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Construction Tips - Details for Roofs

A home's roof provides the first line of defense against the greatest source of unwanted moisture in and around a building: rain. As illustrated in the image, both the overall design of the roof and the finishing materials are important factors in designing a building to withstand moisture penetration.

Roofing systems fall into two categories: near-flat or low-slope and steeply pitched. The roofing system, whether pitched or low-slope, is made up of a number of different components:

• Roof sheathing
• Underlayment
• Roofing material
• Roof intersections
• Flashing Details
• Ventilation

Get the brochure!

APA's Designing Roofs to Prevent Moisture Infiltration brochure, available for free download in PDF format from the APA Publications Library, describes the various weatherproofing methods used to keep water away from the interior.

Steeply pitched roof systems rely on the force of gravity and the surface friction of the roofing materials to direct the flow of water downward and outward. These systems rely on a series of overlapping elements to redirect rainfall:

• Roofing felts
• Shingles
• Tiles
• Flashing details

The pitch of the roof provides the gravity and the detailing provides the redirection.

In low-slope roofing systems, water is kept outside of the building envelope by providing a perfect waterproofing barrier over the entire roof system and around every penetration in the roof. Instead of providing the redirecting force to channel water away from the inside of the building envelope, the force of gravity drives the water into every imperfection in the waterproofing system. Moderately high winds can force water to collect in areas that would normally drain adequately. Once standing water is present, even minor defects can cause major water leaks.