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Building Concrete Block Walls

Laying concrete blocks is hard, physical labor. Not only will you be lifting 40-pound blocks all day long, but often you’ll be handling them in awkward positions. There’s mortar to be mixed and transported, which involves plenty of heavy lifting as well. So have plenty of strong-backed help. Professionals work in fairly large crews, so don’t expect to do this project alone.

Tools: Chalk line, mason’s string, plumb bob, story pole or modular spacing rule, carpenter’s level, pointed trowel, brick set, baby sledge, joint set or sled jointer, circular saw with masonry blade, pointing trowel.

Tips for Building a Solid Block Wall

■ A solid footing or foundation is essential to building a solid block wall. Check local building codes.

■ Choose a Type “S” or “M” mortar. Both have good strength and are resistant to damage from freezing.

■ Use true concrete blocks, which weigh 40 pounds or more, rather than lightweight cinder blocks. Concrete block is stronger and more moisture-resistant; it’s worth the expense.

1. Establish the corners. Mark the locations of the corners. Stretch a mason’s line between the batter boards you set up before excavating for the footing, placing the line on the mark for the outside of the wall. Have a helper dangle a plumb bob from the point at which the mason’s lines intersect at each corner, taking care not to disturb their alignment. Mark the locations on the footings with a thick pencil.

2. Mark for the walls. Sweep the footings clean and snap chalk lines between the corner marks. Check the lines for square, using the 3-4-5 method. If your project involves just one wall, skip Steps 1 and 2. Simply determine the two end points of the wall and snap a chalk line on the footing to designate the edge of the wall. Go on to Step 3.

3. Lay the bottom mortar. Make sure the footing is still clean and lay a 1-inch bed of mortar for the first course of block. Start at one corner, running the length of three or four blocks. Make the bed of mortar about 1 inch wider than the block you’ll be placing.

4. Set the corner block. Carefully place a corner block in position, with the smaller holes in its cores facing up and its smoothfaced, solid end on the corner. Gently press the block into place in the mortar.

5. Measure the block height with a story pole... Use a story pole to check for proper course height. To make a story pole, choose a straight 2x4 and make clear perpendicular marks every 8 inches. As you add the courses, the top of each concrete block should align with one of the marks. If the block is too high, tap it down into the mortar with the trowel handle. If it’s too low, pull the block out, place more mortar on the bed, and re-lay the block.

...or use a folding or modular spacing rule. If you use a conventional folding rule or tape measure, the top of the block should be 8 inches above the footing. Confirm that the mortar joint under the block is exactly 3/8 inch thick. On a modular spacing rule the top of the block should match the line at the “2.”

6. Butter and lay the next blocks. Before laying the second block, butter its flanges (or ears) while it is standing on end. Butter it well because this mortar forms the vertical joint between the first and second blocks. Set the block in place, making sure you have a 3/8-inch joint spacing. Repeat with subsequent blocks.

7. Check for level and plumb. Place a level along the length of the first three blocks. If they are not perfectly level or if their tops do not form a straight line, press or tap down on the high points until they are level. Check for plumb by holding the level against the side of each block. If any block is not plumb, press with the heel of your hand or tap with the handle of your trowel to adjust it. Every three or four blocks, check for level and plumb and make adjustments. Periodically use your level or a long straightedge to make sure the faces of the blocks align.

8. Check for square, build lead. Check corners for square first by holding a framing square against the outside edges of each side. Then use the 3-4-5 method as a double-check. Apply a 1-inch-deep layer of mortar along the top edges of the blocks of the first course. As you place the second corner block, align its outside corner with the corner below it. Press the block into the mortar just enough so its weight compresses the mortar to a 3/8-inch joint. Continue laying blocks until you have built up a corner lead.

Why Build Corner Leads? Whenever you build a masonry wall, whether it’s with bricks, blocks, or stones, it is best to start by building leads at each corner. Then you can fill in blocks between the leads. You may be tempted to skip this step and build a wall one course at a time. But leads are essential to achieve a plumb and straight wall. Without leads, you have to check blocks continually for level and plumb, and there would be no easy way to make sure your wall is straight. With the leads in place, laying the blocks in the middle is simple; stretch a mason’s line from corner to corner and follow it.

