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Foundation Rebar - Reference Cubes

Star Wood cubes are accurately trimmed to the exact height of the slab top to provide a reference.


Placing and trimming wood cubes

The 8" wooden cubes go in the places previously marked with tape.  The top surface of the cube will be the most accurate slab height reference when pouring and screeding the concrete.  The height of the top surface of each cube needs to be set using a self leveling laser and adjusted as needed to be the exact height required for the slab.  It is worth letting the wooden cubes be outside in the rain for a while before doing any trimming so that any expansion will have already taken place.

Make wooden cubes

The cubes are designed to be easy to remove after the concrete has set.  Screws are only used at the top and the heads are covered with tape to stop the heads being filled by concrete.  At the bottom, tape is used to stop the wood warping out.

Wood Block 8in Cube 

Establishing the height

Position a self leveling laser such that it projects a horizontal beam over the whole jobsite at the height of the top of the batterboards.  You will need to do the height measuring at dusk so you can see the red line better.

Self leveling laser

It is worth also using a target on the far side of the jobsite so you can maintain good consistency.

Laser Target

In my case the top of the batterboards also corresponds to the standardized finished (south side) grade height.  The slab height and therefore the height you want the tops of all the wood cubes to be is 2'2" below the grade level laser line.  Use a length of white PVC pipe with a marker at 2'2" and some plus and minus markings so you know how much each cube needs to be adjusted.

Marked Pipe For Slab Height 

If the slab area is a fraction high in some areas then it will be necessary to shave a little off the bottom of the cube sides to ensure the top of the cube is accurately at slab height.  If the slab area is too low in a place then use tape to add wood shims under the sides.

Number the cubes and their locations so that you get all the adjusted cubes back in all the right places.  Also write on the cubes the amount that needs to be shaved off the bottom or added to the bottom.

Here's a cube that happened to already be at the right height...

Wood Cube Found To Be Right Height 

Here's one that is found to be 1/8" too high...

Wood Cube Height Marking 1/8in 

And here's one that was found to be 1/8" too low...

Wood Cube Height Marking 1/8in Too Low Done 

Adjusting cube height

If the wood cube is a bit high then trim the appropriate amount off the bottom.

Trimming Wood Cube 

If the height of the cube is a bit low then tape 1.5" wide shims on the bottom to increase its height by the appropriate amount.

Adding Shims To Wood Cube 

Shimming external Form-a-drain

With a self-leveling laser check the height of the outer form-a-drain.  If it has settled down a bit then use foam adhesive to glue on a long wooden shim of the appropriate thickness to increase the height slightly.

Shims On Outer Form-A-Drain 

The shim wood is used as the concrete screeding edge.