A house roof—to be as free of decay as possible—needs ventilation. One way to achieve this is to install gable vents.
These are window-like, immovable louvered frames in the outside walls of houses. They extend through the siding just blow the roof peak and allow airflow from the house’s attic space to the outside.
An alternative is the ridge vent. The ridge vent, though, doesn’t do the complete job in isolation.
It’s part of a system.
There are a number of ridge vent materials on the market. One type is a coarse, spongy strip of material just under the topmost course of shingles, the shingles that run linearly along the top ridge on the roof’s peak.
The materials’ purpose is to permit airflow from under the roof boards upward and outward to the outside air.
For air to get to the ridge vent from underneath, the sheathing boards must not continue right to the peak. Instead they must fall short, leaving an open space on either side of the peak.
The builder installs the ridge vent strip along this opening, covering it, then installs the last course of ridge shingles over the vent.
Air that flows out of the ridge vent moves along the space between the roof rafters. A potential problem with this is that often interior finish work, including insulation, blocks this air space.
The solution is something called a rafter vent. A rafter vent is a material, typically polystyrene foam or similar material, made into panels that are installed between the rafters just under the roof sheathing.
The rafter vents have formed channels that allow air flow and prevent insulation from clogging the entire air space between the rafters.
There’s one last element in the system and that is the soffit vent. Under the eave below the roof’s bottom edge is a sort of grate that lets air in or out.
Although sealed off from the rest of the house’s interior, this soffit vent completes an air flow path. The path leads directly into the channels formed by the rafter vents and continues to the ridge vent.
Once the ridge vent and the rest of the system is in place, air can flow freely in this space between the exterior roof and the house’s interior. Air flow may be from the soffit vent upward and out through the ridge vent most often but, at times, this flow may reverse.
The important thing is that there is free air passage. This helps keep mold and rot from developing in the roof boards under the shingles.