Roof Overhang Calculations and Latitude

Overhangs and Heating

• Overhangs help control heat levels in your home by regulating the amount of sunlight through windows. In the winter, overhangs allow sunlight to penetrate windows throughout the day. During the summer, these architectural features help prevent sunlight from directly entering your home through most of the day. This helps keep your home warm in the winter and cool during the summer; this, in turn, leads to lower heating and cooling costs and reduced energy consumption.

Overhang Basics

• Two factors that affect overhang performance and calculations are your home’s latitude and orientation. Overhangs only work, as intended, if your home sits more or less on the axis of all directions, with one side each facing north, south, east and west. According to the U.S. Department Of Energy, the overhang on a house that is more than 30 degrees off true-south bearing is significantly less useful than one on a house within 30 degrees of a true-south bearing. The latitude of your home, which describes its basic north-south position on the earth, relates directly to the path of the sun, thereby affecting overhang calculations.

Calculations

• Overhang calculations help determine exactly how far an overhang must protrude beyond the walls of your house to provide optimal protection. These calculations are very complex and require considering the height of your overhang, the distance between overhangs and windows, the angle of the sun at your specific latitude and more. Architects and engineers use sun angle maps made specifically for each latitudinal degree and the intended height of windows in a home to determine the right overhang size for each project. You can contact a local university extension or architecture firm for these charts or information, but the calculations still require complex math.

Simplified Formula

• Alan Jefferis and David Madsen, authors of the book “Architectural Drafting and Design,” provide a simplified formula for an overhang designed to provide 100 percent shade at noon on the longest day of the year. The formula is: Overhang = Windowsill Height/F, where F is a number based on your latitude. At latitude 28 degrees, which covers parts of Florida and Texas, F equals 8.4. Assuming a windowsill height of 15 feet for a second-story window, Overhang = 15/8.4, or 1.79. This means the overhang for this window setup should extend 1.79 feet beyond the walls of the home. You can obtain your “F” value from a local building society or university extension or Jefferis and Madsen’s book.

Additional Information

• The Society of Building Science Educators publishes a free pamphlet entitled “Designing with the Pilkington Sun Calculator.” This publication provides basic information about determining sun angles and overhang requirements based on a chart of the earth and a simple tool you can purchase or build at home. Keep in mind that the further north your latitudinal position, the longer overhang you need. This holds true because the sun follows a more direct path across the center of the sky the closer it is to the equator.