The Length of an Eave for Shade on a Porch

All About Eaves

• Eaves are the portions of a roof that extend out beyond a building’s walls -- or supports and railings, in the case of porches. These roof extensions provide protection from rain, snow and moisture during winter storms. In summer they may protect from rain but also cast shade that keeps walls, windows and covered porch decks cool on hot days.

Style and Regulations

• The length of eaves, such as those for porches, is partially dictated by architectural style. Long or deep eaves, for example, characterize Craftsman-style homes. Eaves on Gambrel-style homes are much more modest. Shed-style porch roofs -- the easiest for do-it-yourselfers to tackle -- have variable eave widths. According to the website Front Porch Ideas and More, in most cases the length of your porch eaves or overhang should match that of your home. It should also have the same pitch or “rise and run” as the other roofs on your home, to avoid detracting from overall architectural style. Check with local building officials to find out if there are specific zoning or other restrictive requirements.

Summer and Winter Sun

• If blocking the intensity of summer sun is a key aim -- and if your home’s architectural style allows it -- you may want to extend your eave overhang to maximize summer shade. Yet you don’t want porch eaves longer than absolutely necessary. Ideal eave length lets in some cheering low-angle winter sunlight. You can figure out an optimal eave length for your porch with the help of a solar angle calculator -- a distance that blocks overhead summer sun yet still allows winter sun.

Calculating Overhang Length

• As the Department of Energy’s Energy Savers website points out, there is no universal formula for figuring out optimal overhang length. This is because one calculation works well in one climate zone but not elsewhere. As a general guideline for the northern hemisphere, in cold climates you locate the summer shadow line at mid-window at the summer-solstice, or June 21, sun angle at solar noon. In moderate climates, the shadow line falls at the windowsill at the June 21 solar-noon sun angle. For hot climates, the shadow line falls at the windowsill but at the vernal-equinox, or March 21, angle of the sun. Keep in mind that “solar noon” is typically not the same as local-time noon. To calculate winter sun angles, use winter-solstice and autumn-equinox sun angles, respectively.