Detailing The Ridge

We must design a detail for the ridge that is consistent with the eave and rake details. We begin by drawing the structural elements of the ridge: the rafters from each side and a ridge board between their plumb cuts. The building code requires only a nominal 1" ridge board, but we know that most builders prefer to use a nominal 2" ridge board because it usually leads to a straighter ridge (Ijocal Skills and Customs). Before we settle on the height of the ridge board, however, we must work out the ventilation opening at the top.

The functional requirements for the ridge detail are that it divert water to the roof surfaces on either side and that it provide screened, water-protected openings for ventilation of the airspaces between the rafters. An easy way to satisfy both of these requirements is to use a manufactured aluminum ridge vent strip that is simply nailed over the top course of shingles on each side. The strip is designed with screened ventilation openings that are protected from gravity-driven water by overhangs and from wind-driven water by aerodynamic baffles. For our building, the disadvantage of the aluminum ridge vent is that we don't feel its appearance is up to that of a roof finished with a high-quality material such as wood shingles. We would prefer to finish the ridge either with a pair of cedar or redwood boards, or with a traditional44Boston ridge," composed of the same wood shingles that are used for the roofing. This leaves us with the problem of providing ventilation openings that arc protected from water penetration and insects.

Some catalog research turns up several proprietary designs for protected ridge ventilation strips that can be covered with shingles or boards. We select one that we know is available locally (Off-the-Shelf Parts) and trace its catalog detail onto our developing ridge detail. We draw the plywood sheathing and hold the top edge of each slope back from the ridge line by the 2"

Ri^e board Rafters

dimension recommended by the vent strip manufacturer. We draw the courses of wood shingles leading up to the ridge, and we cut them off at the upper edge of the plywood. We add the vent strip with its flexible center portion that adjusts to any roof pitch. We look again at the size of the ridge board and decide that, in order to keep the ventilation passages free, it can be only a 2 X 10.

How will we finish over the vent strip? The catalog shows that there is only a narrow zone available for nailing on each side of the vent strip. This is not sufficient for shingles, which would require two lines of nails on either side of the ridge, so we decide to use ridge boards. We will specify that these be made of unf inished Red cedar, to match the shingles. In drawing the ridge boards, we note that because of the taper of the shingles, the boards do not meet at right angles. On the final detail drawing we will add a note to the carpenter to measure the angle and plane the edges of the boards to match it. Because of the difficulty of doing exacting cutting and fitting while standing on roof scaffolding, we will recommend in the written specifications that the ridge board pairs be assembled on sawhorses on the ground and then carried up and attached. To prevent cupping of the boards as far as possible, we show a Relieved Back on each board, and we specify brass screws for all of the fastenings (nails often pull out under cupping stress; screws cannot .

Moving to the interior of the ridge detail, we add the insulating batts. We

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