KNOW-HOW
Cornices and casings get all the attention, but what roofers call the soffit—that exposed area beneath the roof overhang—is important to get right.
The secrets of sexy soffits.
The soffit—the exposed underbelly of any overhead building component, such as a roof overhang or projecting cornice—is often overlooked on many houses. Essentially a ceiled area for the eave, a soffit runs from the top of the exterior wall to the outer edge of the roof. Whether wood, vinyl, or aluminum, the soffit material is typically screwed or nailed to lookout rafters or tail pieces that extend beyond the edge of the house.
The soffit exposure profile (from wall to fascia) on a building’s exterior can vary from 2–3" to 3' or more, depending on construction. It can be non-ventilated or ventilated, to prevent condensation. Soffit vents are usually covered with grilles to prevent insects and animals from entering the roof.
How soffits are built is critical to longevity—one does not want water to penetrate the soffit, where it can percolate into the house. Another function of the soffit is to provide ventilation to protect the roof and attic from moisture buildup. And then there are the aesthetics: This is a finishing touch, visible from the ground, which should enhance the house.
If not designed and built properly, the soffit can contribute to ice dams, flashing failure, and leaking gutters. Squirrels, bats, and birds can chew or peck their way inside, especially if the wood is weakened by water. Bees and hornets are attracted to the spaces beneath the eaves: If you find a nest in the soffit, it’s safest to call a professional bee expert or exterminator (where that’s legal).
A vented soffit provides a continuous flow of air into the attic, preventing moisture buildup. Cold air from the soffits is warmed as it enters through the vents; it travels through the attic and rises by convection to the rooftop, where it is vented out. A general rule of thumb is to allow one square foot of venting for every 300 square feet of attic with a vapor barrier (or every 150 square feet if there is no vapor barrier), with half of the venting in the soffits. Rooftop exhaust vents must be at least 3' higher than soffit intakes for convection to work properly. Crest vents that run along the ridge are the most efficient and unobtrusive system. Be sure to install the vent along the entire length of the roof’s horizontal ridge, including the gables, otherwise it will look unfinished and not function as effectively.
Installation of a soffit is not complicated but you do need to plan ahead. Start by calculating the amount of material needed: Measure the under-eave area, multiplying the length by the width of each overhang, then add up the total for the linear feet of material needed. Add 10% extra for cuts and waste.
If the spaces between the rafters are narrow (usually 16" to 24"), the rafter ends serve to fasten the soffit to the fascia, along with a ledger plate attached to the wall to secure the soffit to the siding.
Perpendicular blocking pieces (also called “lookouts”) should be installed no less than every two feet to allow secure nailing and installation of the soffit material. Soffits trimmed with wood are best for period authenticity: use #2 construction-grade lumber, typically pine, fir, or spruce. Plywood is a less expensive alternative. Tongue-and-groove (beveled or beaded ceiling or siding boards) is
a classic, period-style treatment. The wood should be primed on both sides for durability; prepainting the visible side of the wood before installation avoids paint shrinkage that may expose primer.
Prefabricated vents are usually perforated metal, plastic, or cement panels that can be painted to blend into the soffit unobtrusively. Be sure to check local building codes, as ventilation requirements vary around the country, and are based upon factors including local weather patterns, temperature range, and wind loads.
So-called brick moulding—exterior wood moulding covering the gap between soffits or other casings and the side of the house—is a traditional, straightforward way to abut soffit to house. Depending on the architectural style, more ornate detailing and trim, from crown mouldings to brackets to decorative friezes, may be added as embellishment.
The soffit is one of the first parts of the roof assembly to deteriorate, but also one of the easiest to repair. First take a good look at the gutters, flashing, and roofing material, because soffit damage most likely comes from failure or improper drainage in one of these areas. Then assess areas of the soffit that needs repair or replacement, noting that the ends near the fascia are most vulnerable to water and decay. Look for split ends, peeling paint, mildew or fungus growth, or soft “punky” wood. Poking suspect wood with an icepick will uncover rot.
Prep the area to repair by nailing down loose boards and removing any downspouts, exterior lights, or other fixtures that are in the way. Make sure shrubbery and plantings have been tied back safely or removed. Repair needs will depend on the degree of damage, and range from patching in a section to replacing the entire soffit, or more.