Marking the Forms
Friday, December 25th, 2009
After passing the plumbing inspection, the next thing to do is to mark the forms with stringlines and stakes. The foundation needs to be totally square, which means that both diagonals have to be equal. So, how do we calculate the diagonals?
Do you remember that old school math? If you have a right angled triangle, the square of the two sides equals the square of the diagonal? This is your secret formula.
Say for example that your house is 20′ x 30′. What is the diagonal?
On the calculator, compute 20 x = and put the answer in M+
Then, compute 30 x = and put the answer in M+
To find the diagonal, push MR and then the square root button, and voila, your magic diagonal shows up as 36′ 1″.
So, how do you use this magic number? First, put in two corner pins. Then, put a measuring tape on each pin and move towards the third corner. You’ll need two people for this part. One person’s tape should be the length of the side, and the other person’s tape should be the length of the diagonal. Wherever the tapes meet, is where your third pin goes.
Do the same to find where the fourth pin goes, check that both diagonals are the same (make sure you use metal tape for this since fiberglass tape is useless since it tends to stretch).
After you get your pins in the correct places and the diagonals are perfect, make sure you check that your plumbing pipes are still coming up inside the rigth walls. If everything is just right, you are good to go.
Tags: marking concrete forms
