Sheathing for Slate Roofing | A Commercial Roofing Process Explained
You can use solid sheathing, such as plywood, or spaced wood laths beneath the slate roofing. Some roofers prefer to use solid wood boards instead of plywood. These could be square-edge or tongue-and-groove boards. If tongue-and-groove boards are used, point the tongue towards the ridge. Use two nails in each board at each rafter. Each end should rest on a rafter. Plywood and OSB panels are then installed. Sold-wood sheathing is covered with 30 pound felt unless otherwise specified.
If lath boards are used, they provide an open sheathing. They are generally 1 x 2 boards. Some installers prefer this because it allows air to circulate along the inside surface of the slate, drying any moisture that may have accumulated. Some roofers place a layer of 30-pound felt over the lath boards; this protects the interior of the house while the slates are being installed. Other roofers lay felt on the rafters and place the lath boards on top of it. This last method provides an airspace between the bottom of the slate and the felt, which adds to the insulating value.
It also helps keep the summer heat from entering the attic, while reducing heat loss somewhat in the winter. However, slate roofing takes much longer to install than most other roofing materials, which could be an important factor in making your selection. Lath boards do not provide the quality insulation value given by a roof lined with solid sheathing. In colder climates, this is a factor to consider as the roof is designed.
When lath boards are used, solid sheathing is nailed along the eaves. It is usually set flush with the eave. The slate overhangs 1 inch on the each and a little less along the rake. The lath boards are spaced center to center at a distance equal to the exposure of the slate. The slates are overlapped 3 inches and each end rests over a lath board.
Most slates come with two holes punched in them by a machine at the quarry. Large slates require four holes. Typical layouts for holes in slate are available on site. The location of the hole varies and this location determines the spacing of the lath in open sheathed roofs. It is possible to specify the spacing of holes when you order the slates or have the manufacturer send several samples over. Once the starting sheathing has been installed, the rest of the laths are placed on a center-to-center spacing equal to the slate exposure.
Slate roofing longer than 20 inches should have four holes. Holes should be punched rather than drilled in this instance. Punching produces a countersunk opening around the hole in which the head of the nail will rest. Drilling leaves the surface flat and the head of the nail rests on top of it.