Building a block wall fence requires a poured in place concrete footing to support the wall, #4 steel re-bar in the footing (known as horizontal reinforcement), and #4 vertical rebar reinforcement that will be set in the footing and project upward to approx 3" below the finish height of the wall, which in most cases is 6 ft for a standard fence. This vertical reinforcement is required at a minimum of 48" apart for non reinforced block walls.
The concrete footing should be 12" wide x 12" deep with (2) runs of horizontal rebar, one at 4"up from the bottom and the other 4" down from the top, along the center line of the footing. Rebar lengths should be lapped a minimum of 16" and tied in (2) places at each lap with #4 wire. Rebar cannot contact the soil at any point.
The type of block commonly used is CMU, (concrete masonry units) and the standard size is 6" wide x 16" long x 6" tall, each. The number of block required will be the length of the wall, x's the height, divided by 1.75 and this total divided by 1.4. This allows for a 1/2" horizontal mortar joints along the runs and at the heads of each block. "End" or finish blocks are sold in 16" and 8" lengths, which is in keeping with the staggering of the CMU block as it resolves at the ends of the fence. Once the concrete footing has cured for 24 hours, the block may be installed.
- To begin, apply mortar to the underside of a block and place one block at each end of the run.
- Then, drawn a string line over the top of these blocks, as a guide for the intermediate blocks. For long walls, a mid block may be set to prevent any sagging of the string line.
- Now you are ready to install the first course of block.
- Apply mortar along the top of the concrete footing (approx 4' to 6'at a time to keep it fresh). Butter one end of each block with mortar and set it, tight to the previous block, using a short level and mallet to tap it level at the top and plumb at the face of the block. Where vertical steel is encountered, slip the block over the top of the bar, place it and tap it level and plumb.
- Some masons install the block one course at a time, others stagger, by running a course out 6' or so, and then falling back and beginning another course. This creates a stair-step pattern. However, leveling of each course is more difficult and requires experience. For novices, one course at a time is suggested.
- Be sure to install the end blocks at each course.
- As the mortar begins to cure, the joints can be "raked" if you prefer recessed joints, or smoothed with a trowel if you desire flush joints. Raking can be done with the round curve of a steel foundation bolt or similar.
- Once the wall is complete, a masonry cap can be installed using mortar at the base and heads of each cap piece.
Mortar is a common mix of sand, lime and cement and can be hand mixed on site, either by hand or in a mixer. The concrete for the footing can be ordered already mixed and placed directly from the mixing truck. The Red-mix company will provide the correct mix formula for block wall fence at no additional charge.