Roof Rafter Analysis

Dead Load

• Dead load on a roof is the weight of the roof itself -- the lumber in the rafters, the sheathing and actual roof covering -- and will vary with the size and pitch of a roof. A bigger, steeper roof will have more material and thus more weight. Composition shingles or metal roofing will be far lighter than clay tile. Rafters of 2-by-6-inch lumber will be stronger but also heavier than 2-by-4s, and 5/8-inch decking will weigh more than 1/2-inch. These weights are calculated based on size, material and pitch.

Live Load

• Live load is weight added from accumulations of water, snow and ice. The International Building Code, used in all states, has specifications for live loads in various areas. Some tables break down loads by ZIP code, based on U.S. Weather Bureau statistics. Steep roofs will have smaller live loads because they shed snow and ice better than low-slope roofs. Shingle roofs accumulate larger live loads than metal, which sheds snow and ice more efficiently than the rougher surface of shingles. Some structural engineers also figure "drift" loads on roofs for sections prone to snow accumulations, like valleys where two roof lines meet.

Wind Load

• Wind load is the force of the wind against a vertical element like a roof. Flat and low-slope roofs will have smaller wind loads. These loads also are based on weather bureau statistics, by region. Roofs in areas subject to strong winds and tropical storms will have higher wind loads than those in colder, less windy regions. Most building codes now require metal "hurricane" ties to connect rafters and wall caps as added strength against wind loads.

Loads for Lumber

• Lumber plays a big part in the ability of rafters to support loads. Flat or low-slope roofs will need larger rafters than pitched roofs, which direct the load to the outer supporting walls. No. 1 grade yellow pine is the strongest rafter lumber, but No. 2 pine, fir and spruce also are frequently used. Lumber associations and building codes provide tables for maximum length of rafter spans by size and type of lumber and spacing of rafters.

Examples

• A 2-by-4 rafter of No. 1 pine on a low-slope roof with a live load of less than 20 pounds per square foot with 24-inch spacing will have a maximum span of 8 feet 6 inches. That drops to 5 feet 11 inches with a live load of 40 pounds per foot. A 2-by-6 rafter with similar loads will have maximum spans of 13 feet 4 inches and 9 feet 4 inches, respectively. Maximum spans for fir and spruce are less in every situation.