9. Install spacers if necessary. Spacers made of cut blocks can be used to extend leads if you cannot make the number of blocks come out even by slightly enlarging the mortar lines. Cut spacer blocks and install them early, rather than trying to cut the closure block in the middle to fit.

10. Cut concrete blocks. If you need to cut blocks to fit, be sure to take into account the 3/8-inch width of mortar lines when measuring. To cut by hand, place the block on sand or loose soil and use a brick set and hammer to make a line about 1/8 inch deep on both sides of the block. Then work along the line again, hammering harder and moving the brick set each time you rap it. Continue until the block breaks along the cut line. For precision cuts, use a circular saw with a diamond or masonry cutting blade. Be sure the block is dry when you cut it.

11. Fill in between the leads. Once you have two corner leads in place, hook line blocks around the corners and stretch a mason’s line between them. Align the mason’s line with the top of the blocks in the course being worked on and hold it about 1/16 inch away from the blocks’ outer edges. With the mason’s line stretched in place, begin setting the blocks between the leads. Check the line often to make sure no blocks or mortar are touching it because they might otherwise push the line out of alignment.

12. Install the closure block. Butter both ends of the closure, or final, block and lay it to complete the course. If some of the mortar falls off the flanges, lay the block anyway. Fill gaps in the joints by tucking mortar in place from the sides with your trowel.

13. Strike the joints. Use a joint strike or a sled jointer to finish each joint after you have laid two courses above it. Tool the head (vertical) joints first then the bed (horizontal) joints. Brush off loose mortar, then restrike. For blocks below grade, simply strike off excess mortar with your trowel.

Install window or door lintels. Plan door and window openings so you’ll have to do minimal cutting of blocks to fit around them. For doors, it’s best to use metal units designed for use in masonry walls, but you can build wood frames as well. You can make your own lintel with two angle irons and U-blocks. Cut or purchase angle irons 16 inches longer than the opening is wide, allowing for an 8-inch overlap on either side of the opening. Set them back to back and place U-blocks on top. Place windows as the wall reaches the appropriate height for the opening.

Add reinforcing wire when needed. For retaining walls and foundation walls on which there will be considerable lateral pressure, beef up the wall with truss-type or ladder-type reinforcing wire. Embed the wire in the mortar of every other horizontal joint. As you imbed it, overlay the ends of the sections by at least 6 inches. position the wire when reinforcing a corner. Simply cut two of the wires in the truss and bend the remaining one to form the comer. To tie intersecting walls together with an S-shaped, 3/8 inch rebar. Once covered with mortar and the next course of blocks, the rebar is secured firmly to the webbing of the blocks.

Tie a new wall to an old one. If you’re building a new wall adjacent to an old one, tie the two together. Every other course, knock a hole through the existing wall, stuff newspaper into the cavity, and place an S-shaped piece of rebar in the hole. Fill the hole with mortar. Stuff newspaper in the cavity under the other end of the rebar, lay the second course, and fill the cavity with mortar.

Cap the wall. If the foundation wall supports brick or stone veneer, use a combination of brick and smaller-size block as the cap, as shown here in the top example. For example, if the foundation blocks are 8 inches, stack 4-inch block and bricks on top. If the wall serves as a foundation for a wood frame building, lay wire mesh on top of the next-to-last course of blocks. Embed anchor bolts in the mortar in the cores of the top course of blocks. Once the mortar sets, drill holes in a pressure-treated sill plate, set the sill plate over the anchor bolts, and fasten it in place with washers and nuts.

Applying stucco on concrete block. To apply a stucco finish on concrete blocks, first paint the blocks with latex concrete bonding agent. Apply the scratch coat with a finishing trowel and scratch it with a plasterer’s rake or a scratching tool made of a piece of 2x2 and 4d nails. Keep the scratch coat moist for two days, then apply a finish coat.

Surface-Bonded Block - Special mortarless concrete blocks can form a strong wall when simply stacked on top of one another, reinforced, and grouted. This type of block, however, is expensive. A more practical solution is to build a surface-bonded wall, which uses standard concrete blocks. With this process, you only mortar the first course of blocks. Then you simply stack the rest of the blocks on top of one another. Once all the blocks are stacked, apply a coat of ready-made bonding agent made of Portland cement mixed with pieces of fiberglass. The bonding material is available in many colors. The resulting wall actually is stronger than a standard mortared wall.

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