Laying A Block Foundation That Won't Crack!

If you are pouring a raised slab or building a crawl space in your block foundation, after you have your footings completed, you will lay concrete blocks around the perimeter of the house. A regular concrete block is called 8 inches wide, 8 inches high and 16 inches long. It is actually smaller than the dimensions to allow the installed dimension to work right. This permits the mortar joint to maintain the correct spacing.

STEP 1: If you have a few hundred blocks to lay, you will want a dump truck load of sand. Be sure to specify mortar sand. Order three bags of mortar mix per hundred blocks for your block foundation.

STEP 2: To figure how many blocks you will need take the total length of the perimeter in feet, multiply by .75. This effectively divides your length by 16” which is the length of a concrete block. This number will tell you how many blocks it will take to encircle your house one time. A level or layer of blocks is called a “course”. The highest or top course will need to be of a special kind of block called a “header” or “L” block for raised slab block foundation.

STEP 3: Multiply your number of blocks per course by how many courses you will need. Each course will be 8 inches high. On sloped ground, there will be some number of partial courses, due to the steps in the block foundation. Figure the length of each subsequent level or course and add the numbers for a total. If you have any areas that are higher than 6 courses, you must lay 12” wide blocks from course 7 and below.

STEP 4: When you take delivery of the blocks be sure to have them placed around the house, near where they will be used.

STEP 5: Remove the 2x8 bulkheads from the footings and sweep the footings clean.

STEP 6: Mix a bag of mortar. A general guide line is 15 or 16 shovels of sand to one bag of mortar. Mix the sand and mortar really well, dry, before you add any water. This will simplify the mix tremendously. You can mix the mortar by hand in a wheel barrow or mixing pan, but if you have many blocks to lay you will want a mixer. You can rent gas powered or electric mixers to use when building your block foundation.

Word to the Wise: If you are using a mixer, spray the inside with WD-40 or wipe it down with oil before and after each use. This will keep it clean and rust free.

If you mix by hand, learn to rock your body when moving the hoe in and out, rather than just using your arms. Rocking back and forth is much less tiring. You want the mortar to be sticky, without being runny. After you dry mix really well, add about a gallon of water. Work the mix back and forth repeatedly, taking care to scrape the bottoms and sides to insure no missed areas. As you work back and forth you want the mortar to flow around your hoe smoothly. Add water in very small amounts as it will surprise you how easy it is to get it too runny. The more you work it, the runnier it will get. If you do get it too runny, just add a little sand and/or mortar to stiffen it up.

Word to the Wise: I highly recommend a mixer for mortar. If you try to do it by hand you will tend to under-mix and weaken the mortar. Mixing mortar by hand is many times harder work than mixing concrete. You will enjoy building your block foundation far more with a mixer.

You can keep the mortar in a wheel barrow or place it on 2 foot by 2 foot pieces of plywood that you space out along a footing. Of course, these are called “mortar boards”. If you use mortar boards, wet them down before you place mortar on them.

STEP 7: Begin by using a four foot level and plumbing down from the layout string intersection to the footing surface. If this distance is over four feet, use a straight board and hold the level against the board. Mark the corner in both directions on the footing, then move down the string a couple of feet each way, plumb down and mark the footing again. Then lay your level down and draw a line between the marks, making a three or four foot line on each side of the footing corner. Repeat the process on the other corners in the block foundation.

STEP 8: Laying the corners correctly is critical to your success so take the time to get the corners right. Using a brick trowel, scoop some mortar up and tap it off onto the footing just inside the lines you drew. Place a line of mortar a couple of feet long each direction and then place another line of mortar 6 or 7 inches farther inside, parallel to the first. Both edges of the block must have mortar under them.

STEP 9: There are usually two types of 8” block in a pallet. Some will have square edges and most will not. On most blocks the “body” of the block is not flush with the end of the block. The edges stick out a little. This design makes the block lay and hold better in the mortar joint. The square edge blocks for block foundations are used for corners and openings.

Select a square edge block and lay it on your mortar. You can use your trowel handle to tap it down level. Place a two foot level on top of the block in both directions and work with the block until it is perfect. It must be level in both directions, exactly on the mark you made, and exactly ending on the other corner mark. The closer to perfect you lay this first block, the better your corner will end up and the easier your entire laying time will be for the block foundation.

