One way you can prevent problems with the roof of your historic house is to improve ventilation in your attic. Ventilation helps to regulate air temperature. Good ventilation in the attic space under your roof will help to ensure the air temperature under your roof is similar to the outside air temperature. Since the heat inside your house rises up to the roof, you want to make your roof cold.

Prevent Two Roof Problems

Improved ventilation in your attic will help prevent these two problems:

• Sun damage to your asphalt shingles. Good attic ventilation will help to cool off your sun-heated roof shingles so they last longer. As the sun beats down on your roof, your attic becomes like an oven. High temperatures can bake your asphalt shingles, making them brittle over time. Without ventilation, summer heat can play havoc on the wood structure and even increase your cooling costs.
• Ice damming on your roof and winter condensation in your attic. One significant winter concern for Wisconsin homeowners is ice damming. The primary cause of ice damming on a roof is inadequate ventilation. If you provide proper ventilation from your eaves to the peak of your roof, the temperature in your attic will be less variable. The more even temperature should prevent most ice damming.

Add Vents to Improve Your Ventilation

You can improve your roof ventilation by adding more soffit vents (to the underside of your eaves), ridge vents, or standard box vents. Follow the guideline of providing one square foot of net free ventilation for every 150 square feet of your attic floor space. Your vents should be distributed evenly between the soffit area and the high side of the roof.

A standard box vent provides about one-half of a square foot of net free ventilation. Box roof vents are visually intrusive, so place these vents on a side of your roof near the peak where they will not be visible from the front of your house. The best box roof vents come with a flashing system that slips under the high side of a shingle. Follow the manufacturer’s recommendations for installation.

Invest in the Best Ventilation Solution

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Retrofitting your soffit with a continuous soffit vent, installing insulation with a baffle and roof vents is the best way to prevent ice dams. Source: WHS - State Historic Preservation Office

Your best ventilation solution, both visually and functionally, is to install continuous-ridge vents capped with a low-profile shingle plus a continuous soffit vent running around the entire perimeter of your eaves.

• Continuous soffit vents. Continuous soffit vents are louvered strips with screening on the backside. They measure 1-1/2 to 2 inches wide. Most lumber yards sell continuous aluminum soffit vents that can be painted.
• Continuous-ridge vents. Continuous-ridge vents are low profile and easily installed by a professional roofer. A good-quality continuous-ridge vent is made of mesh-like matrix matting with shingles on top and can barely be seen from the ground.

TIP: Make certain your shingle-capped continuous-ridge vent runs completely from one end of the roof to the other and covers 100% of your roof ridge. If your roofer installs a short ridge vent on a long ridge, it won't give you enough ventilation and will ruin the look of your roof. Also make sure your roofer runs the ridge vent all the way out to the eave edge (even if the roof boards do not extend to the edge) to avoid creating a “hump” in your ridge.

Check with the manufacturers of both of these venting products to determine how much net free ventilation each type of vent will provide. A continuous-ridge vent that allows 18 square inches of net-free ventilation per running foot will give you one square foot of net-free ventilation every eight feet.

Consider Power Roof Venting

Power roof venting is a more complicated solution to roof ventilation issues. Power roof vents have an electrical fan and a thermostat or humidistat to control the fan motor. Therefore, you must run electricity to the fan and install a device to control the fan operation. Power roof vents must be carefully balanced to ensure even ventilation in your attic. The motor can burn out without your knowledge, so you must be able to regularly check the fan’s operation.