How to Calculate Mortar Requirements

Written by larry simmons
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Performing any building project requires that you have enough mortar on hand at the start for the entire project. Stopping in the middle to obtain more mortar can lead to problems in uneven setting, or with colour mismatches in the dried mortar. Calculating the mortar needed depends heavily on the type of materials being laid as well as required thickness. To deal with the multiple variables involved, builders use what is known as “Rule 38” to estimate mortar usage on smaller projects. Rule 38 finds that in order to make a cubic yard of mortar, 38 cubic feet of raw materials will be necessary.

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  • Mortar ratios for the project
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  1. 1

    Find the ratio of mortar to sand required for use in the mortar mix for the project.

  2. 2

    Add the ratio numbers together and divide 38 by the sum to determine the cubic feet of cement needed for the mortar mix. For example, if a mortar mix to lay flagstones requires a 1:3 ratio of one part cement to three parts sand, add the ratio to get a four and then divide 38 by that number to get 9.5. You’ll need 9.5 cubic feet of cement for each cubic yard of coverage.

  3. 3

    Multiply the amount of cement needed by the ratio for the sand, which in the example is three. So 28.5 cubic feet of sand will be needed for each cubic yard of coverage. The total amount of raw materials needed with 9.5 cubic feet of cement and 28.5 cubic feet of sand is 38 cubic feet.

  4. 4

    Calculate the total amount of mortar needed to complete the job by estimating the coverage necessary for each 100 square feet of wall being built. Coverage estimates will vary according to site conditions, but a general rule provides that for 100 square feet of wall built using 4-inch blocks, 13.5 cubic feet of mortar is necessary. To build 100 square feet of wall using 8-inch blocks you will need 8.5 cubic feet of mortar.

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