What Is a 2/12 Roof Slope?

2/12 Roof Slope

• The slope -- also know as pitch -- of a roof is calculated by the vertical rise in inches for every 12-inch horizontal length, or run. For example, a roof with 6 rise/12 run slope, or 6/12, is a roof that rises vertically 6 inches for every 12 horizontal inches. A 2/12 roof slope, with a 2-inch rise for every 12-feet run, is is considered a flat roof. Although some flat roofs are even flatter than 2/12, to avoid drainage problems, no roof can be entirely flat.

Determining Slope

• To find a roof's slope, Roofing Calculator suggests using a ladder and placing one end of a 12-inch level on the roof's surface, holding the tool level. Then, using a tape measure or ruler, measure from the other end -- the end of the level facing you -- down to the the roof surface. If that measure is 4 inches, for example, the roof slope will be 4/12. The rise is always first and the run second.

Flat Roofs Pros and Cons

• Flat roofs are often found in cities because they offer more options when there isn't a lot of room for expansive, sloping roofs. Flat roofs provide rooftop promenades and space for various building equipment, such as that related to heating and cooling. In other areas of the country, flat roofs are typically used on secondary buildings, such as sheds. According to Max Alexander in his "This Old House" magazine article, "Building a Flat Roof Right," flat roofs don't last as long as higher-sloped roofs because higher-sloped roofs are less affected by snow, rainfall and water runoff. However, some modern flat-roofing materials can last up to 20 years, almost comparable to those of sloped roofs.

Flat-Roof Coverings

• Flat roofs require certain types of coverings, such as built-up or rolled roofing. Higher-sloped roofs need shingles or tiles, according to Buyer's Choice Inspections. Materials appropriate for a flat roof each have their pros and cons. They tend to be low cost, but most are not well-suited for use in areas that receive extreme amounts of snow and rain.

  Built-up roofing involves alternating layers of heavy roofing felt and hot asphalt or tar. The layers are coated with a mineral, such as gravel. Modified bitumen is roofing asphalt that includes bitumen with layers of polyester or fiberglass for additional strength.

  Roll roofing is similar to asphalt shingles in composition but is available in rolls. It has a relatively short life expectancy of less than 10 years because it is a single-ply material, according to Home Improvement Helper. Roll roofing is also known as 90-pound felt.

  EPDM (ethylene propylene diene monomer) Rubber Roofing consists mainly of oil-based by-products with modifiers and stabilizers. It's made to resist damage from sunlight. A longer-lasting covering, it is more expensive than other flat-roof coverings.