corner in each course. Table 8-4 lists the average number of concrete masonry units by size and approximate number of cubic feet of mortar required for every 100 square feet of concrete masonry wall. Table 8-5 lists the number of 16-inch blocks per course for any wall.

You should always use outside measurements when calculating the number of blocks required per course. For example, a basement 22 feet by 32 feet should require 79 blocks for one complete course. Multiply 79 by the number of courses needed. Thus, a one-course basement requires a total of 790 blocks for a solid wall, from which deductions should be made for windows and doors. If any dimension is an odd number, use the nearest smaller size listed in the table. For example, for a 22-foot by 31-foot enclosure, use 22 feet by 30 feet and add one-half block per row.

As a Builder, you might find yourself in the field without the tables handy, so here is another method. Use 3/4 times the length and 3/2 times the height for figuring how many 8-by-8-by-16-inch blocks you need for a wall. Let's take an example: Given: A wall 20 ft long x 8 ft high

ESTIMATING MORTAR

You can use "rule 38" for calculating the raw material needed to mix 1 yard of mortar without a great deal of paperwork. This rule does not, however, accurately calculate the required raw materials for large masonry construction jobs. For larger jobs, use the absolute volume or weight formula. In most cases, though, and particularly in advanced base construction, you can use rule 38 to quickly estimate the quantities of the required raw materials.

Builders have found that it takes about 38 cubic feet of raw materials to make 1 cubic yard of mortar. In using rule 38 for calculating mortar, take the rule number and divide it by the sum of the quantity figures specified in the mix. For example, let's assume that the building specifications call for a 1:3 mix for mortar, 1 + 3 = 4. Since 38 4 = 9 1/2, you'll need 9 1/2 sacks, or 9 1/2 cubic feet, of cement. To calculate the amount of fine aggregate (sand), you multiply 9 1/2 by 3. The product (28 1/2 cubic feet) is the amount of sand you need to mix 1 cubic yard of mortar using a 1:3 mix. The sum of the two required quantities should always equal 38. This is how you can check whether you are using the correct amounts. In the above example, 9 1/2 sacks of cement plus 28 1/2 cubic feet of sand equal 38.

Figure 8-6.-Planning concrete masonry wall openings.