A ridge board or beam runs through the tops of rafters or trusses from end to end along a gable roof, tying those framing members together. The rafters bear most of the weight of the roof; the ridge board holds them in place. (Trusses have rafters tied together with other structural elements and are installed as a single unit, while rafters are installed separately). Ridge boards typically are one dimension larger than rafters, a 2-by-6-inch ridge on 2-by-4-inch rafters, for instance. But sometimes a roof is too long for a single ridge board; then a splice is in order.
Calculate where a ridge beam splice will be needed. Measure rafter spacings of 24 inches with a tape measure and plan the splice to fall between rafters. A 30-foot roof, for instance, will need at least two boards. A 16-foot board will end at a rafter, with 24-inch spacing. Cut each board to 15 feet with a circular saw, so the splice will fall between rafters and each end of the splice will be fully supported by a pair of rafters.
Splice rafters with gussets or overlapping boards. Use metal gussets for a strong, even splice; these overlap the butt joint of the two boards and have nailing points on each side. Nail gussets on both sides of the splice to make a firm connection. Use plywood gussets as alternative; the principal is the same, but strong plywood is substituted for the metal gussets, which must be bought. Make a similar splice of lumber the same width as the ridge board, overlapping the splice joint by 1 foot on each side. Use a hammer and framing nails to fasten the splice.
Brace the ridge board halves beyond the splice joint while connecting the splice to keep the ridge beam straight and level. Splice the boards on the ground as an alternative, then raise the entire spliced board to set between rafters tops. Slide the ridge board between rafter top cuts from the bottom, if installing on standard rafters; set it in place in the pre-formed openings of trusses. Splicing on the ground is easier, but a very long ridge board can be hard to lift and install.