Staying the course: Building a cinder-block wall

HOME WORK

September 24, 1994|By Karol V. Menzie and Randy Johnson

A well-built cinder-block wall can be a thing of beauty -- sides that are straight, courses that are level, space that is neatly enclosed.

Cinder-block walls are the foundation of choice in new construction, but rehabbers might find a need for such a wall in an addition, in rebuilding a failed masonry wall, or in walling in an urban garden. The outside of the block wall can be finished with facing bricks, or parged or stuccoed for a more finished look.

The first step in building a block wall is pouring a concrete footing for the blocks to rest on. The footing has to be below frost line and as level as you can make it. For a foundation wall with 7 courses (rows) of block, the footing was 30 inches below grade, 24 inches wide and 12 inches deep. Footings require a lot of concrete, so if you're building long walls, it might be a good idea to hire a contractor to pour the footings.

The technique of building a block wall is the same for large or small walls. Calculate wall size based on the size of a block. Standard block size is 8-by-8-by-16 inches. You can break blocks in half fairly easily, but it's hard to get 1/4 -block or 3/4 -block sizes. (Split a block by tapping a groove on each side with the claw

end of a masonry hammer, then break it in half.) The best course is to base wall length on increments of 8 or 16 inches. Add half an inch between blocks for mortar. Thus a wall with 10 blocks laid end to end would be 10 times 16 plus 4 1/2 inches, or 164 1/2 inches long.

Figure how many courses you need based on block depth of 8 inches, plus half an inch for mortar. A wall 25 inches high would be 3 courses. (Three times 8 plus 1 inch.)

Once you know how long each course will be, convert the measurement from feet to inches and divide by 16. That will tell you how many block per course. Multiply that by the number of courses for the total number of blocks.

The block supplier should be able to tell you, based on the number of blocks you order, how much mortar you'll need. You can buy premixed dry mortar -- that is, it already has sand in it -- but it's somewhat expensive. However, if you're building a small wall, the extra expense will probably be outweighed by the convenience. Otherwise, it's cheaper to buy dry mortar and sand and mix them yourself. (Randy uses a mix of 2 parts mortar to 3 parts sand.) Add enough water to make the mixture thick but not stiff. You want the mortar thick enough to stick to the trowel and the blocks, but not so thick you can't adjust the blocks as you set them in place. It's not unlike icing for a cake: It needs to be stiff enough not to slide off the cake, but not so thick you can't spread it easily.

You can mix the mortar in a mortar pan or wheelbarrow. A pan is fine if you're working in a small space; a wheelbarrow works better for longer walls, because you can push the mortar to where you need it.

The first course of blocks should be set into a full bed of mortar -- that is, mortar spread under the entire block. Subsequent

courses will be mortared only on the sides and ends.

Courses are "locked at the corners with corner blocks, which have flat ends. (Regular blocks have flanges on each side of the end that butt against flanges on the next block.) The corner blocks alternate courses, so the first layer may have a corner block that runs east-west, and the next course will have a corner block that runs north-south. That way each corner block overlaps the block below by half a block.

To lay the second, and subsequent, courses, the sides of the first course are "buttered" with about an inch of mortar. It takes some practice to develop the buttering motion. For the outside edge of the block, you load the mortar on the trowel and slide it across the edge of the block and down. For the inside of the block, put the mortar on the other side of the trowel and repeat the motion. You're trying to get a quick, consistent bed of mortar on each side.

One trick in building a block wall is to build up the corners two or three blocks high, then stretch a string along the outside edge at each course as you work upward. The string will give you a "level" line along the outside edge -- though you'll have to keep checking with a level on both sides and top.

Practice setting the blocks in gently. Butter the ends and ease it gently into place. This will mean holding on to it, though the temptation will be to drop it, because it's heavy. But dropping it would compress the mortar too much. You want the mortar compressed to about half an inch. You can make small adjustments in the height of the blocks, to keep them level, by compressing the mortar less or more. Scrape off the excess mortar as you go. Then go over each joint with a "striking tool," a sort of double-ended reverse scoop designed to compress the joints laterally and leave them slightly concave.

That's it. Just keep repeating the courses, leveling at each step, until you reach the designated wall height.

Once the wall is in place, let it sit for 24 hours so the mortar can set. Then you can begin setting joists, or parging or whatever final treatment the wall will get.

The pause will also give you 24 hours to stand back and admire your handiwork.

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