead to an installation in which the exposure is less than the maximum permitted. Attempt to have the bottom of a board placed over the top of a window aligned with the top of the window cap (Figure 111).

Bevel siding must have a butt thickness of at least 12 mm (1⁄2 in.) for widths of 184 mm (8 in.) or less and 14.3 mm (9⁄16 in.) for widths greater than 184 mm (8 in.). The top edge should not be less than 5 mm (3⁄16 in.) thick. Tongue-and-groove matched siding (see Figure 109) is 184 mm (8 in. nominal) or less in width. The first board is face-nailed near the grooved edge and angle-nailed through the tongue. Each successive board is fitted tightly to the preceding board and angle-nailed through the tongue. A nail set is used to finish off the nailing.

Drop (or matched) siding (see Figure 109) should be at least 14.3 mm (9⁄16 in.) thick and 184 mm (8 in. nominal) or less in width. It is produced in a variety of patterns with matched or shiplap edges. Stagger the butt joints between boards in adjacent courses. Seal the ends and fit the siding so it is in close contact with adjacent pieces. Loose joints allow water to get behind the siding, which can cause paint deterioration around the joints and lead to decay at the ends of the boards and inside the wall.

A tight, waterproof joint can be made by placing a small bead of caulking compound or putty along the end of each board after it is installed, then pressing the next board into the compound and removing the excess compound. Joints at window or door trim can be made the same way.

Bevel and drop siding should be face-nailed to structural sheathing (plywood, OSB or lumber) or to the wall studs. The size of the nail required depends on the thickness of the siding and the type of sheathing used. A good way to minimize cracking from dimensional change is to drive nails through the siding just above the lap so that they miss the top edge of the piece of siding beneath (see Figure 111).

Vertical Application

Lumber siding that can be applied vertically includes: plain matched boards; patterned matched boards; square-edge boards covered at the joints with a batten strip; or square-edge boards spaced apart and covered with another board. Vertical siding is usually 14.3 mm (9⁄16 in.) thick. Boards should not be wider than 286 mm (12 in. nominal). Vertical boards may be fastened to 14.3 mm (9⁄16 in.) lumber sheathing, 12.5 mm (1⁄2 in.) plywood or 12.5 mm (1⁄2 in.) OSB or waferboard, or to horizontal furring strips.

The furring (strapping) should be at least 19 × 64 mm (1 × 3 in. nominal) lumber where the framing is spaced not more than 400 mm (16 in.) on centre or 19 × 89 mm (1 × 4 in. nominal) lumber where the framing is spaced not more than 600 mm (24 in.) on centre. Butt joints in lumber siding should be cut at 45 degrees (mitred) and overlapped to prevent the entry of water into the joint. When the spaced (board-on-board) method is used (Figure 109), the boards next to the wall are normally wider than the cover boards and are fastened with one row of nails near the centre of each board. The cover board is then applied so that it laps the edges of the underlying boards at least 25 mm (1 in.). Fasten the cover boards with one row of nails in the centre. Use a vertical furring behind so that nailing does not split the cover board. This arrangement allows all the boards to adjust to changing moisture without splitting.

The board-and-batten method uses square-edge boards that are ordinarily 184 mm (8 in. nominal) or less in width. The boards are applied with the edges at least 6 mm (1⁄4 in.) apart and fastened with one row of nails near the centre of each board. A batten is used that laps the edges at least 12 mm (1⁄2 in.). The batten is fastened with one row of nails driven in the joint between the two boards. This allows all boards to adjust to changing moisture without splitting. Since the batten also serves to prevent the board edges from curling outward, the nailing should be secure and closely spaced.

Figure 111

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