A Building Foundation: Making it "Square"

A perfectly square building foundation begins with a square layout. A rectangular house must also be a "square" rectangle, meaning each set of two sides are equal.

STEP 1: Position the four far corners of the house roughly in place where they will go. Stick a screw driver, stick or flag in the ground approximately where the corners will be. Use a 100 foot tape measure and place these four corners how you want the house to face in relation to the lot, the neighbors and the road. If you are within a subdivision or city area, there may be covenants and restrictions on where you may place the house. Be sure to check this out before you start, or you may be very sorry later.

Word to the Wise: When working with forms in your building foundation, batter boards and etc., you will find that if you screw the pieces together instead of nailing them, you will have a much easier time. If you are using nail guns, it is not as big a deal, but never try to hammer these pieces together as you will loosen the supports.

There are several ways to find “square” in a building foundation. All methods use some sort of diagonal measurement. A true square will measure the same on all four sides, and the two diagonals will be exactly the same. It is very easy to have a four sided box that has all four sides of equal length, but it is not square. This is a parallelogram. Pulling the two diagonals will reveal that they are not the same distance. See Figure # 1 below

Figure #1 Parallelogram

Equal length sides, but not square. .

“Pushing” the longer diagonal corner back so that the distance shortens will bring the layout into square. This may take several tries, “walking” the dimension in. When you are within an eighth of an inch, you are close enough in your building foundation.

Word to the Wise: When pulling dimensions across long distances, use a steel or fiberglass tape measure to eliminate any stretching and hold the “start” point at 1 foot instead of at the end of the tape. Measuring an eighty five foot distance requires pulling the tape quite firmly to prevent it sagging and changing the true dimension. When the person on the other end is pulling that hard, you will find it impossible to hold the end of the tape secure and still enough to get an accurate measurement. By “cutting a foot” you can hold the tape against your leg and lean into the pull, holding the one foot mark right on the proper point. Of course, you must remember that you added a foot to the length.

STEP 2: So how do you square something that isn't square in a building foundation? First of all, you need to understand what a right triangle is. A right triangle has three sides and one 90 degree angle. You can “place” a right triangle anywhere you need it to determine dimensions and “square-ness”. In a right triangle, you can use a simple 3,4 5 formula, which means 3 units along one side, 4 units along the other side, yields 5 units across the hypotenuse (diagonal). That means if you measure 3 feet down one side and four feet down the other side, when the diagonal between them is exactly five feet, the two lines intersect perpendicularly, creating a “square” corner. This is not a very accurate method of setting to square in a building foundation, but the larger you make it, the better it is. It will remain accurate as long as you maintain the proper ratio. You could go ten units of 3 down one side (thirty feet) and ten units of four down the second side (forty feet) and the diagonal would be fifty feet ( ten units of five).

A more accurate method is to figure the diagonal measurement mathematically and then set it to it. In a right triangle, “A” squared plus “B” squared equals “C” squared. In other words, side “A” multiplied by side “A”; added to side “B” multiplied by side “B”; will equal the diagonal “C” multiplied by the diagonal “C”. This sounds more complicated than it is. Say your house is 40 feet wide by 60 feet long. You will multiply 40 x 40 to get 1,600; then 60 x 60 to get 3,600. Add the two together to get 5,200, then find the square root of that, or 72.111 Hmn . . .remember we used feet so our answer is in feet. Seventy two point 111 feet. So how far is that? Well seventy two feet is obvious enough. How far is .111 feet? Multiply .111 x 12 to get inches. That yields 1.332 inches. .332 is the decimal equivalent of 21/64ths. However, that is a little more precise than we need. 5/16ths is .312. So, you can look for a diagonal of 72 feet 1 and 5/16 inches.

This method may be used to locate any corner in any house in a building foundation, no matter how cut up or crazy the design may be. If you have a solid grasp of this concept, you are ready to move ahead.

In order to obtain an accurate “square” you need to install batter boards & strings in your building foundation.

STEP 3: Once you have the four corners roughly square, with sticks or screw drivers or something in the ground marking the corners, you are ready to drive your stakes. Depending on the height of your floor, you will cut 2x4s the floor height plus 24 inches or so. The extra is to drive into the ground. Depending on how high your floor level is, you may need to brace these stakes. If your floor is within three feet of the ground, you should be OK. The worst house I ever did had 23 feet of fall from the front corner to the back corner. Can you imagine batter boards 25 feet high in a building foundation? It was a challenge, to be sure!

