OSB, short for oriented strand board, is widely used in roofing applications under felt paper and roof tiles. It is comprised of wood chips glued together and is less expensive than regular plywood. OSB comes in varying thickness, ranging from 6 mm to 1.8 cm (1/4 inch to 3/4 inch). When installing OSB on a roof it is recommended to use at least 11 mm (7/16 inch) boards. It is also common to use 1.2 cm (1/2 inch) and 1.5 cm (5/8 inch) sheets for this application. These sheets come 1.2 by 2.4 m (4 by 8 feet) in size and can be installed with just a few tools.
Measure the size of the roof to determine how many sheets of OSB will be used. Multiply the length by the width of each side of the house and add them together to get the total square metres. Divide that number by 2.88 to get the total number of sheets necessary. Add a few extra sheets to the total as cuts may need to be made to fit to size.
Inspect the trusses to make sure everything is intact and line the first sheet of OSB up beginning at the bottom corner of either side of the house. Set it down so it is covering the trusses and is 1.2 m (4 feet) in width and 2.4 m (8 feet) in height. Hammer a 6.5 cm (8d) nail at the bottom of the board. Insert a nail every 15 cm (6 inches) up the first truss.
Nail into the truss on the other end of the OSB in the same manner as the first one, making sure to space the nails 15 cm (6 inches) apart. Insert nails every 30 cm (12 inches) into the trusses underneath the OSB.
Space the second sheet 3 mm (1/8 inch) from the first sheet to allow for expansion of the board and install it in the same manner. Continue this process until the end of the house has been reached. Cut the last sheet to fit if necessary with a table saw.
Move up the roof and start another row above the first, installing the OSB the exact same way as the first row. Move to the other side of the house and repeat the same process until all the OSB has been properly installed.
It is necessary to install the felt and roofing on top of the OSB as soon as possible to prevent weather damage to the OSB. A sheet of OSB is 2.88 square metres, which is why the total square footage of the roof is divided by 2.88. If the trusses are spaced 40 cm (16 inches) on centre, there will be two trusses underneath each sheet of OSB and one on each end. If they are 60 cm (24 inches) on centre there will be only be one truss underneath.