Flashing is an integral factor in a building's ability to resist water intrusion. Flashing is required at the head, jambs, and sill of all windows, doors and other similar wall openings, roof penetrations, copings, above wood trim, porches or decks, and at built-in gutters. The intersection of a roof above a wall must have flashing stepped up the roof slope. Flashing is also needed at the meeting of two different roof slopes, such as at projecting gables and dormers.

There exist many different and effective ways to detail flashing conditions and wall assemblies to prevent water intrusion. Attention to this important aspect of building performance is necessary in both the design and installation phases of your building project. This section seeks to provide a broad overview of the most general conditions and a few resources for further assistance, as it is assumed that those experienced in designing and installing flashings have a working knowledge of pertinent considerations and guidelines.

Walls

Installing flashing and layering the wall assembly elements to drain water to the exterior are the primary techniques used to control water infiltration. In order to be effective, the drainage plane must provide continuity, especially at windows, doors, service penetrations, vent stacks and chimneys. Flashing must direct water toward the exterior of the building.

Flashing, Fig. 1

Sill and jamb flashing installation diagrams

Flashing, Fig. 2

Flashing detail at exterior deck or porch framing

Roofs

Roof coverings and related flashing must be installed to prevent moisture infiltration. Flashing is required at wall and roof intersections, changes in roof slope or direction, around roof openings, and at gutters. Parapet walls must be capped with a noncombustible, weatherproof material, with joints protected from intrusion of moisture. At wall and roof intersections, through-wall flashing is required, if the exterior covering is a moisture permeable system.

Flashing, Fig. 3

Detail at roof-wall intersection Flashing, Fig. 4

Flashing locations at floor framing.

Flashing, Fig. 5

Diagram of roof-wall intersection assembly Flashing, Fig. 6

Section detail at roof-wall intersection

Building Science Corporation
70 Main St.
Westford, MA 01886
(987) 589-5011
www.buildingscience.com

See "Building Contractors", "General Contractors", "Siding Contractors", and "Roofing Contractors"

National Roofing Contractors Association
10255 W. Higgins Rd., Ste. 600
Rosemont, IL 60018-5607
(847) 299-9070
www.nrca.net