Today we set up batter boards and put out marking strings to prepare for constructing a round deck.
We wanted 2 strings that marked the center lines, crossing each other at 90 degrees. We set up batter boards and ran strings between them. Then we measured out a 3/4/5 triangle from the crossing. We looked at the error, adjusted the strings, and repeated until we thought it was good enough.
The strings were not at the same height, so measuring the crossing was tricky. Measuring against the middle of the string is hard, because the string moves so easily.
Each time we adjusted the strings, the crossing point moved, so we had to measure all 3 legs of the triangle each time. If you get measure the hypotenuse and find it's off by some distance d, you have to move the ends of the string by some value that's larger than d, but you don't know how far. You just have to adjust, measure, and repeat.
In this case, the deck is 30' diameter. We started with legs of 6'/8'/10', then realized we had enough room for 12'/16'/20'.
If I was in my workshop building with wood, I would use a carpenter's square or factory corner of a piece of plywood, but at this scale that doesn't work.
I get that 3/4/5 is a good way of checking a right angle, but it's a painful way to create one. Is there a good way to create a right angle with strings, getting it right the first time?
Best Answer
Step #1: Finding the center.
Never built a round deck before, but to find out where you want to put the deck, and to mark the center point, I would drive a stake or nail, tie a string to it that is as long as the desired diameter (30' here), and then walk in a circle with it, keeping it taut, and putting some long nails into the ground, at the end of the string, every 3' or so. Then I'd put a string around all of those nails, wrapping it around each nail a few times. Alternatively, if you are working on concrete, clean it up and use chalk instead of the nails & strings for the outside.
Step #2: Dividing the circle into quadrants.
I'd imagine that this is the most accurate method outside of buying survey equipment. Good luck!
(Note: Unfortunately, as you swing a string around a nail/stake, it will shorten in length. If you decide to trace out the entire outside of the circle, your distance from the center will decrease as you go. Tying a loop in the string, allowing it to rotate around the nail/stake will prevent this.)