The best way to square up a deck is to use the 3-4-5 or 6-8-10 method. Using your ledger board and a rim joist attached to one end of the ledger, measure 3 feet from that corner along the ledger and 4 feet from the same corner down the rim. Make two marks. Measuring between the two marks. You'll want the marks to be exactly 5 feet apart. Adjust the rim joist to get that measurement for a square deck.
A stable foundation and level framing are needed for a square deck. If you don't level your framing, you won't be able to build a square deck. In the same way, if your foundation moves, your deck will no longer be square.
There are other ways to get a deck to be square, but the 3-4-5 method is the easiest and fastest. There are other ways to get a square that are even more accurate, which we'll talk about next.
1. Use the 3-4-5 or 6-8-10 Rule
Pythagoras' theorem lets us get a square deck. In a triangle with one right angle, summing the squares of the two shorter sides equals the square of the longest side. Traditionally, a2+b2=c2. When you enter those numbers in that sequence, the equation balances. That means you may have a 3x4 triangle. Connect the terminals of these sides to form a triangle with a 5' side. Also, 6-8-10.
After installing the beam and ledger board, we only need to install one rim joist, attaching it to the ledger and resting it on the beam or beams. Measure the ledger where it meets the rim joist. Mark the ledger's inside 4' top edge. Measure the rim joist from the inside of the ledger down 3'. Mark the top inner rim joist. Check for squares. Start measuring from the ledger. Move the rim joist so its mark reaches the 5' mark on the measuring tape. After that, connect the joist to the beams. A square deck. The 6-8-10 approach is more accurate due to the longer distance. This procedure is for 8' or longer joists. Crooked joists could throw off measurements using this method. Be sure your first rim joist is true before utilizing it as a template.
Read more: Framing a Deck Parallel to a House
2. With a Tape Measure
Measuring opposite corners to ensure they're equal is another frequent method. To do this, place rim joists on both ends of your ledger board. Attach rim joists to ledger board. Attach them to the deck beams afterwards. Once you've placed the rim joists, measure their diagonals. Start at the right inside corner where the rim meets the ledger and measure diagonally to the opposite corner rim joist. Repeat with the other corners. Equal measurements are required. Otherwise, check each side's square. Method 1 will do this. You'll find an off-angle. Repeat until the deck is square. Join beams and joists. To square your deck, apply both procedures. First, check your first rim joist is square, then your last. Using this approach first is more effort because you'll need to attach two rim joists instead of one, and if you need to change, you'll use 3-4-5 or 6-8-10 anyhow.
3. Using a String
Floating decks don't have ledger boards for measuring, levelling, and squaring. In that scenario, use a string. You've (hopefully) previously drawn a floating deck using batter boards and string. You'll use string and the other two methods to square your deck. Batter boards are posts with L-shaped boards that simulate deck corners. They're staked beyond each corner and hold your strings. Since they're outside your deck's footprint, you can modify the strings to find your square. After installing the beams, use string to locate the rim joists. String the batter boards perpendicular to your beams. Tie two more strings parallel to the beams' edges. First string set: 3-4-5 technique. Check diagonals. Once the strings are square, install the rim joists.
4. With Builders Square
Using a builders square is an even easier way to use the 3-4-5 method. For this method to be worth it, you'll need to buy a big 3'x4'x5' square made just for square structures like decks and other framing. Most of the time, these can be folded up and can be bought at most home improvement stores. Once you've put the rim joist on one end of the ledger board, wedge the right angle of the square into the angle made by the rim and the ledger. Keep the square tightly against the ledger. Adjust the rim joist so that it is also flush with the square's side.
For several reasons, this tool can be more accurate than a measuring tape. First, some people forget which side of the thickness to measure along the tops of the ledger and rim joists, the inside or the outside. The builder's square makes you measure from the inside, so you can't mess up when you're trying to make your deck square. Lastly, this type of builder's square is very useful in many other ways, so the tool's low price is well worth the investment.
5. Build a Box
If you're building a floating deck, you can use your rim joists to build a box on the ground. Then, you can lift the box and put it on top of your beams. This lets you square your deck while it's on the ground, which might be faster and easier than trying to square it while it's on the beams.
To do this, cut the rim joists to the right length and fasten two of them together at a right angle. Use the 3-4-5 method to make the two numbers equal. Put a piece of plywood or a 24 across the corner to keep it from falling back out of shape. Do the same thing with the two other rim joists. Brace them, and then connect the two sets of rim joists.
Make sure the box is square by measuring the diagonals, and then lift it onto your deck beams. This method works well for smaller floating decks where two people can easily put the box on the deck beams.