How to Calculate a Corrugated Metal Roof

Corrugated metal roofing is used through out the world, from Alaska to Australia to India. Its appeal owes much to the simplicity and strength of the material, as well as its availability and low cost. Corrugated metal sheeting is available in different thicknesses, styles and sizes, but regardless of the shape and size, the sheets must be overlapped, like roofing tiles, to provide a continuous and watertight finish, with no gaps through which weather can penetrate. The overlap must be taken into account when determining how many sheets to use.

  1. Determine the length and width of the area requiring a roof. Obtain the dimensions from the builder's plans or stretch a tape measure along the structural members of the roof to directly measure the length and width. Record the values.

  2. Add on any required overlap at the edges, then round up the dimensions to the next larger foot to simplify calculations. For example, if the measured length is 19 feet 10 inches, and an additional overlap of 6 inches is required at both ends, the final figure will be 21 feet -- 19 feet 10 inches plus 6 inches times 2 equals 20 feet 10 inches, which is rounded up to 21 feet.

  3. Multiply the length of the roof by the width to establish the surface area in square feet. For example, if the length is 21 feet and the width is 10 feet, the surface area is 210 square feet -- 21 times 10 equals 210.

  4. Determine the width and length of the metal sheets to be used. Obtain the data from the manufacturer's specifications or by direct measurement. Standard sheets in the U.S are 36 inches wide but the length can vary.

  5. Determine the overlap required so that each roofing sheet can rest on the adjacent sheet to avoid gaps in the roof. This value varies according to roof angle, local climate, type of sheet and exposure to the wind. If in doubt, seek expert advice.

  6. Subtract the overlap value from the length and width of the sheet. For example, if sheets laid side by side overlap adjacent sheets by 6 inches, subtract 6 inches from the width. The result is the effective width of the sheet, the distance of roof that the sheet will actually cover.

  7. Multiply the effective length and width of the sheet to obtain the effective surface area. For example, if the effective width is 2.5 feet and the length is 7.5 feet, multiply 2.5 by 7.5. The result, 18.75 square feet, is the area each sheet will actually cover.

  8. Divide the total roof area by the effective area of a single sheet and round up the result to the next whole number. The result is the number of sheets required. Using the example values of a roof area of 210 square feet and an effective sheet size of 18.75 square feet, 12 sheets are required, as 210 divided by 18.75 equals 11.2.

  9. Tip

    Order 10 percent extra to allow for damaged sheets and errors in your calculations. This is the standard safety margin builders apply to everything from flooring to bags of cement. If your supplier will provide sheets on a use or return basis, order several extra sheets to ensure that you will have sufficient sheets to complete the job. Return any that are unused when the roof is finished.

    If you are unsure about overlapping the sheets, seek advice from a qualified expert who can advise you about climate factors that influence the size of the overlap.

    Local regulations, including the San Francisco Building Code, should be consulted before installing a new roof. Strict rules apply under some circumstances, such as when reroofing historic buildings.


    Sheet lengths vary considerably, so always check with your supplier before determining the number of sheets needed. For example, many sheets are available in 8-, 10-, 12- and 20-foot-long versions, and also can be produced to the length specified by the builder.