Appropriately sized roof overhangs have two major benefits: They keep unwanted, hot summer sun from heating a home, and they help protect the home from moisture damage caused by precipitation.
While protecting the walls and foundation from excess moisture, roof overhangs over entries and windows are also convenient for the occupant during foul weather. This architectural feature that can also enhance a home’s visual appeal.
An overhang over an entry, such as a porch or even an eave, protects occupants from precipitation, but also protect the door’s finish from moisture around jambs, trim, and thresholds, thereby minimizing the need for maintenance.
Overhangs above windows allow the resident to enjoy the sound of falling rain without worrying about the rain coming inside.
Studies have shown that the larger the size of overhang for windows or doors, the less frequently moisture penetration problems will occur on the exterior and foundation walls.
The local climate will determine the minimum size of overhangs. In moist climates with significant rainfall, liberal use of overhangs is strongly recommended.
Recommended Minimum Roof Overhang Widths for One- and
Two-Story Wood Frame Buildings
|Climate Index (See Figure Below)
|Eave Overhang (Inches)
|Rake Overhang (Inches)
|Less than 20
|12 to 40
|40 to 70
|71 and above
|24 or more
|12 or more
Source: Modification of Prevention and Control of Decay in Homes by
Arthur F. Verrall and Terry L. Amburgey, prepared for the U.S. Department
of Agriculture and U.S. Department of Housing and Urban Development,
Washington, DC, 1978.
Use the overhang widths in the table above if all walls have a properly constructed weather barrier, roofs are adequately guttered, and normal maintenance of the exterior will occur. For overhangs protecting more than two stories of walls with exposed windows and doors, consider using larger overhangs.
Rake (gable end) overhangs deserve special consideration because more costly “outrigger” framing methods will be required for overhangs exceeding about 12 inches in width, and the appearance may not be acceptable to some home buyers. For sites subject to frequent wind-driven rain, larger overhangs and drainage plane techniques that include an air space behind the siding should be considered. For non-decay-resistant wood sidings and trim (e.g., windows and door casings), larger overhangs and porch roofs are recommended.
The climate index map does not directly account for wind-driven rain, a condition that varies with local climate or site exposure.
As with rain on the building envelope, properly sized roof overhangs can minimize the exposure to solar radiation and radiation-related problems such as fading of furniture and carpeting.
It is possible to block unwanted direct summer sunlight from entering windows while allowing the heat gain through windows from winter sunlight. The width of a roof overhang that allows this seasonal solar shading depends on where the building is located with respect to the equator. Buildings situated farther south receive greater protection from the summer sun by roof overhangs because at higher latitudes, the sun is lower in the sky than at lower latitudes.
To determine the exact size a south-facing overhang that allows winter sun into a home but protects the interior from direct summer sun, visit Durability by Design or see the instructions below.
Overhang Sizing Rules
- Draw the wall to be shaded to scale.
- Draw the summer sun angle upward from the bottom of the glazing.
- Draw the overhang until it intersects the summer sun angle line.
- Draw the line at the winter sun angle from the bottom edge of the overhang to the wall.
- Use a solid wall above the line where the winter sun hits. The portion of the wall below that line should be glazed.