Roof sheathing and roofing are usually applied before the siding to make the addition weatherproof as soon as possible. Be sure the walls are braced well and the rafters are securely tied into the house roof.
Sheathing for most roofing materials is usually ½-inch plywood, but some roofing materials, such as wood shingles, require spaced sheathing of 1 by 4 boards.
In planning the roof sheathing, determine whether the bottom of the sheathing will be visible along the eave and rake edges or whether soffits will conceal it. If it will be visible, se an attractive material for the edge panels, such as plywood siding or rustic wood siding, and sheathe the rest of the roof with normal sheathing material.
Install plywood panels perpendicular to rafters. Leave gaps at ends and edges to allow for expansion; recommended distances are usually stamped on plywood. Double these measurements in a humid climate. Stagger end joints by 4 feet. Special H-clips can be slipped between panel edges to lock them together between rafters. Nail plywood with 8-penny (8d) nails every 6 inches at ends and every 12 inches in field.
Sometimes the 1 by 4 boards may be installed over a plywood roof deck for purposes of insulation or shear strength. In most cases, however, they are nailed directly to rafters, Just for spaced according to the required exposure of the roofing material.
For instance, if wood shingles are applied with a 5-inch exposure (the bottom part of the shingle which is exposed to the weather), sheathing boards must be spaced exactly 5 inches on center or shingle nails will not align. On shake and shingle roofs, it's good to cover 18 inches just below the ridge with solid boards for a good nailing surface.
If sheathing is visible from inside the house, as in cathedral ceilings, use 2 by 6 tongue-and-groove roof decking. Install first board along eaves with tongue edge facing ridge. Use 2-foot-long scrap wood as a hammering block to knock successive boards into place. Stagger butt joints, and leave 1/16-inch for expansion.