Rafter ties are designed to tie together the bottom of opposing rafters and form a triangle. This helps keep walls from spreading outward due to the load of the roof or from ice and snow and can strengthen a roof against lateral forces such as a strong wind. Rafter ties may be required for old existing buildings or in the design of new structures. However rafter ties should be installed by an experienced person for the stability of the structure and the safety of its inhabitants.
Frame your roof as directed by the building plans, if starting a project from scratch.
Count the number of rafters to determine the amount of lumber you will need. Rafter ties are required every 4 feet on centre by Section 803.2.1 of the International Residential Code, meaning that you only need one tie for every other 24-inch on-centre rafters or for every third pair of 16-inch on-centre rafters.
Cut the rafter ties to the appropriate length using a circular saw and angle the ends so that they fit the slope of the roof.
Install the rafter ties. The ends must be placed against the rafters leaving a 1/2 inch from the rafter's edges.
Use a level to double-check that it is flush before installing permanently.
Nail the rafter tie into place using three 10d nails, on both ends.
Framing requires a group effort and should not be attempted alone.