View Full Version : How to square forms for a slab??

11-30-2007, 07:24 AM
Looking for some good reading on how to set up and square either cement forms or a building. At one time I saw how to do it with strings and so on but I can't
find any info on it now. Any help would be appreciated.

Thanks Doug

11-30-2007, 07:50 AM
Assuming it's rectangular or square, measure from corner to corner. Measurements should be the same.

11-30-2007, 07:51 AM
If it is square or rectangular in shape, just measure from corner-to-corner on the diagonal (ie RF to LR and LF to RR). You can use a tape measure or string that doesn't stretch too much (like masons string). Once the distances are identical, you will have 90 degree corners, guaranteed.

11-30-2007, 08:10 AM
The Pythagorean Theorem

A(squared) + B(squared) = C(squared)

Works great if you don't have complete squares, rectangles, etc.

11-30-2007, 08:10 AM
Initially, you can use the 3-4-5 method to get the corners square.

Measure out 3 feet from the corner on one side, and out 4 feet on the other. The diagonal between those two points will be 5 feet if the corner is square. Of course, you can scale this up - 6-8-10, 9-12-15, 12-16-20, etc.

It's the Pythagorean theorem, if you're interested.

11-30-2007, 08:11 AM
Measuring across corners is a good start, but it will not guarantee 90° corners, unless you can be assured that all parallel runs of the foundation are exactly the same lenght. which can be hard to do on garage foundation that has an open end for the door. Cross measuring make sure the both corners on the same end/side are the same angle, but you could still end up with a trapizoid. In addition to measure across corners use a 3-4-5 measurement at each corner to ensure 90° corners.


11-30-2007, 08:12 AM
3,4,5 method at a corner. Just picture a triangle. One straight leg is 3' the other straight leg is 4', and the diagonal line will measure 5'. Or if a square, corner to corner will be equal distance on the measurment. Quite a few people set up batter boards. That is the 3 stakes and boards set up at an "L" out past where the foundation goes. You do this at all four corners, then run string to the boards all four ways. You can then adjust the string until you get your square or corner set. At the point the string crosses you then drop a plumb bob down to where you need it. Setting up batter boards is probably the easiest and most accurate. Easiest being the ease of adjustments until you get the settings you want. If you are interested I can sketch you up showing a little more detain as to what I mean.


11-30-2007, 09:18 AM
measuring trianges sucks.
it's an outdated method. Go to Home Depot. in the tool asile, buy a laser square. Stanley makes them, they're about $30 and they save about 4 hours of set-up time when you hand your strings.

The way I've been laying out my string lines is to use batter boards and mason string. basically, all you do is run the edge of the line about 3' past where the corner lies and hammer two 2x4 stakes in the ground about 2' apart and span between them with something like a 2x6. do this at both ends of every line run. then you can adjust the lines left/right and up/down very easily. it also means that you can drop the lines and the excavator can dig out the corners without knocking over your stakes.

11-30-2007, 09:23 AM
Thanks, the 3-4-5 thing that is what I have seen done before. I just did not know quite what the purpose of it was. Or how it worked. When it comes to this building stuff I'm at a loss. I'm trying to gather info to figure out how much of my project I will be able to do on my own or with the help I have available to me. Again this place is great and thanks for your help. I'm sure you'll be seeing more questions from me. :headscrat:headscrat

11-30-2007, 09:28 AM
Sundowner is this what you use?

11-30-2007, 09:43 AM
yep, that's the bugger.

Craftsman also makes a self-levelling laser level/ plumb line that comes with a tripod and mounting bracket. I think I paid $89 for it, but it's a bargain at 3x the price. I f'n hate spirit levels.

11-30-2007, 02:14 PM
if its craftsman brand have already paid 3 times the worth.

11-30-2007, 08:21 PM
3, 4,5 or 30, 40, 50 its the same, and if you do cross measure, it will be square, no other way for it not to be. measure opposite to opposite and if you are 1/2 inch diffenebts, that is a lot, get it to a 1/4

11-30-2007, 08:55 PM
if its craftsman brand have already paid 3 times the worth.

aw, stuff a sock in it. ;)
it works for what I need it to work. :thumbup:

12-01-2007, 05:02 AM
1. Use a surveying instrument (theodolite)
2. set the instrument over a nail (A) at one corner of the proposed slab
3. measure out the length of the slab and place a nail at that corner (B)
4. sight the nail (B) with the instrument on (A)
5. turn a 90 degree angle and set a nail (C) on the 90 degree line at the width distance of the slab.
6. set the instrument over nail (B), sight (A) and turn 90 degrees and set nail (D) on the 90 degree line at the width distance of the slab
7. check distance D-C and the diagonals B-C and A-D.
8. Run your forms along string lines connecting the four corners