I'm laying out batter boards and strings for a 24 by 40 addition foundation.

To ensure your 24×40 addition foundation is square before digging, use the diagonal measurement method based on the Pythagorean theorem (3-4-5 triangle principle scaled up).

Steps:

1. Set up batter boards beyond the foundation corners and attach strings to mark the perimeter.

1. Measure and mark the two known sides: 24 feet and 40 feet.

1. Check the diagonals: Measure from corner to corner. For a perfect rectangle, both diagonals must be exactly equal. - Use the formula: √(24² + 40²) = √(576 + 1600) = √2176 ≈ 46.65 feet (or 46' 7.8")

1. Adjust the strings until both diagonal measurements match. This ensures all corners are 90 degrees.

1. Double-check by re-measuring all four sides and both diagonals.

Tip: Use a laser distance meter or taut string with a line level for accuracy. Also, verify the 24' and 40' sides are correct after squaring the diagonals.

Once square, stake and brace the corners securely before excavation.

This method is standard in residential foundation work and aligns with typical framing square techniques. No special tools required—just precise measuring.

Use the 3-4-5 method (or 6-8-10 for better accuracy) on each corner.

Step-by-step:

1. Set the first wall: Drive batter boards 3–4 ft past each corner of your 24' wall. Run a string line; this is your baseline.
2. Square the first corner: On the baseline, measure 8 ft from the corner stake and mark the string. On the perpendicular string (roughly set), measure 6 ft from the same corner. Adjust the perpendicular string until the diagonal between marks is exactly 10 ft. Pin it.
3. Square the second corner: Repeat at the other end of the 24' wall.
4. Set the 40' walls: Measure 40 ft along the squared perpendicular strings from both corners. Run the back string line between these points.
5. Final verification (Critical): Measure both diagonals corner-to-corner. They must be equal (≈ 46' 7-5/8"). If not, shift the parallel walls (keeping them parallel) until diagonals match.
6. Check dimensions: Verify all four sides: 24', 40', 24', 40'.

Code/Standard: IRC R403.1.6 requires foundations to be "level and square." Equal diagonals are the field proof.

Pro tips:

• Use a 100-ft fiberglass tape (steel stretches/kinks).
• Plumb bob from string intersections to mark exact corner points on grade for excavation.
• Batter boards must be level and sturdy (2x4 legs, diagonal bracing) so strings don't sag or move.
• Keep strings tight and at same height on all boards.

If the site is tight or slopes > 12": Hire a surveyor to set pins. Excavation errors cost far more than a layout fee.