Before a single post goes in the ground, the site itself needs to be checked, marked, and squared. Skipping this step is one of the most common reasons a deck ends up out of square, unevenly graded, or sitting on top of a buried line it shouldn’t be anywhere near.
Check the Site First
- Grade and Drainage: The ground under and around the deck should slope gently away from the house, not toward it, so water doesn’t pool against the foundation.
- Access and Clearance: Confirm there’s enough room to work around the perimeter, and that nothing overhead (eaves, wires) will interfere with framing or tools.
- Soil Condition: Soft, loose, or recently disturbed soil may need extra attention when you get to footings, since it won’t bear load the same way as undisturbed ground.
Call Before You Dig
Before placing or digging any footings, submit a locate request through Ontario One Call at least five business days ahead of time. It’s free, and it has utility owners mark buried gas, electrical, and other lines on your property so footings don’t go in on top of them.
Marking and Squaring the Layout
- Set batter boards a short distance outside each corner of the planned deck, and run mason’s line between them to mark the actual edges.
- Check that the layout is square using the 3-4-5 method: measure 3 feet along one line and 4 feet along the perpendicular line, and the diagonal between those two points should measure exactly 5 feet (or any multiple of 3-4-5, such as 6-8-10, for a larger deck).
- Adjust the lines until both diagonals of the full rectangle measure the same length, which confirms the whole layout is square, not just one corner.
- Mark each post location directly below the string lines before you start digging.
If you’d rather skip the math, the corner-to-corner method works just as well on a simple rectangular layout: measure diagonally from one corner to its opposite corner, then repeat the measurement from the other two corners. If both diagonal measurements come out equal, the layout is square. It’s a quicker check than the 3-4-5 method on a straightforward rectangle, though 3-4-5 holds up better if your layout isn’t a clean rectangle.
Common Mistakes to Avoid
- Skipping the utility locate because “it’s just a small deck”
- Checking only one corner for square instead of both diagonals
- Digging footings before confirming the finished grade and drainage direction
- Losing the string-line layout partway through digging, so later posts drift out of line
Once the site is checked, marked, and squared, you’re ready to move on to sizing and setting your beams and joists. The layout you’ve just confirmed is what everything above it depends on.
