How To Square A Deck: 5 Simple Ways

Years ago, when I built my deck, I was concerned about getting it exactly square. I had the tools to level it, but I didn’t have a tool to help me achieve a perfectly square deck. After some effort, I realized there were several techniques to achieve a square deck that was simple.

The best way to square a deck is to use the 3-4-5 or 6-8-10 method. Using your ledger board and a rim joist fastened to one end of the ledger, measure 3 feet from that corner along with the ledger and 4 feet from the same corner down the rim. Make two marks. Measure between the two marks. You want the marks to be exactly 5 feet apart. Adjust the rim joist to achieve that measurement to get a square deck.

Achieving a square deck depends on having a stable foundation and level framing. If you don’t level your framing, your measurements to get a square deck will be off. Similarly, a foundation that shifts will throw your deck out of the square.

There are several other methods to achieve a square deck, but the 3-4-5 method is the simplest and quickest. There are other ways to get an even more accurate square, which we’ll go over below.

How To Square A Deck

Why Do I Need a Square Deck?

No, we’re not referring to a deck with all equal sides. We mean a deck whose corners are all perfect 90-degree angles. But why do they have to be exactly 90 degrees? Sure, a corner that is 89 degrees won’t affect the integrity of your structure much, but it will change how it looks.

Let’s say an angle of your deck is off by a degree or two. Without deck boards, you probably wouldn’t notice with the naked eye. Once you start installing the deck boards, you quickly find that each successive board either needs to be a bit longer or shorter.

While you could custom cut each board to account for the differing lengths required of each row due to the out of square angle, the boards would still look staggered. Plus, taking the time to cut each board a different length would lengthen your install to account for an out-of-square corner that you could fix easily before installing the decking.

Another reason for squaring your deck is that your joist hangers and beam to joist connections are meant to fasten at 90-degree angles. If you use hurricane ties or hangers to affix your joists, and the deck isn’t square, then it means your joists, or some of them, are out of square. Thus the lumber isn’t sitting square in the brackets, which means the connection is compromised.

Of course, your deck won’t immediately fall apart if it is just slightly out of square. But the issue is longevity. Fasteners and connectors rely on solid 90 degree angles. Screws and nails do too. Fasteners that are not able to snugly connect two pieces of lumber will fail faster.

How to Square a Deck: 5 Ways

There are several ways to square your deck, and the chances are that you’ll find more ways than the methods below if you talk to enough contractors or old-timer DIYers. I’ve compiled the list below according to simplicity and accuracy. You don’t need any special tools with these methods to accurately square a deck.

1. Use the 3-4-5 or 6-8-10 Rule

Square deck frame with 3 4 5 rule

Pythagoras and his handy theorem allow us to precisely obtain a square deck. If you don’t know Pythagoras or any theorems, then let’s recall: in any triangle with one right angle, adding the squares of the two shorter sides will always equal the square of the longest side. Traditionally the equation is written as a2+b2=c2.

We call this the 3-4-5 rule because when you input those numbers into the equation, in that order, the equation balances. That means that you could have a triangle with sides of 3’ and 4’. Then, if you connect the two endpoints of those sides, you’d have your complete triangle with a side of exactly 5’. The same goes for 6-8-10, as well.

This helps us when squaring a deck because after we install our beam and ledger board, all we need to do is install one rim joist, connecting it to the ledger, and resting it on the edge of your beam or beams.

Using your measuring tape, measure from the inside edge of the ledger where it meets the rim joist. Measure to 4’ and make a mark on the top inside edge of the ledger. Do the same with the rim joist, starting your measurement from the inside edge of the ledger down along the rim joist 3’. Make your mark on the top inside edge of the rim joist.

Now you will check for square. Measure between the two points, starting from the ledger. You will move the rim joist until the mark on it meets the 5’ mark on your measuring tape. Once it does, fasten the joist to the beams. You’ve got a square deck.

Using the 6-8-10 method will yield a more accurate result, as the greater distance will make the measurement more precise. Opt for this method if you will have joists that are 8’ or longer.

Drawbacks to this method are that you could have a crooked joist, which could throw off subsequent measurements. Be sure your first rim joist is true before using it as a template for the rest of your joists.

2. With a Tape Measure

How to check for square with a tape measureThe other most popular way to square a deck is measuring opposite corners to ensure they are equal. To do this, you must have both your rim joists installed on either end of your ledger board.

Attach both rim joists to the ledger board. Do not affix them to the deck beams yet. Once you’ve set the rim joists where you want to install them, measure the diagonals with a tape measure. Start at the right inside corner of where the rim meets the ledger, and measure diagonally to the opposite corner, terminating at the inside top edge of the opposite rim joist.

Do the same to the other pair of corners. You should have equal measurements. If not, you need to check the square of each side. To do that, you’ll use method 1 as described above. Likely you’ll find one angle slightly off. Re-measure and repeat this procedure until the deck is square. Attach joists to beams.

Ideally, you’ll use both methods to square your deck. You’ll use method one to ensure your first rim joist is square, then the second method to ensure your last rim joist is square. Using this method first is more work because you’ll need to affix two rim joists instead of one and if you need to adjust, you’ll end up using the 3-4-5 or 6-8-10 method anyway.

