Construction Tips on How to Check the Foundation for Square and Plumb

Carpenter's levels commonly use alcohol as the fluid within their sealed tubes.Jupiterimages/Comstock/Getty Images

Having a foundation that is square and plumb is essential for ensuring that the rest of the elements you add to a structure, like walls and floors, fit on smoothly and don't require extra work. While square entails having foundation corners that form perpendicular or 90-degree angles, plumb entails having foundation sides that are perfectly vertical.

Make Right Triangles

Creating right triangles in the corners of a foundation is one of the simplest methods for ensuring that a foundation is square. People have been using the method for thousands of years. Start by choosing a corner and measure 4 feet along one of the foundation's sides and 3 feet along the adjacent side. Mark these points on the foundation's floor and then connect the points by drawing a line. This line will form a triangle with the two sides and -- if your foundation is square -- will measure exactly 5 feet. You can use other measurements for the sides as well as, like 6 and 8 feet or 12 and 6 feet, lengths easily workable within the Pythagorean theorem. The theorem holds that one leg of a right triangle (one wall length) squared added to the other leg (the other wall length) squared will equal the hypotenuse (the line you drew) squared.

As Building Foundation Tips notes, another strategy for checking that a foundation is square, which you can use in addition to the triangle method, is to measure diagonally from one corner of the foundation to the other. If you drew these measurements on the foundation floor, they would form a large X. For a foundation to be square, the two diagonal lines comprising this X need to have identical lengths.

To ensure that a foundation is plumb, one of the best strategies is to hold a carpenter's level vertically against the foundation's sides. A carpenter's level is a ruler-like tool that utilizes a sealed tube filled with liquid. If a foundation side is plumb, an air bubble inside of this tube will float into a central or "dead center" position when you apply the level. For best results, Renovation Headquarters recommends using a long level, such as one that is 48 inches.

Using a level clamp -- a device that holds a level in place against the side of a structure -- will free-up one hand for making adjustments to the foundation until it becomes perfectly plumb.