The stakes you drive in must be at least as high as the finished floor level. Drive the stakes in at least two feet away from the corner stakes. You must allow room to dig the footings without disturbing the stakes. Refer to figure #2 below. You will need three stakes per corner. If you end up with a stake that is too short, you can simply screw an extension on it to reach the height you need for your building foundation.

STEP 4: Use an 8 or 10 pound sledge hammer to pound your stakes in the ground of your building foundation. It’s best to buy one with a fiberglass handle, so you don't have to replace the handle a few times during the project. Keep your feet out of the striking area, so when you miss the stake, you don't hit toes.

STEP 5: Once the stakes are in, use your laser level or builder’s level to mark the floor height on the 2x4 stakes. If you set a pin as a floor height reference, set your laser reader unit on that pin and adjust the unit until it finds the laser beam. Come off the pin and back again to make sure the unit is reading correctly. Then, go to each of the stakes and hold the reader unit in position where it finds the beam and beeps, and mark the stake at that height. Mark the stake on the side away from the house. If you find the unit difficult to hold in place while marking the height, get someone to help you.

STEP 6: When you have all the stakes marked, cut some 2x4s long enough to reach from stake to stake. Using a cordless drill and 3” drywall screws, screw these boards in place (or fasten with a nail gun) from one stake to another horizontally. Place these boards on the outside sides of the stakes and set them just a little higher than the marks.

Word to the Wise: If you are perfect at setting these boards you can place them right on the marks in your building foundation. However if you are a little off, you are asking for headache. However, if you set the boards a little high, you can easily tap the stakes down to “fine tune” the final height.

STEP 7: Once you have all the horizontal boards in place, use your laser level again and place the unit on top of the boards at the three points where they are attached to the stakes. Tap the stakes down to get the height perfect.

STEP 8: Now set a screw or nail in the top of the horizontal board, in line with your “rough square” stick marking the corner. Do this on all horizontal boards, then stretch a mason line (heavy string) between these points, across to the opposite batter board point. When you finish, your strings should cross roughly over the sticks you placed on the ground for corners.

On each end of each string, turn a loop around your finger of about 5 or 6 turns, slip the loop over the nail and pull the string tail away from the running length of the string. This will bind the string in place, but will also allow you to adjust the string length very easily. Once all strings are in place, tighten them up by pulling the string tail back toward the running length and pulling it tight. Once tight, pull the tail back away from the running length and it will “lock” in place.

STEP 9: OK, you have a set of strings and batter boards that are roughly establishing a square, and in line with the walls of the house. Next we need to set the strings parallel. Measure from your nail or screw to the far nail or screw on the opposite batter board. Make a mark on the batter board at your target dimension. Do this on each end of the layout. Then set your screw or nail on the marks. This should make your strings the correct distance between, and running parallel.

STEP 10: Now we need to bring the strings into perfect “squareness” for our building foundation. Make sure all the strings are tight enough to not droop, then measure from string intersection, diagonally, to string intersection. “Cutting a foot” on the tape measure is very helpful when doing this, just remember the extra foot. Refer to Figure # 4 below.

I had it happen once, in many hundreds of houses, that the first placement of strings just happened to fall in place perfectly. That means in 5 or 6 hundred others it didn't, so don't be expecting it to!

STEP 11: Figure how far off your dimension is from correct – and which way – short or long. I generally select the front wall string as being a reference line, meaning it will not ever be moved. All the other strings will be placed relative to it. Now, you must adjust the screw or nail that the string is looped around, to make the string intersection change the amount you need it to move. Remember, the corner is where the strings cross, not on the batter board. The easiest way to do this is to mark the string with a fluid ink pen at the point where the strings should cross. Then hold the string across the batter board so that it intersects at the marked point. Move your screw or nail to this point on the batter board.

Word to the Wise: Never allow more than one nail or screw to stay on your batter board. If you do, it will inevitably become confusing at some point in the project.

STEP 12: Do each corner, one at a time.

STEP 13: As you adjust strings for square, remember that you also must maintain the correct base dimensions and the strings parallel to each other in your building foundation. If you move one string end an inch, you must move its opposite string an inch, as well, to maintain the proper spacing between them.

STEP 14: Double check every dimension to be sure.

STEP 15: OK, now you have an array of boards and strings that hopefully represent the house layout accurately for your building foundation. If your batter boards have been set to height correctly, all the strings will touch each other where they cross. There should not be any vertical gap between any strings, nor any deflection of the strings where they cross. If there are, a height has been messed up and you need to reset those batter boards.

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