3. Using a String

If you have a floating deck, then you don’t have the benefit of a ledger board as the basis for measuring, leveling, and squaring your deck. In that case, the string may be your best friend.

When building a floating deck, you’ve (hopefully) already outlined where the deck will go using batter boards and string. Keep in mind that using string isn’t the sole method – you’ll use the above two methods in conjunction with string to square your deck.

Batter boards are stakes with horizontal boards formed in an L-shape that roughly mimics your deck corners. They are staked beyond each corner and serve as the material for you to affix your strings. Since they sit outside the footprint of your deck, they allow you to adjust the strings along the boards to find your square.

Once you’ve installed your beams, you’ll use string to find the outside edges of the two rim joists. Tie the string to the batter boards perpendicular to the outside edges of your beams. Tie two more strings that will show the other two outside edges parallel to the beams.

Use the 3-4-5 method to square the first set of strings. Double-check by measuring the diagonals. Once your strings are square, install your rim joists along the inside edge of the strings.

4. With Builders Square

builders squareAn even easier way to use the 3-4-5 method is to use a builders square. To make this method worthwhile, you’ll want to invest in a large 3’x4’x5’ square designed for the express purpose of square structures like decks and another framing.

Typically these are foldable and can be found at most home reno stores. Once you’ve installed your rim joist on one end of your ledger board, wedge the right angle of the square into the angle created by the rim and ledger. Hold the square snug to the ledger. Adjust the rim joist so that it also flush against the side of the square.

This tool can be more accurate than just using a measuring tape for several reasons. First, when measuring along the tops of the ledger and rim joists, some forget which side of the thickness to measure – the inner or outer edge. The builder’s square forces you to measure from the inside, which negates the possibility of messing up squaring your deck.

Finally, this type of builder’s square is highly useful in several other applications, so the investment is well worth the tool’s moderate cost.

5. Build a Box

If you are building a floating deck, then you can build a box using your rim joists on the ground, then lift it and place it atop your beams. This allows you to work on the ground squaring your deck, which in some cases might be faster and simpler than trying to square the deck when it is on the beams.

To do this, you’ll need to rip your rim joists to length and fasten two together at a right angle. Use the 3-4-5 method to square the two. Cross brace the corner with a piece of plywood or 2×4 to ensure the corner doesn’t fall back out of square. Repeat the process with the other two rim joists. Brace them, then fasten the two pairs of rim joists together.

Measure the diagonals to make sure the box is square and lift it onto your deck beams. This method works well for smaller, floating decks that allow two people to easily place the box on the deck beams.

How to Fix an Out of Square Deck

An out of square deck that is already fully constructed can be a time-consuming fix.

To properly square an out of square deck, you’ll have to first remove all your deck boards and detach all the joists from the beams. Square the deck using the 6-8-10 method. Refasten one rim joist and then space your joists accordingly based on the rim joist you just squared.

Manipulating deck joists while they’re all still attached is no small feat. You could push the joists back into the square in a variety of ways, such as using ratchet straps or a come-along. You could also get four guys to push the other end. Regardless, you want to avoid taking all the joists off, if possible.

You’d use the same process for a floating deck, except you’d leave all the joists attached to one beam, so you could push or pull the framing into square on the second beam and re-fasten.

How to Square a Floating Deck

Square deck design

To square a floating deck, you’ll attach both your rim joists that are perpendicular to your deck beams. Once in place, measure the diagonals. Make sure you are measuring from inside corner to inside corner or outside to outside. Otherwise, your measurements will be off. Once both measurements are equal, you can attach the other two rim joists.

It is important to attach the last two rim joists because you can then use the 3-4-5 method to ensure square corners. If you find the corners still need adjustment, you can detach one rim joist and move it accordingly.

You can also use the box method to square a floating deck, as described above. While this isn’t ideal for a 20×20 deck, it can work very well for a small 10×10 deck footprint.

How to Square Deck Posts

Squaring your deck posts requires mason’s string and stakes. This is critical because if you don’t have perfectly square deck posts, you won’t have a square deck no matter how square you try to get your joists.

Mark your deck post locations. Start with two outside posts and stake them. Put a stake in another post location at a right angle to the first two. Tie a string to the first stake, wrap it around the second and then bend it at a right angle to the last post. Use a measuring tape and the 3-4-5 method to ensure the post locations are square to one another.

If you have many posts, you can work your way through the post locations or use the 6-8-10 method if you have many deck posts. Either way, you’ll still get an accurate measurement.


Whether you use string or a builder’s square, you’ll need to make sure the foundation of your deck is square before you try to square up the joists and decking. Ensuring your deck posts are square to one another will guarantee your beam is parallel to your ledger board or that both beams on your floating deck are parallel.

Once your beam or beams are parallel and square, you can use the above methods to square your rim joists to the ledger board or the other rim joists. Be extra careful how you measure. Take care to be consistent and take note whether you measure from the inside of your lumber or the outside – an inch and a half out of square can make a big difference!

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