When performing layout work for posts to support a deck, a concrete slab, or a future foundation, the corners of the layout must be square.  Because standard hand squares are too small for these tasks, contractors use a transit or builders level to achieve a square layout. These instruments are expensive to buy, but fortunately, there are two other methods that can be used to achieve a square layout. This article explains the 3-4-5 method which is performed with the use of two tape measures.

The 3-4-5 method 

This method involves using the variables 3-4-5 to plot the first corner of the layout.  From there, lines can be extended to locate the second and third corners, and from those, locate the forth.

If it used correctly, the 3-4-5 method will create a square 90 degree corner.  To prove this, let's apply the variables 3-4-5 to the Pythagorean Theorem.  

• The theorem states: (a) squared + (b) squared = (c) squared 

• 3 squared (3 x 3 = 9) + 4 squared (4 x 4 = 16) = 5 squared (5 x 5 = 25) or 9 + 16 = 25

• The a and b sides produce the square corner and the c is the longest side or slope

Using the 3-4-5 as a base, you can double the variables to 6-8-10, and again to 12-16-20 and so on.  The general rule is, use the variables closest to the size of your proposed layout.  

Step by Step Process

Note: All measurements from the stakes are made from the nail in the stake.

1. Establish the location of the first corner and drive a stake into the ground (stake A).  Place a 4d common nail halfway into the top of the stake.
2. From stake A, measure 4 foot and drive a second stake into the ground (stake B).  Measure again from stake A and place a 4d common nail halfway into the top of the stake at the 4 foot mark. 
3. Using two tapes, simultaneously measure 3 foot from stake A and 5 foot from stake B.  Where the 3 and the 5 intersect, drive stake C into the ground.  Measure the 3 foot and the 5 foot again, this time marking the top of stake C with an intersecting  pencil mark at each distance.  Place a 4d common nail halfway into the top of the stake where the two lines intersect. 
4. If the nails in your stakes measure 3-4-5 when you have finished, you have established corner 1 square.  Again, if used correctly, the 3-4-5 method will produce corners as square as the transit method.    
5. From the established corner 1(stake A) nail, extend a string line across the top of the stake B nail and drive your corner 2 stake at the correct distance for the proposed layout. 
6. Using the extended string line and tape measure again, mark a cross hair on the top of the corner 2 stake and place a 4d common nail halfway into the top of the stake.
7. From the stake A nail, extend a string line across the top of the stake C nail and drive yourcorner 3stake at the correct distance for the proposed layout. 
8. Using the extended string line and tape measure again, mark a cross hair on the top of the corner 3 stake and place and place a 4d common nail halfway into the top of the stake.
9. From corner stakes 2 and 3, measure the correct distance to corner 4.  Where the two tapes intersect, drive a stake into the ground.  Once the stake is located, measure again from corners 2 and 3 and mark the top of the stake with intersecting pencil marks.  Where the lines intersect, place a 4d common nail halfway into the top of the stake.
10. The final step is to measure diagonally from stakes 1 - 4 and from stakes 2 - 3.  For the layout to be square, the dimensions must be equal.

When performing any type of layout work, the corners must be square. The 3-4-5 method is performed with the use of two tape measures and while it is more time consuming than the transit method, if performed correctly, it is just as accurate.