Laying a perfectly square foundation is critical step before erecting a building. As an example, suppose corner posts on your foundation or slab are 3/4 inch out of square. Regardless of how you try to compensate during framing, as the walls get higher, the error compounds itself. By the time you reach roof height, the top of the building will be is as much as 1 1/2 inches out of square, and after framing and setting the roof trusses, the square roof panels cannot fit the roof. Consequently, your roof becomes noticeably lop-sided and the building's appearance is ruined. Avoid this mistake by following the steps outlined below.

House foundations
credit: Hemera Technologies/AbleStock.com/Getty Images
Use the 3-4-5 and cross diagonal methods to square a building foundation.

Building Layout

Step 1

Have the lot surveyor mark the location of the two front corners of the lot. Drive 2-inch square wooden stakes, or "hubs," into the ground at these points, with the top of the stakes level with the ground. Ask the surveyor to mark the top of both hubs with the precise location of the front corner property points, using a fine-tipped felt pen. Drive a 2-inch nail halfway into the mark on each stake to establish the front corners of the lot.

Step 2

Tie a loop on the one end of a braided nylon builder's line with a figure-eight knot and slip the loop over the first nail. Stretch the line across the front of the lot and secure it to the second corner nail. Ensure that the line is stretched tautly across both points.

Step 3

Refer to the construction plans. Measure and mark the location of the intersecting points of the front corners of the building onto the builder's line across the front of the lot with a fine-tipped marker pen; use a reel type steel measuring tape at all times. Transfer these locations to the ground.

Step 4

Remove the looped end of the builder's line from the first nail and drive a second pair of 2-inch square stakes into the ground to mark the projected front corners of the building. Replace the looped end of the builder's cord over the first lot corner nail and remeasure to ensure that the marks are positioned accurately. Drive a 2-inch nail halfway into each stake to mark the projected front corners of the building.

Step 5

Measure and mark a point on the line 6-foot from the center of the left corner nail toward the right-hand corner line. This represents two units of three of the 3-4-5 method of establishing a 90-degree angle leading away from the right angle corner of a scalene triangle.

Step 6

Tie a loop on the end of a roll of builder's line with a figure-eight knot. Slip the loop over the nail marking the left corner of the building. Have a helper stretch the builder's line across the lot at approximately 90-degrees to the line across the front of the lot. Place the triangular shaped metal loop of the steel tape over the left corner nail. Measure and mark a point on the builder's line stretching across the lot, 8-feet -- representing two units of four -- from the left corner nail to establish the second mark of the 3-4-5 method of establishing a 90-degree angle.

Step 7

Keep the end of the tape on the corner nail and walk toward the helper holding the end of the line crossing the lot. Stretch the tape and measure a point 6 feet from the back of the proposed building. Cut the builder's line at this point, tie a loop on the end of the line, and hand the helper a hammer and a 6-inch nail. Have him insert the nail through the loop to act as a handle while stretching the line.

Step 8

Wind the steel tape onto the reel as you walk back toward the line across the front of the lot. Leave the end of the tape over the left corner nail, stretch the tape toward the line across the lot marking the right corner of the building. Measure and mark a point on this line 8 feet -- two units of four -- from the left corner nail.

Step 9

Have a second helper hold the tip of the tape against the 6-foot mark on the line stretched across the front of the lot. Stretch the tape diagonally across both marks to form the hypotenuse of a scalene triangle.

Step 10

Ask the first helper holding the end of the builder's line stretched across the lot to hold the loop on the end of the line close to the ground. Tell him to stretch the line as much as possible while moving the end gradually to the right or left. Keep monitoring the tape until the distance of the diagonal between the 6-foot and 8-foot marks is precisely 10 feet, or two units of five. When this happens, tell your first helper to hammer the 6-inch nail into the ground without moving the line while maintaining tension. At this point, the line down the length of the lot will be stretched at precisely 90-degrees to the line across the front of the lot.

Step 11

Repeat the above procedures and install the right-hand line across the lot to mark the right edge of the proposed building. As a precautionary measure, double check the distance between the lines at the back of the proposed building to ensure that they correspond with the distance between the corner lines secured to the stakes across the front of the lot.

Step 12

Measure the distance from the left corner line across the front of the lot to the left front corner of the proposed building, and mark this point on the line stretched across the lot with a fine-tipped marker pen. Measure and mark the distance from the left corner line across the front of the lot to the left rear corner of the proposed building, and mark this point on the line lightly with a soft pencil. Repeat this marking procedure on the second line stretched across the lot, but draw light marks with the soft pencil.

Step 13

Spray a 6-inch diameter circle with an aerosol can of quick drying bright orange paint on the ground directly below each building corner mark. Hold a plumb line on the front left corner mark on the line with the tip of the plumb bob almost touching the orange circle below the corner mark. Mark the front left corner of the building by driving a 6-inch nail into the ground directly below the plum bob pointer.

Step 14

Establish the right front corner position where a line at 90 degrees from the left front corner dissects the line on the right of the proposed building. Use the same 3-4-5 triangulation method employed on the line across the front of the lot.

