A lean-to shed on a building that you already have can be a handy structure for a variety of storage purposes. A lean-to can shelter firewood and lawn equipment, allowing it to stay dry in all kinds of weather. Constructing a lean-to shed requires a certain amount of planning. One consideration is the slope of the roof. The right pitch will allow rain and snow to slide off the roof, keeping what's inside your shed safe and dry.
Constructing a lean-to requires consideration of the size, location and function of the structure. For example, the ground should be level, and a shed used to store farming equipment may need to be larger than a shed used to store garden equipment. The pitch of the roof should be determined by how much rain and snow you get in your region of the country. Areas with heavy snowfall may require a steep pitch to allow water and snow to slide off the roof. A more gentle pitch may be fine for an area that doesn't get as much bad weather.
About Roof Pitches
Roof pitch is a measure of the slant of a roof. This measure is calculated in the number of inches the roof rises vertically for every 12 inches it extends horizontally, according to Joseph Truini of the "This Old House" website. The greater the vertical measure, the steeper the pitch.
Determining the Minimum Pitch
For construction of a lean-to shed, you need to consider the type of weather in your region. A roof pitch of less than 3/12 -- that is, 3 inches of rise for every foot of horizontal run -- cannot shed rainfall and snow effectively, according to the Minnsnowta Roof Razor, a company that sells roof snow-removal equipment. Too much snow can cause a roof to collapse. A 4/12 pitch or higher would be needed in this type of climate. In the South, where snow isn't a problem, a pitch of 3/12 is fine.
Measuring Roof Pitch
To measure the roof pitch on a lean-to structure, you will need an 18- or 24-inch level, a tape measure and a pencil. Measure 12 inches from the end of the level and mark this point with the pencil. Hold the level straight from the slope of the roof so that the bubble is centered. Then measure the distance down from the level to the roof below. This is the pitch.