3-4-5 to square a roof, wall, floor or deck

Using 3 4 5 to square a roof or floor

Can also be used to layout a wood framed foundation.

Use this method to square up your concrete pour forming as well!

What are the advantages of having a square foundation to work from. Accuracy is always priority one! Other things you gain are optimum use of materials having consistent measurements at all points. Another thing is the ability to cut multiple framing members to the same dimensions, over and over as needed, because you have an exacting area that does not change. It doesn’t start out at 12 feet wide and end up 11 feet and seven inches.

Layout of a square foundation also allows the use of 4’X 8′ sheet-goods to be placed directly on the framed foundation and to line up the edges with the exterior perimeter.

Think for a moment if you have taken no time to square up a foundation for your shed you are building for all your home improvement tools and home overflow items. You frame this foundation up build and frame walls, stand the walls and secure them. Whenever you start cutting joists and rafters you get an irritating sense of “what is going on here”?

Diagonal Measurements to establish square base, form or foundation area


Because the foundation is racked out of square the walls transfer this out of squareness to the upper top plates. Your widths may be different lengths for your joists and your rafter pairs have to be cut individually due to the constant change in measurements from this oversite. Even the plywood that lays on the final framed rafters will have to be cut individually because the rafters will tend to drift out of layout and also may be higher and lower on plane.

Simple measuring technique is to measure across the diagonal corners. In a perfect world measuring across a diagonal or square area should produce the same measurement from one cross corner to the next.

But a perfect world is hard to produce. So if you are out of square 1/4″ ( quarter inch ) across 21 feet you should be alright. Remember this is the starting place, the foundation. You will continue to check the structure for squareness at every stage as you build.

It is more beneficial and much easier to square the foundation, re-square the wall layout and have it all work out. And if this is your first DIY framing job, you may not be able to think your way out of these issues. And what will the neighbors think!


framing square - verify square

One of the biggest headaches you, as a do it yourself-er, can get is from trying to install a square wall into an unsquare room. Or to attempt to cut rafters to match an out of square top plate.

There are other areas of building that require the carpenter to square up a particular assembly, so we have to look at the several different ways to approach this important aspect of building to develop a true, plumb/level/square project.

The easiest method to establish is something in or out of square is to measure across the corners diagonally.

As an example, if you had a simple frame measuring 12 inches on all sides, if you measured across the corners diagonally, and the frame is square, your diagonal measurement will be approximately 17 inches from one corner to the farthest corner away.

A simple way to verify this is to take a framing square and measure across from the 12 inch mark on the short side to the 12 inch marked on the long side. This method can be used on either square or rectangular surface areas, so it is not limited in how you can assure a proper squared up area.

framing square - 3-4-5 method

Next comes the much used 3-4-5 method. A typical triangle with a base of 3 inches and a right angle leg at 4 inches will measure across the diagonal, 5 inches.

You could simply measure 3 feet on one side and 4 feet on the right angle side and this will generate the 5 foot hypotenuse. You can use this method for an alignment tool when in doubt or in need of another way to verify the squareness of your forms or wall assemblies, etc.

The beauty of this method is that you can use multiples to get your end result. you can use one number and multiply it by all the root numbers (3, 4, 5) and this will generate a much long area that you can cover to square up.


  • 2 X 3 = 6 ft.
  • 2 X 4 = 8 ft.
  • 2 X 5 = 10 ft.

One more example:

  • 4 X 3 = 12 ft.
  • 4 X 4 = 16 ft.
  • 4 X 5 = 20 ft.

Again, a simple way to verify is to measure across the blades of your square using 12 inches and 16 inches as the baseline for your measurement.


Using the above example, let us say that we are forming up to pour a concrete slab 12 feet wide by 16 feet long. Measuring across the framing square locating the number 12 then the number 16 will produce the resulting number 20.

framing square - verify square slab
So you should be able to measure across the 12 foot by 16 foot form and get a 20 foot reading.

So your form of 12 feet on the one side and 16 feet on the other, when measured diagonally across the form should be 20 feet.

Simple enough?