Step 15

Measure and mark a point 6 feet from the left front corner position toward the left rear corner pencil mark on the left line. Tie another line to the left front corner position on the left boundary line with a tight overhand knot. Slide the knot right or left along the line to ensure that it's positioned directly over the left front corner mark on the left boundary line. Have the first helper hold the end of the line at roughly 90 degrees to the left boundary line without stretching the line enough to bend the left boundary line inward.

Step 16

Measure and mark a point on this line exactly 8 feet from the center of the knot. Have the second helper hold the tip of the measuring tape precisely over the 6-foot mark on the left boundary line. Position the tape diagonally over the cross line to form the hypotenuse of a scalene triangle. Ask the first helper to move the end of the cross line slowly forward or backward until the distance between the 6-foot and 8-foot points measures exactly 10 feet. Mark the point where the cross line dissects the right hand boundary line with a fine point felt pen to establish the front right corner point.

Step 17

Drop a plumb line from the newly established front right corner point and drive a 6-inch nail into the ground directly under the tip of the plumb bob.

Step 18

Calculate the length of the diagonal between the left front and right rear corners of the proposed building as follows. In this example, we'll use a 30-foot by 50-foot building. Use a scientific calculator and square the measurement of both the building's width and length, and then add the results together as follows: 30 x 30=900. 50 x 50=2,500. Therefore, 900 + 2,500=3,400. Now, calculate the square root of 3,400, and that equals 58.3 feet, or 58 feet, plus 3/10 of a foot. Multiply .3 x 12 to convert tenths of a foot into inches and you get 3.6 inches, or 3 inches, plus 6/10 of an inch. Convert the 6/10 of an inch into sixteenths by multiplying .6 x 16, which equals 9.6, and the answer is 58 feet, 3 inches, plus 9 1/2 sixteenths of an inch.

Step 19

Place the end of the tape over the center point of the front left corner nail driven into the ground. Stretch the tape diagonally toward the right rear patch of orange paint and mark a point in the orange patch 58 feet, 3 inches and 9 1/2 sixteenths of an inch from the center of the front left corner nail. Mark this point by driving another 6-inch nail into the ground.

Step 20

Repeat this procedure and mark the positions of the left rear corner of the building. Drop a plumb line toward the left rear nail and move it along the line until the tip of the plumb bob dissects the center of the nailhead. Mark this point on the left boundary line with a fine-tipped marker pen. Repeat this process on the right rear corner. You now have four corner points of a perfectly square foundation. However, since the builders will soon obliterate the marks on the ground when they excavate the foundation trench, you must transfer the building corner points to "batter boards" situated 3 feet outside the foundation trench.

Batter Board Installation

Step 1

Prepare 12 stakes of 2-inch by 4-inch lumber, or 2-by-4s, with the length of the stakes determined by the required foundation grade line. Sharpen the stakes by removing two end corners at a 45-degree angle so that they can be driven into the ground.

Step 2

Anchor temporary parallel builder's lines surrounding the building line on all four sides 3 feet away from the foundation corner points; ensure that the lines stretch at least 4 feet past the building's corner points so that all four lines intercept and form corners. Drive the first stake about 1 foot into the ground in the left corner where the temporary outer builder's lines intercept. Position the board with the 4-inch side facing toward the front of the building. Drive a second stake against the temporary builder's line in front of the building 3 feet to 4 feet from the first stake so that the gap between the stakes straddles the foundation's left front corner point.

Step 3

Drive a third stake into the ground at 90 degrees from the first two stakes to form a corner resting against the side temporary builder's line. Ensure that the 4-inch side faces the building and that the gap between the side stakes straddles the front left corner point.

Step 4

Repeat this procedure on the remaining three corners of the building.

Step 5

Have the site surveyor determine the highest point of the foundation grade line with instruments. He must then use a laser level to mark the same grade level on all vertical batter board stakes.

Step 6

Cut 1-inch by 6-inch board into cross pieces wide enough to join all three stakes to form a corner after the surveyor has marked the grade lines. Position the top edge of the cross pieces flush against the grade lines and nail the cross pieces to the vertical stakes using the corner stake as the common post for both directions. Repeat this procedure on the three remaining corners.

Step 7

Have your two helpers stretch a builder's line over the top of two opposing batter boards at a time. Line up the upper lines as close as possible with the existing lines. Drop a plumb line from the upper builder's line down to the corner nails driven into the ground. Have the helpers maneuver the lines sideways across the top edges of the batter boards and adjust the position of the plumb line until the point on the end of the plumb bob dissects the center of the nailheads. Repeat this procedure on both opposing batter boards.

Step 8

Drive single 2-inch nails into the top edges of both opposing batter boards where the line dissects the cross pieces. Secure both ends of the line to the nails to act as reference points during the rest of the procedure.

Step 9

Continue this way until all eight opposing arms of the corner batter boards contain reference nails, and you have double checked the accuracy of the four corner points. Once the trench is excavated and the foundation or slab is laid, builder's line stretched across the foundation and tied to these nails mark the corner datum points and the outer edges of the building line to ensure accurate positioning of sill plates and exterior corner posts.