How to Square a Foundation for a Shed in 3 Simple Ways

If you’re a fellow DIY enthusiast, you already know that with a little bit of time, effort, and know-how, you can complete just about anything in and around your home. This even includes building your shed! Building a shed occurs in a few different steps. And firstly, you’ll need to know how to square a foundation for a shed

There are three extremely common types of shed foundations, and we’ll discuss how each of these three foundation types influences the shed-building process. So without further adieu, let’s get started! 

How to Square a Foundation for a Shed

The 3 Most Common Shed Foundations

  • Deck Blocks
  • Concrete Slab 
  • Concrete Blocks 

Deck blocks are a great foundation option for several reasons. They’re suitable for a wide variety of shed sizes, they’re relatively easy to install, and the cost of the materials required is relatively inexpensive. A shed foundation built from deck blocks involves evenly spacing out precast concrete blocks, most of which have notches in the top for wood joists. 

The one downside to using deck blocks is that they can sink into the ground over time, creating slopage. Because of this, it’s best not to use deck blocks as a foundation if you’re planning on storing heavy equipment in your shed. 

Concrete slab foundations tend to be the most long-lasting and sturdy. They create excellent support for the base of any size shed and are strong enough to make storing heavy equipment, machinery, and even vehicles completely doable. 

It’s important to note that pouring a concrete slab isn’t necessarily an easy task and can be quite expensive. Concrete slabs are also permanent, so it’s vital that if you choose to pursue this method, it’s done correctly. 

Concrete blocks are another very suitable option in terms of shed foundations. Like deck blocks, they can also support any shed size, including prefab sheds. As long as you’re leveling as you go, concrete blocks are relatively simple, quick, and inexpensive to install. 

Similar to deck blocks, concrete blocks aren’t the best option if you’re planning to store heavy equipment in your shed, due to structural weight-bearing concerns and the possibility of vibrations. If you need to store heavy items in your shed, installing a concrete slab is always the best way to go. 

3-4-5 Square Method – Triangle Method

Any time you’re working with corners, whether you’re installing carpet, laying flooring, or in this case, creating a shed foundation, you’ll want those corners to be square. Ensuring your corners are as close to 90 degrees as possible will make the rest of the shed-building process go that much more smoothly. 

That’s where the 3-4-5 method comes in. The 3-4-5 method has its roots in the Pythagorean Theorem, but thankfully, you don’t need to be a math wizard to understand it. The general rule of thumb behind this method is that if a triangle has three sides, which are three, four, and five feet long, that triangle is a right-angle triangle that includes a 90-degree angle. 

3-4-5 Method For Squaring Foundation Corners

If you can find this 3-4-5 triangle within the corner, you are trying to square, and you’ll know that the corner is, in fact, square. If you measure two sides at three and four feet, and the third, the longest side doesn’t total five feet, you’ll know the corner isn’t square, and you’ll have to adjust your measurements. 

For measuring larger surface areas, measure the corner in question with any multiple of the 3-4-5 method (for example, 6-8-10, 9-12-15, 12-16-20, and so forth). Using a larger multiple to measure a larger surface area will make the corner measurements more accurate. Carpenters and contractors use the 3-4-5 method all the time to check that their corners are square as they go, and to avoid a base or foundation issue before progressing to the next steps. 

How to Square a Foundation for a Shed with String

Another back-to-basics method for squaring a shed foundation involves using string. Once you’ve chosen the spot for your shed, use batter boards and mason’s string to measure out the perimeter of the shed’s foundation. Once you have the perimeter marked and the string in place, it’s time to employ the 3-4-5 method. 

Working with string can be a little finicky, especially when dealing with a large surface area. That’s why it’s best to enlist the help of someone else when proceeding to the next step of squaring off the corners (aka using the 3-4-5 method). 

In each corner, measure out your 3-4-5 triangle (or a larger multiple if it’s easier) and look for that distance of five feet between the smaller sides of your triangle. If your third, longest side isn’t measuring out to five, adjust the string and batter boards as needed until you end up with four square measurements in each corner. 

How to Square a Foundation with a Tape Measure 

If you don’t want to go about setting up batter boards and string to measure out your foundation’s corners, another way to determine if you have a square foundation is to use a tape measure and the diagonal measuring method. 

This method is extremely quick and simple, and the only tool you’ll need is your tape measure. Begin by holding your tape measure in one corner of the foundation and measuring across the diagonal to the opposite corner. When you’ve recorded this measurement, hold your tape measure in the next corner and measure across the diagonal to its opposite corner. 

If both of these diagonal measurements are the same, your foundation is square. If the measurements are not the same, you’ll need to adjust the foundation measurements accordingly. This is an extremely effective, tried-and-true method to practice during the planning stage and before you get to pouring a foundation. 

6-8-10 Square Rule

Speaking of larger surface areas, let’s talk a little more in-depth about using larger multiples of the 3-4-5 method. As previously mentioned, sticking with the 3-4-5 feet measurements may not give you an accurate reading when dealing with large foundations and in turn, large corners. 

That’s when using the 6-8-10 method for squaring comes in handy. Dealing with an even bigger surface area? Measure your corners using the 9-12-15 method instead. A great rule of thumb for squaring out extremely large areas is to use meters as your unit of measurement instead of feet. Since one meter is equal to three feet, you’ll still be working in units of three, hence the original 3-4-5 method will still apply. 

How Far out of Square is Acceptable? 

Even if you’ve been extremely focused and have practiced the 3-4-5 method throughout the process of creating your shed’s foundation, mistakes can happen. If you’ve completed the foundation, only to measure it out one final time and realize it’s not square, don’t panic. You may have a little bit of wiggle room to resolve the issue. 

Where to Compensate

If you’re about ¼” out of square, everything is likely going to end up working out just fine. If you’ve poured a concrete slab that’s out of square by this measurement (or less), you can adjust the bottom plate as you go to build the shed walls. The bottom plate refers to the 2×4 (or 2×6, depending on your preference) that will be set on the slab. 

Although some people choose to build their shed walls flush with the edge of the concrete slab, it’s best practice to leave ½” to 1” of the slab around where the walls will be sitting. This is because once you’ve installed your strapping, siding, exterior finishes, etc., the thickness of these materials will end up extending past the slab anyways. 

And so, giving yourself a little bit of room in terms of the edge around the slab will help to resolve an out-of-square issue (as long as it’s not astronomical). You’ll also be left with shed walls that end up being more-or-less flush with the base, which tends to look far more aesthetically pleasing. 


You can save yourself a good chunk of change if you choose to build your shed (or at least, the foundation for it). When planning out and installing the base, it’s extremely important that you continuously measure as you go. Remembering to use the 3-4-5 method will help ensure that you end up with a square foundation, which is an essential part of the shed. 

Hopefully, you learned some great tips on how to square a shed foundation, and you’ll be able to take on the project confidently. If you enjoyed this article, don’t forget to share it and leave us a note in the comments!

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