The 3-4-5 rule for decks is a layout check for square corners: measure 3 feet along one side of a corner and 4 feet along the other, and if the straight line between those two marks is exactly 5 feet, the corner is a true 90 degrees. It comes straight from the Pythagorean theorem, and it’s the first thing we check when we lay out a new deck in Kenner.
A square layout matters more than most people think. If the first corner is off, every joist and every footing that follows inherits the error. On the flat, open lots around Kenner, where a deck often runs the full width of the house, a small miss at the ledger can grow into a gap you can see from the kitchen window. Here’s how the rule works, how we use it on a real layout, and what it doesn’t cover.
Key takeaways
- The 3-4-5 rule checks that a corner is square: 3 feet one way, 4 feet the other, 5 feet on the diagonal.
- It works because 3 squared plus 4 squared equals 5 squared (9 plus 16 is 25).
- Bigger multiples, such as 6-8-10 or 9-12-15, give a more accurate check on a large deck.
- Equal diagonals across the whole frame