How to Determine Cement & Sand Quantities for Screeding a Roof

Cement beds are installed onto roofs that will receive tile roofing shingles. The installation of this cement bed is called roof screeding. The process requires that wet cement be poured onto an established roof and leveled with screed boards. However, before any screed work is accomplished, you must determine the amount of sand and cement needed to complete the project. The average do-it-yourselfer can perform the required measurements and calculate these amounts in an hour or two, depending on the size and scope of the project.

Things You'll Need

• Tape measure
• Calculator

Instructions

1. Determine the area of coverage to which the screeded cement will be applied. Measure the length and width of the roof and record these measurements. Include the length and width of overhangs, dormers and extensions that are not included in the larger part of your roof. Determine the thickness of the cement bed. Generally, one to four inches of cement thickness are required to support tile roofing, one inch for lighter roofing loads and four inches for heavier loads.

2. Multiply the length times the width of the coverage areas to determine square footage. For example, if your roof is 20 feet long and 20 feet wide, multiply 20 times 20 to get 400 square feet. Multiply the total square footage times the cement bed thickness. Convert inches to a decimal notation by dividing the thickness by 12 inches. For example, if you are laying a 2-inch thick bed, divide 2 by 12 to get .16. Multiply 400 square feet times .16 to get the cubic feet needed for the screed bed -- 64 cubic feet in this example.

3. Calculate the amount of cement and sand needed for the project. One 94-pound bag of cement is equal to 1 cubic foot. Additionally, the cement is combined with sand at a ratio of 2 to 1, or two parts of sand to one part cement. So, to mix one bag of cement, you will need to combine it with 2 cubic feet of sand. This mixture totals 3 cubic feet. Divide the total cubic feet of the project, which is 64 cubic feet, by 3 cubic feet to get the total mixture needed (64/3=21.3). Round up 21.3 to 22. Since there are 2 cubic feet of sand per unit of mixture, multiply 22 times 2 (sand) and 22 times 1 (94-pound bag of cement) to get the totals for each. For this example, you would need 44 cubic feet of sand and 22 bags of cement.

References

• "Masonry Skills"; R.T Kreh; 2002
• "Masonry & Concrete"; Benjamin W. Allen; 1997
• "Building with Masonry"; Dick Kreh; 1998