A rat wall is a sub-grade concrete wall. It is dug inside the perimeter of a foundation or slab, and adds depth to the footprint. The purpose of a rat wall is to prevent burrowing vermin from entering a house, basement or crawlspace under the footers of the foundation. In addition, a rat wall provides structural integrity. If the rat wall is dug beneath the frost line, it prevents heaving and slumping of the foundation as the ground temperature changes.
Set up the laser level a few feet outside the excavation footprint. Pace the site with the grade rod and locate the lowest area of the footprint. The lowest elevation is grade. Excavate the footer or slab footprint to that elevation or grade with an implement or pick and shovel. Pace the entire footprint with the grade rod to make certain the whole of the footprint is at grade after excavation.
Stake the perimeter of the footprint. Drive 2-foot rebars into the ground, 1 foot deep, at each corner of the footprint. Square the footprint. Measure diagonally across the footprint from one corner to the opposing, then measure diagonally across the footprint a second time, the second diagonal measurement perpendicular to the first. Compare the measurements.
Pull and reposition the stakes until the diagonal measurements are equal in length and the legs of the footprint are equal to the lengths outlined in the blueprints. Run nylon string from stake to stake around the perimeter of the footprint. Drive a stake into the ground, outside and adjacent to the string, every 6 feet around the perimeter.
Drive stakes 1 foot into the ground at each corner inside the footprint perimeter. The width of a rat wall is generally between 1 foot and 18 inches.To make certain the stakes are exactly 1 foot -- for example -- inside each corner, hold the end of a tape measure against the string on one leg of the footprint. Measure out 1 foot. Hold the stake on the 1-foot mark and measure 1 foot from the string on the other leg of the perimeter. Adjust the stake until it is 1 foot from both leg strings. Connect the stakes of the inside perimeter with string.
Dig a trench between the perimeter strings, to the depth the rat wall requires, with an implement or pick and shovel. For vermin, 8 inches to 1 foot is sufficient. Depending on the geographical location of the site, a rat wall may require a depth of up to 2 feet to sit below the frost line. Check the bottom of the trench with a grade rod to verify level.