How to Calculate Roof Overhang for Solar Heat

Determine the latitude of your city. Generally this information is found on maps, at your local agricultural or cooperative extension office, city hall, the library or online at websites such as MapQuest or City-Data.

Measure the distance from the bottom of the window to the soffit, or the bottom of the eaves.

Determine the "F" factor for your latitude. According to the Arizona Solar Center, the "F" factor is: 20 degrees: 5.6 to 11.1; 32 degrees: 4.0 to 6.3; 36 degrees: 3.0 to 3.5 degrees; 40 degrees: 2.5 to 3.4; 44 degrees: 2.0 to 2.7; 48 degrees: 1.7 to 2.2; 52 degrees: 1.5 to 1.8: and 56 degrees: 1.3 to 1.5. Use the smaller number to shade the window on June 1 and the larger number to shade the window until August 1.

Multiply the height measurement by the "F" factor to find the depth of the overhang.

Use a simpler calculation. In 20- to 36-degree latitudes, the depth of the overhang is roughly one-quarter of the measurement from the window to the eaves. In northern latitudes, 48 to 56 degrees, the overhang depth is one-half the measurement of the window to the eaves. In latitudes between 36 and 48 degrees, average the difference between one-quarter and one-half the measurement.

Add 10 percent to the width of the overhang if your windows face southwest or southeast to ensure 100 percent shading at noon.