How to Square a Building Foundation

When a building is being erected it is critical that the foundation is very square. A rectangular house must also be a “square” rectangle, meaning each set of two sides are equal. A perfectly square building foundation begins with a square layout. It is very important before you order the concrete for you building foundation to check the square of the forms. Bringing your forms into square is not a complicated process, but it takes some time. A good method to use on small buildings, 8’ x 8’ or smaller is to use the framing square and string. For larger buildings you need some tools (hammer, framing square, string line, wood stakes, measuring tape) and follow some steps.

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Start by drawing a straight line (use a string) on one side. It will be the base line of your foundation, and its length will be from outside the wall to outside of the wall. Draw a similar line in the opposite side.  Your opposite sides should be equal.

Then you pick a point on that straight line to be your first corner point. Position the other three corners of the house roughly in place where they will go.  Stick a wood stake in the ground where the corners will be (use string from a stake to another stake, to form a parallelogram).

Now the idea is to square something what isn’t square, and maybe a good thing is to understand what a right triangle is. A right triangle has three sides and one 90 degree angle. Go back to the first corner point that you marked and make from boards a right triangle. It’s very easy.

You can use a 3, 4, 5 formula (or 6, 8, 10 formula). That means 3 units (or 6units) along one side, 4 units (or 8 units) along the other side, yields 5 units (or 10 units) across the hypotenuse (diagonal).

That will create a 90 degree angle, a square corner. This is not a very accurate method of setting to square in a building foundation, but the larger you make it, the better it is.

Do the same thing in the other corners of your foundation and then measure from either of the existing lines to the size of base or foundation you want.

Tip: A way to check your work is to pull a measure tape from a corner to the opposite corner. If you are square the diagonals are equals.