Size Overhangs to Provide Specific Amounts of Shading

As with rain on the building envelope, properly sized roof overhangs can minimize the exposure to solar radiation and, hence, minimize radiation-related problems. The width of a roof overhang that will protect walls from excessive solar exposure in the summer while allowing heat gain through windows from winter sunshine depends on where the building is located relative to the equator. The sun is higher overhead in the summer than in the winter. In addition, for any day of the year, at the more northern higher latitudes the sun is lower in the sky than at lower latitudes. Therefore, buildings situated farther south receive greater protection from the summer sun by roof overhangs, as shown in Figure 6–2.

The solar angle factors of Table 6–1 can be used to help determine overhang width to achieve the desired shading effect on south-facing surfaces. An example calculation which follows shows how the solar angle factor is used.