STEP 10: Next lay a second block on the other side of the corner. Before placing the second block, place mortar on the end of the block on both sides. As you lay the second block, ease it up against the first block. Again, spend the time required to get this block as close to perfect as you can. Make sure that you did not move the first block when laying the second one.

STEP 11: Lay a third block, placing this one on top of the first two, and turning the long part so that it turns the corner opposite from the first course. That is, it should lap over the lower joint. Again, get it perfect for a perfect block foundation.

STEP 12: Measure down from your layout string to the top of your block. You need to maintain 8” increments as you come up. This is critical and the sooner you get “on the mark” the better off you will be. In the event that your footing ended up too high and there isn't enough room for 8 inch increments, you will have to lay spacers to get to the required spacing. You may use 4 inch blocks, two or one inch solids, or any combination required to get to the 8 inch increments.

Alternatively, you may end up with the footing too low and need to “pick it up” or raise your blocks to get to the required spacing. Again, you may lay any combination of blocks to get to where you need to be. Very small adjustments may be made by thickening or lessening the mortar joint.

STEP 13: Continue stacking your corner up to the string. As you get higher, you will also get longer out from the corner. As you lay the blocks, use your trowel to scrape off excess mortar from the outside face of the blocks in the block foundation.

STEP 15: Once you have a corner up to four or five courses and it is as perfect as you can get it, move to the next corner. If you are going higher than four or five courses, you will still want to stop at that height and fill in the wall runs before going higher. Remember to use 12 inch blocks for any lower courses that are farther than 6 courses below the layout string. As the corner grows in height, place your four foot level against the sides of the block stack to insure that the group of blocks are standing plumb. Again, get the corners as close to perfect as you can for a perfect block foundation.

STEP 16: Once you have the corners built, use your string line blocks to run a string from corner to corner. Set the blocks so that the string is positioned at the top edge of the block. This establishes where the block will lay – in and out on the footing, and also set s the height of the block when laid. Be careful to level the block sideways, as it is easy to lay the block in the right place against the string, but have it leaning one way or the other. Your eye will begin to see this as you work.

STEP 17: Using mortar boards for this step makes it easier. Move along the footing, troweling two lines of mortar into place for the first course.

STEP18: Return to one end and begin laying the first course. Place mortar on the two edges that will butt up to the previous block, then position the block in place. You may find it easier to lay several blocks, and then come back and tap them into perfect.

STEP 19: Continue laying blocks until your first course is filled in between the corners. You may need to cut a block to finish the run. You can do this by sharply hitting the block with the trowel edge or hammer point where you want it to break.

STEP 20: When you finish the bottom course, raise your line blocks up one course and go again. Be sure that the blocks over lap the joints below them.

STEP 21: Continue laying blocks until you reach the top course. The top course must be “header” blocks which are shaped like a “L”, if you intend to pour a slab. They are 8”on the outside and 4” on the inside. This allows the concrete to pour into them and into the wall below them, and finish off at 4” thick. Regular 8” blocks are correct for crawl space foundations.

STEP 22: Repeat the process for all sides.

STEP 23: When all the blocks are laid, lay a small piece of block to fill in the holes at the corners of the header blocks.

Word to the Wise: remember that until the concrete is poured, the block foundation wall is very weak and you must be careful around it or you will be laying it again.

STEP 24: Now it's time to place the fill, if you are pouring a slab. To figure how much fill you need multiply the width of the area by the length and then by the height. This will give you square footage required – divide by 27 to get square yardage needed. Estimating fill requires some guess work. If you can, have the material supplier help you with this calculation. They usually have a good eye for figuring this.

STEP 25: If your block foundation walls are higher than four courses, you will need to brace the outside of the wall(s) before you place the fill. Drive stakes in every four feet and place a 2x6 flat against the block wall halfway between the top course and the course below.

STEP 26: Place kickers from the stakes to the 2x6 to hold it in place against the wall. If your wall is higher, place another 2x6 for every two more courses of blocks. Remember that block walls higher than 6 courses should have 12” blocks below that.

If you are building a crawl space, remember to place the scuttle hole opening frame in the block wall of your block foundation.

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