How to Build a Low CMU Block Wall

Footings and Forms

• Concrete-block walls need a foundation, or footing, of poured concrete to support them. The footing must be below the frost line and about twice as deep and twice as wide as the wall. Using wooden forms made from 2-by-4s allows you to cast the poured concrete into the proper shape and dimensions. Before attempting to build the wall, the concrete must dry completely, which could take up to three days.

Dry Run

• A dry run tells you the number of blocks you'll need for the courses. The corner block must be placed an equal distance from the sides of the footing and have a smooth face on the exposed end. Putting pieces of 3/8-inch scrap plywood in between the blocks will take the place of the mortar. Lay all of the necessary blocks onto the footing, and cut any blocks to fit.

Courses

• Set string lines as you lay the courses to keep everything straight. You can check with a spirit level as you go along. Spreading the mortar about 1 inch deep and 8 inches wide works well for a typical wall that rests on a 16-inch footing. Always make sure the corner block lies perfectly level before laying the rest of the course. Place blocks with the hollow cores facing up.

Tips

• The blocks must be staggered so that the joints on adjoining courses do not intersect. Mixing more mortar than you can use in 90 minutes might result in the mortar starting to set before you place it on the wall. You can reuse surplus mortar by taking it off with a trowel and returning it to the mortar board. According to Ace Hardware, when checking for levelness, use a 3-foot-long level for best results.