How to Construct Basement Walls With Core-Filled Concrete Blocks

Lay out the basement walls, mark the outline with stakes and builder's twine and square it by measuring corner to corner with a tape measure and adjusting the outline until the diagonal dimensions are identical. Dig trenches with a shovel or excavator for a concrete footing twice as wide as the block width and at least as deep as a third of the height of the planned wall. Compact a base of gravel in the trench with a compactor.

Set a 1/2-inch steel reinforcing bar — or rebar — upright in the footing at every corner and every 4 feet in between along each wall. Space the rebar so it conforms to holes in the concrete blocks. Drive the rebar into the ground at least as deep as the base of the footings. Fill the footing trench with concrete. Tie rebar with twine if necessary to stabilize it while the concrete sets, which takes about three days.

Place a test course of blocks on the footing to fix the spacing of blocks and corners. Put this row all the way around the foundation to make sure corner blocks overlap properly and there are no serious gaps or other problems. Set those blocks to the side once the base is proper.

Spread an inch of mortar over the footings with a rectangular mason's trowel and set the base layer of blocks in it. Press blocks firmly into the mortar but without mortar between blocks. Butt block edges tightly together. Use a level to make the base course of blocks level on all walls. Double-check the corners for square with a framing square and make sure the line of blocks is straight. Set a string line around the walls as a guide.

Start the second layer of blocks by setting a block over a corner rebar so it overlaps the base block in an L fashion. Add blocks along each wall, keeping joints firmly together. Use the string as a guide to keep walls straight and use a level to keep block tops level and sides plumb. Place blocks over rebar at those locations, but keep block edges tight.

Fill the walls with blocks. Work from the corners to the center of each wall. Stack blocks five or six courses high at each corner, stretch a string guideline between the corners, then fill in the centers. Build all walls at the same time, so one wall is not seriously lower than all the others, until the proper height is reached on all walls.

Pour concrete grout into the holes around the rebar first, then fill all other block holes with concrete grout. Use a board to push grout firmly into the cores but without disrupting the level or plumb of the blocks. Insert any fasteners that will be needed to secure wall plates to the foundation top. Smooth the grout level with the top of the block. Let the grout cure for a couple of days.

Spread a 1/8-inch layer of surface bonding cement over both sides of each wall with a rectangular mason's trowel. Smooth it with the wide straight blade of the trowel and make sure the entire wall is coated with cement to an even depth. Cut the tops of rebar flush with the top of the wall.