How to Lay a Concrete Block Wall
Learning to lay concrete blocks is a valuable DIY skill and one that could make many projects a breeze. Here are some tips and instructions that will help make your first concrete block project that much easier. This information will help create a neater and superior installation that produces far less waste, and it will help you know how to choose the best blocks, pour the footing, estimate the materials, prepare the area, and lay the blocks.
Step 1 - Choosing Your Blocks
Various types of concrete blocks exist to meet your needs in whatever project you have in mind. These include two-core units, three-core units, steel sash units, wood sash (jamb) units, single bullnose units, capping units, header units, and return or corner angle units. If you’re unsure of which type your job calls for, any home improvement sales associate should be able to help you select the correct ones.
Eight-inch standard blocks are actually only 7 5/8-inches wide, which allows the mortar to expand while drying. Standard blocks are appropriate for most projects, with half-blocks with square or rounded corners usually installed at the end of a run. Single and double corner units provide smooth surface at the corner.
When laying blocks for a doorway, make sure you use jamb joist blocks. Sash blocks are good for casing windows, and header blocks provide a space for wooden supports or other structures. Special blocks are also available for almost any building requirement.
Step 2 - Pour the Footing
Every concrete block wall should be laid on a secure footing of poured concrete, regardless of height or length. Both block barbecue grills and outdoor fireplaces require a large concrete pad, so keep this in mind.
When you pour a footing, the base should always reach below the frost line. To be safe, you should probably dig a couple inches below the known frost level in your area. It should also be two times width of the wall as well, with the wall placed in the center. So, if you are using eight-inch blocks, then the footing should be and 16-inches wide in order to provide a four-inch rim on either side of the block.
Use 2x4 boards to build the frame where you will pour the footing, and hold the boards in place with wooden stakes driven into the ground every three inches. If the area has drainage problems, install a line along the outer edge of the form, providing a one-inch drop for every 20-feet of drain line. Then, backfill the drain line with one foot of crushed stone or gravel. Be sure that your footing will not disrupt a natural runoff of water or divert the flow into a neighboring property.
Create an opening to accommodate any gas, electric and water supplies you need by nailing four 2x8 boards in the proper position before pouring the footing. Four-inch drain tile will also often suffice. Mark the depth and location of this utility opening on your foundation plan with a stake in order to easily locate it later.
Larger footings will have to be poured in pieces, rather than all at once, so before proceeding, divide the total frame into smaller sections with 2x4s. Then, to ensure your footing will be level, whether it’s big or small, place a level across the frame every eight feet and adjust the 2x4s by raising or lowering the stakes as needed. Fill the form to the top edge and level the concrete by scraping a 2x4 across the top in a sawing motion. Add more concrete to the low spots if necessary. When the footing is poured and leveled, allow it to dry for a couple days, three if it’s carrying a heavy load.
Step 3 - Planning the Amount of Blocks and Mortar
You can generally estimate the amount of blocks and mortar you’ll need for a given project. For every 100 square feet of wall, you will need about 225 4x4x16, 6x4x16, or 8x4x16 blocks. If you’re using 4x8x16, 6x8x16, 8x8x16, or 12x8x16 blocks, you will need about 112 and one half blocks for completion.
You can also estimate your required number of blocks by figuring you’ll use three eight-inch blocks for every four feet on each course.
For mortar, 100 square feet of wall made with four-inch blocks will require about 13 1/2 cubic feet (or about six cubic feet per 100 blocks). A wall made with eight-inch blocks will require about 8 1/2 cubic feet of mortar per 100 square feet of wall (or 7 1/2 cubic feet per 100 blocks).
Please note that these figures are based on a 3/8-inch mortar joint.
Step 4 - Preparing the Area
Before starting to lay blocks, build a form at each corner by driving stakes made from scrap wood. Locate the exact corner by stretching lines from one corner form to the other. The exact corner will be the point at which the two lines cross. Drop a plumb bob at the cross point and about two feet out in each direction.
Lay out a course of concrete block on the dry footing. Do not use mortar for this test run since you’re only trying to get an idea of what you need. Use corner blocks where they are needed and cut blocks as required to make it all fit. Use a piece of scrap 3/8-inch plywood to fill in the mortar joint between each block. After you’ve finished the text run, clear everything away and prepare for the real thing.
Step 5 - Lay the Blocks
Drop plumb bobs from the corner string and at a point three feet from the corner. Mark the location of the corner block on the footing base and then spread the mortar in the marked area about one inch deep and eight inches wide. Extend this mortar by three blocks in one direction and put a furrow in the center of the mortar with a trowel. This furrow will force the mortar to the edge of the block when it is laid.
Lay the corner concrete block first and make sure you’re using the correct finished end. Thoroughly check the starting corner block and take time to position it correctly. All other blocks will align with this starter block, so it's very important that it is set right. Follow the same procedure for all remaining corners, laying about two or three blocks of the first course in each direction. Tie a line between two bricks and stretch it between the corner blocks on the first course.
Continue to lay the base mortar as the course continues, and also apply mortar to the ends of the blocks with a trowel before placing them in the correct position. Make sure to keep all mortar joints about 3/8 of an inch. Use a masonry chisel if you must cut a block. Mark a line on both sides where the cut is to be made and then strike the chisel with a hammer. It’s tricky, but you’ll get the hang of it.
Once several concrete blocks have been laid, use a long level to verify the alignment. Check both the tops of the blocks and the outside edge. Tap the blocks to correct the alignment while the mortar is still wet. Never attempt to move a block after the concrete begins to set.
Continue to build the corners first and keep them higher than other runs until you finish the job. Also keep the guidelines between the corners at all times. They will help you keep the blocks level at all points in each course and keep the guidelines tight. Correct any misalignments as you go.
Use a trowel to remove surplus mortar and dispose of it onto the mortarboard. Keep turning the mortar with your trowel throughout the project so small portions will not harden. Use 3/4-inch plyboard or the bed of a wheelbarrow as a mortarboard. Always wet the board or the wheelbarrow bed before placing the mortar while laying concrete block.
Do not mix more mortar than you can use in 90-120 minutes, and on hot days, keep it covered with a piece of plastic to retain moisture while you’re working. Also, to keep it workable, use as much water as the mortar will absorb while retaining its elasticity.
Stagger the blocks from the corners and build the wall to the desired height. Take time to level the blocks after each course, using a level at least three feet long. The thicker end of each block should face up. Also, keep a leveling string at the top of each course on each run of block.
Measure not only the length, but also the height every three runs, and hold your level diagonally along the block corners to check for accuracy. If the blocks are being laid right, then the corners will strike evenly along a level held in this position.
Once all the concrete blocks have been laid, while the mortar is still pliable, use jointer to finish the joints. Keep the jointer wet at this time. Reinforcing rods may also be necessary for walls that are built high. Lay 1/4-inch reinforcing with the ends overlapped two to three inches. Mortar can be placed directly over the rods as well.
If wood framing will be attached to the top run, set an anchor bolt every three feet in the last run. If a load-bearing wall intersects with the exterior wall, use metal tie-bars to secure it to the main wall. Set the bars at least every other run. Non-load-bearing walls that are connected to main walls can be tied with a 16-inch strip of 1/2-inch mesh galvanized hardware cloth directly over the block. A strip of this cloth should be used for every other course at the tie-in point.
If the concrete blocks are being laid as a fence or barrier, the top course should be finished by filling the holes with fresh concrete. For a neater and more enduring finish, use patio blocks, as they are very attractive with the added benefit of being waterproof.
Now that your concrete block wall is finished, you can add some personality! If you would like to paint your new wall, read this article to learn how.