What Goes Under What's on Top

When designing, replacing or building a roof, remodelers must pay as much attention to preventing moisture intrusion as they do to framing or shingles.

September 30, 2003

One of the hardest-working elements of a house, the roofing system shelters all the materials and people within from rain, snow and baking sun while withstanding the elements within the home. When designing, replacing or building a roof, remodelers must pay as much attention to preventing moisture intrusion as they do to framing or shingles.

Using good construction techniques will reduce callbacks and improve your odds of getting good insurance coverage. This article focuses on some broad techniques as well as problem areas to watch out for in reroofing, roofing an addition or building a roof. We'll assume a pitched roof, asphalt shingles and engineered wood sheathing.

Underlayment and flashing

Underlayment (or roofing felt) and flashing help prevent bulk moisture intrusion. "Anybody who claims they don't need underlayment is missing the boat," says Brad Oberg, director of technology at IBACOS Inc., a research and consulting firm specializing in building science.

Under asphalt shingles, Oberg recommends using 15-pound felt at minimum. Different climates and roofing materials call for different types of underlayment, so check manufacturer specifications and building codes.

The underlayment should be in-stalled parallel to the eaves, starting at the bottom of the roof, with each upper layer overlapping the layer below to keep water that might leak or be blown through the shingles moving to the edge of the roof rather than allowing it to seep in. Edges require extra detailing, with the underlayment going over the drip-edge eaves flashing but under the drip-edge flashing along the rake or side.

Roof-line interruptions require extra attention to detail, especially flashing. Skylights, chimneys, vents, valleys and particularly the intersections of vertical planes (walls) and horizontal planes (roofs) are the most likely locations for leaks.

Properly installed, flashing directs water toward the roofing material's surface rather than the bottom of the underlayment. Oberg says underlayment should run under side-wall flashing and potentially even turn up at wall intersections.


Roofing ventilation minimizes moisture buildup, as it lets air flow through and dry the roof assembly if minor leaks or condensation develops. It also lowers the roof deck's temperature in the summer. Oberg says standard building codes require that vented attics have 1 square foot of free ventilated attic area for every 300 square feet of attic space. If the home has only gable vents and no ridge vents, the ratio should be 1:150.

However, he adds, "Research is bringing into question whether attics need to be ventilated." IBACOS prefers that attics be treated as conditioned space, like the rest of the home. Acknowledging that this can be impractical, Oberg says that when ventilation is used, any ceiling openings for electrical fixtures, chases, etc., should be sealed to prevent air leakage between the living area and the attic.

When putting a new roof over a vented attic, don't assume it was done right the first time. Make sure eave vents are not blocked by insulation or painted shut. Also, check gable vents to be sure the sheathing was cut out and the building paper cut away when the home was built, and clean out any bird nests or dust.

Existing ducts can be sealed with mastic and buried in blown insulation, Oberg says. Otherwise, create a cathedralized attic with rigid insulation and sheathing over existing sheathing, and blown or batt insulation between framing members at the underside of the roof sheathing.

Ice dams

Ridges of ice can build up at the edge of a roof or below roof penetrations and prevent melting snow from draining off the roof, causing water to back up and leak into the home. Warm air rises through the attic and melts snow at the top of the roof. When the water reaches the cold eaves, it freezes. With repetition, an ice dam forms. Water pooling behind the dam can get under shingles, the underlayment and the sheathing and into the attic, the framing and the walls.

One long-term solution is to increase the attic insulation to decrease heat loss. Another is to improve ventilation so the roof keeps a more uniform temperature. Oberg's preferred method is to use an ice and water membrane as underlayment, installed per manufacturer instructions.


Common Sheathing Mistakes

Fulton Desler, senior engineer with APA - The Engineered Wood Association, says he frequently sees the following sheathing errors:
  1. Butting the sheathing panels too close together when installing. "It's the easiest way to do it, of course, but it doesn't leave the panels room to expand when they pick up moisture," Desler says. He recommends leaving a 1/8-inch space on all sides, using an 8-penny nail or spacer-type panel edge clips to assist in measuring. "After they've been on there for a few days or weeks, the panels pick up the moisture."
  2. Fasteners missing their mark. Nail guns make roofing a less strenuous task, but Desler says some roofers move too quickly with the guns, so not every nail hits a support. In high-wind areas, that can be especially costly. To ensure contact, Desler suggests standing on the panel over the framing near the fastener location.
  3. Switching directions over the eaves. Panels should go down perpendicular to the supports. Roofers sometimes lay them parallel over the eaves, Desler says. "It changes a multiple-span support condition to a single-span support condition," which weakens the roof's strength. He proposes laying them perpendicular and trimming the excess material.
  4. Allowing sheathing to bend during nailing. Standing between supports can bend a panel. "The person doing the nailing will stand on either side of the support he's nailing to, he'll hit one of the trusses, and his own weight will cause the sheathing to deflect," Desler says. "He'll nail it right into the sheathing so it captures the deflection." Even when covered by shingles, these dips are noticeable in the right light.

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