A gable roof consists of two sloped planes that meet at the ridge or “peak” and rest on the top plates of 2 parallel exterior walls. In wood-frame construction, the primary structural members of a gabled roof frame are the ridge board and the rafters. If the lower section of the roof rafters, or “rafter tail,” extends beyond the exterior wall, the projection forms an overhang or “eave”. It is possible to shorten the eave of an existing roof frame by cutting back the rafter tails. It is also possible to enlarge the eave by installing “sister” or “scab” rafter tail extensions onto the existing rafters in line with the plane of the roof. Whether doing remodeling or new construction, the eave has the same construction assembly, which is described as follows.
The end of the rafter tail is shaped by 2 cuts that form the corner of a right angle. When the rafter is installed onto the roof, the bottom “level” cut, onto which the soffit will be installed, is now horizontal, and the end “plumb” cut, onto which the fascia is nailed, is now vertical. The size of the eave or the length of the overhang is determined when the pattern is laid out for the pre-cut roof rafters. With all the rafters in place and the sheathing installed on the exterior walls, you’re ready for the first step.
Step 1 - Make Sure that the Rafter Tails Are Aligned
Inspect the rafter tails to see if they’re aligned. You can check the plumb cuts by nailing a linear 12 foot or longer 2 x 4 to each rafter tail as you would the fascia. Look down the edge of the 2 x 4. Large bows indicate places where the rafter tails will have to be trimmed.
Step 2 - Attach a Nailer Board
Once you’re satisfied that the fascia will go on straight, align the bottom edge of a 2-foot level with the level cut of a rafter tail. Make a pencil mark on the sheathing where a horizontal line would meet the exterior wall. Do this at every fourth or fifth rafter, and snap a chalk line connecting the marks. Align the bottom edge of a linear 2 x 4 with the chalk line and nail it flat against the exterior wall through the sheathing into the studs for the entire length of the wall. This will be the nailer to which the “lookout” blocks are nailed. The nailer is installed before the exterior wall material (brick, siding) is applied, so that the wall material butts up against the bottom.
Step 3 - Install the Lookout Blocks
Hold a 2 x 4 lookout block flat and level against the side of a rafter tail, perpendicular to the nailer. Scribe it and cut it flush with the plumb cut. If the upper corner of the lookout block penetrates the roof plane, it will have to be chamfered. Using this block as a template, cut one block for every rafter tail. Level each block with a 2-foot level then install the lookouts by toe-nailing down into the nailer, nailing through the block and into the rafter tail.
Pre-drilling toenail holes in this instance using a drill and wood bit will help ensure the nails do not miss the mark as can often be the case where a nail appears to have a bite in the adjoining wood but doesn't. Ring-shanked nails will bite and hold a predrilled hole without slipping out later making them ideal for a permanent structural application such as this or for deck framing and scabbing in dock stringers.
Step 4 - Install the Fascia Boards
Install the fascia onto the plumb cuts. Fascia boards vary in size from 1 x 6 to 1 x 10 and can be made of wood or composite material. Make the top edge of the fascia even with the top corner of the rafter tail and nail into each rafter tail using headless finishing nails. The bottom edge of the fascia should extend below the bottom edge of the rafter tail so that a ½ inch “lip” is provided after the soffit is installed. Sight along the top edge of the fascia to ensure that it is going on straight.
Step 5 - Install the Soffit
Finally, fasten the soffit onto the bottom edge of each lookout block. The soffit can be made of solid wood or perforated vinyl. The perforations are designed to vent attic air to the outside.
When an eave overhangs the gable end it is known as a rake. It is made by extending the ridge board beyond the gable end, doubling the last rafter to provide a nailing surface, and installing one additional rafter a distance equal to the desired overhang. It is secured to the frame by lookout blocks nailed perpendicular and flat to the 2 adjacent end rafters, flush with the rafter's top edge. The rafters are notched so that the blocks are in line with the roof plane. Soffit and fascia are installed directly onto the end rafter.