Install a roof or ridge vent

[edit] Roof vent

Ventilating your attic greatly reduces the amount of accumulated heat which otherwise radiates down into your house. Roof vents are inexpensive ($5 to $10 each) and easy to install. They should be located at each end of the roof and every 12' between ends. Installing roof vents will not make your house cooler in winter; they will help remove moisture from the attic.

[edit] Ridge vent

For even more effective attic ventilation, a continuous ventilation system, Coolvent, can be installed along the ridge, beneath the ridge shingles.

Coolvent is lightweight and durable, and it eliminates the need for turbines or louvered vents. It's also designed to keep out bugs and wind-driven rain.

Coolvent comes in 20' rolls, in several different widths, and can be installed by the homeowner on new roofs or as an easy re-fit to existing shingle-style roofs. With re-fits, you can lift the ridge shingles without damaging them by working in cool weather or early in the day. Coolvent is available in building supply stores throughout the US and Canada. Cost is approximately $30 (US) per roll.