Setting out and stringlines.

[DJ’s Timber]

Keep it on thread please boys, we're not after a geometry lesson here.

DJ
Super Moderator
Woodworking Australia's
WOODWORK FORUMS

[echnidna]

if both sides are the same length and
both ends are the same length,
the sides are square to the ends when both diagonals are the same length

[China]

Thanks Dj's I was begining to think I'd woken up 30 years ago

[Chumley]

Keep it on thread please boys, we're not after a geometry lesson here.

DJ
Super Moderator
Woodworking Australia's
WOODWORK FORUMS

Aw, spoilsport!

I'm not really into geometry, but it was the only time SilentC was going to lose an argument (picture of smiley face fishing)...

Sealed lips,
Adam

[ComboSquare]

if both sides are the same length and
both ends are the same length,
the sides are square to the ends when both diagonals are the same length

That's true, however the wall might be 5000mm and the length 7000mm, so using 3,4,5 or the method outlined in my second pic seems to be the way to go?

[Ivan in Oz]

if both sides are the same length and
both ends are the same length,
the sides are square to the ends when both diagonals are the same length

Good to see someone answered the question

though you beet me to it

[silentC]

Several of us have answered the question. The way to do it is measure 3 metres across the back wall, 4 metres perpendicular to the wall, then measure 5 metres from the 3 metre mark and where it meets the 4 metre mark, you put a peg and then you run your line out whatever distance from the house, put in your hurdle and then slide the string across until it meets the peg and there you have a straight line perpendicular to the house. You can then measure across the back wall from the first line to the width of your extension and run the second line out parallel to the first. You don't need to mess about with diagonals, unless you want to check it for square after the fact.

[ComboSquare]

Yep I'm aware of that method, just that the guy I've been working with seems to adopt the 'diagonals' approach.

[silentC]

Seems like a lot of trial and error to me. Unless you work out what the diagonal is first and then use it to find the end point of the line. It's basically the same, but with 3,4,5 you don't need a calculator to work out the square root of the square of the hypotenuse.

[ComboSquare]

Seems like a lot of trial and error to me. Unless you work out what the diagonal is first and then use it to find the end point of the line. It's basically the same, but with 3,4,5 you don't need a calculator to work out the square root of the square of the hypotenuse.

Yeah you're right, essentially using A squared plus B squared = C squared is an extension of the 3,4,5 approach. Thinking about it more, simply measuring the diagonals could give you a huge variety of shapes aside from a true square if you don't know the EXACT length of the diagonal correct? (ie both stringlines might run at 85 degrees away from the wall, not necessarily 90 degrees).


Or if you don't maintain the same length at all times between the two points away from the wall.

[silentC]

If the diagonals are identical, it will be square. It's just that if you don't work it out in advance, there would be a lot of trial and error as you move the line and check the diagonals. Your first starting point would be a guess and you could be a metre off and not realise it.

Last time I did this, here's what I did (I was laying out a driveway but the same rules apply):

I put a peg (A) at the wall where one of the lines started. I tied a string line to it and pulled it out to 4 metres then I put a knot in it. Then I measured 3 metres across the wall from peg A and put in a second peg (B). I hooked my tape to peg B and with the string line in one hand and the tape in the other, walked out until the knot in the line and the 5 metre mark met. I put a peg (C) in the ground at this point and put a nail in the top at the exact point.

Then I walked out with the string line to the length of the line I was marking (5 metres) and, holding the string on top of the peg and watching the line until it just touched the nail in the top of peg C put in another peg (D). Then I slid the string line across the top of peg D until it just touched the nail in peg C, and put in a nail the top of peg D at that point. It helps to have nice wide pegs, or use a hurdle.

To get the other side, I just measured the width (4.8 metres) across the wall from peg A, put in another peg (E), ran the line out, tied a knot at the length I wanted, then put the tape on the nail in peg D held the string in the other hand and where the knot met 4.8, I put in the last peg with a nail at the exact spot. Then I checked the diagonals.

.

[Ivan in Oz]

Depends on the job.
Each to their own.

Just done a 35mm recess in my Slab to be layed.
1200mm x 1500mm sides.

Second Diagonal correction had it spot on.

Time is of the essence.


Ivan

[Skew ChiDAMN!!]

Laying out by running diagonals is fairly easy: if you check the length of the run parallel to the wall after setting the diagonals the first time and then halve it's difference to the length of the wall this will provide you with how far you have to move each peg (in opposite directions) to get it nearly spot on. If you're happy with around a 1/2-1" error, you can get away with only one correction most of the time.

Laying out by 3:4:5 is, IMHO, more accurate (about 1/4-1/2" error) and quicker. Especially if you already have a string knotted to 3:4:5 proportions... As Silent said, once you've squared the ends the only measurements are marking off their lengths from the wall. Checking the parallel length and the diagonals is only a double-check, but I still recommend it. Measure twice, peg once as it were.

Still, Ivan is right: to each his own.

[silentC]

1200 x 1500 is a pretty small area. I did a 1200x1200 slab last week. I just made up the boxing, dropped it in position, got one side parallel to the wall (it's set out from the wall by 1200) and adjusted the diagonals. I definitely wouldn't bother with 3,4,5 for that.

5000 x 7000 is a much bigger area and farting around with string lines and diagonals of that proportion by yourself is a different story. If you had a couple of helpers, you could get them to hold the ends of the string line while you run back and forth checking diagonals and moving things left and right I suppose

Time is of the essence