Setting out and stringlines.

Just had a query regarding setting out. If you are setting out from say, an existing brick wall, and you measure out and make a stringline at the end of your extension parallel to the wall, when it comes to setting out the perpendicular stringlines at the edges you have to be careful to avoid creating a parallelogram instead of a square correct?

For example, if you set out one stringline angling to the right away from the house, and another angling away to the left, then when you come to measuring the diagonals you won't have a square when you make them the same...you will have a parallelogram correct? So to avoid this potential problem is it best to angle the perpendicular stringlines by eye in the same direction away from the house THEN square them up by measuring the diagonals?

As a secondary question, how do you get the stringline running parallel to the house to be perfectly parallel? I was working with someone and from memory we just measured out the distance from the existing bearers, drove stakes in the ground then set up hurdles, levelled them with a laser level and did the same on the other side. Then we ran a stringline between the hurdles parallel to the house. Is that correct?

CS,
if you can run a stringline along one of your end walls (one that's square to the one your deck will be parrallel to) then you can simply measure off this. If this is not possible then you'll need to use pythagorus (3,4,5) to square your initial line.

For your parrallel line to be strictly parrallel you'll need to make your measurements square to the building, pythagorus to the rescue again. I hated maths at school and had no idea I'd end up using it so often in daily life.:rolleyes:
Mick

Thanks, I'd thought about using pythagoras for the second part of my question, and yeah that would get you a square line off the wall to then measure off to your hurdles and run your parallel line from (or yeah running it along an existing wall a way then out to your extension length).

However does anyone have any idea about the first part of my question, ie avoiding a parallelogram?

Just remember that most houses are out of square to start with.

To be picky... squares and rectangles are parallelograms. :U

I assume you mean avoiding rhomboids (squashed rect's) & trapezoids (tapered rect's) but if you use Pythag to lay out both end lines before measuring off for your hurdles and ensure both diagonals are the same length before placing the hurdles, then out-of-square won't be a problem.

Just take your time and get it right.

Of course, if one end wall is going to be a continuation off an existing corner, you're often better off running a string line along that wall, even if it means that the result isn't properly squared. As Bricks said, many houses aren't squared and if you build an extension "properly" you end up with a dogleg in the wall, which looks ugly. A case of reality versus theory, I'm afraid. :shrug:

Technically a square is a rhombus too :) But a rectangle isn't...

But actually what he's talking about avoiding is a parallelogram that isn't square - the opposite sides are parallel. (A parallelogram is only a rhombus if all four sides are of equal length).

But all the jargon aside, perhaps if you post a picture of what you have now and what you're trying to lay out, someone might be able to give you a step by step on how to get there.

A rhombus is a squashed square.

A rhomboid is a squashed rectangle. :U

Technically, rhomboid is not all encompassing in this situation, because it does not cover square extensions, where as parallelogram does :p

A rhombus is a quadrilateral with four equal and parallel sides - a square is a rhombus with right angles, so it doesn't have to be squashed to be a rhombus, so we still have the same problem as you are trying to solve with parallelogram. In fact I don't know if there is a term for a quadrilateral with parallel sides and oblique angles - except rhomboid, which, as I said, doesn't cover parallelograms with 4 equal sides.

Damn I hated geometry at school, why are you making me remember all this?

Ok guys, I've posted a couple of pics here to help outline the question. In the first one, assuming that both those stringlines were at the same angle out from the wall then when I measure the diagonals they will be the same measurement, but it's obviously not a square.

If however, I set out the stringlines like the second picture, then I can bring them into a square by checking the diagonals and adjusting accordingly...this wouldn't happen in the first scenario. Considering that the initial measurement out from the wall is often just with a tape and a stake, is it a good idea to bear that in mind?

In that case, you have a fixed reference point, which is the rear wall of the house, so I would just use 3,4,5 to set both lines and then check the distance at both ends to make sure they're parallel.

Quote:

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Originally Posted by silentC

A rhombus is a quadrilateral with four equal and parallel sides

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If we're being technical...:D

Can you have a 2 dimensional geometric shape with 4 parallel sides? Rhombus and rhomboid shapes have opposite sides parallel.

A rhomboid is an oblique-angled parallelogram with only the opposite sides equal in length (and the opposite angles equal). A rhombus is an 'oblique-angled' equilateral parallelogram, so a square is not really a rhombus. And a rectangle is not really rhomboid.

Cheers,
Adam

No, a rhombus (according to my sources) is an equilateral parallelogram. The angles don't have to be oblique. So a square is a rhombus. A rectangle is not a rhomboid but it is a parallelogram.

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Can you have a 2 dimensional geometric shape with 4 parallel sides?

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Yeah yeah, I was taking a short cut because I assumed everyone would get that bit by now... :rolleyes: :)

"Of quadrilateral figures, a square is that which is both equilateral and right-angled; an oblong that which is right-angled but not equilateral; a rhombus that which is equilateral but not right-angled; and a rhomboid that which has its opposite sides and angles equal to one another but is neither equilateral nor right-angled. And let quadrilaterals other than these be called trapezia."

Euclid - Elements, Book I, Definition 22

:p

Quote:

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Originally Posted by ComboSquare

Ok guys, I've posted a couple of pics here to help outline the question. In the first one, assuming that both those stringlines were at the same angle out from the wall then when I measure the diagonals they will be the same measurement, but it's obviously not a square.

If however, I set out the stringlines like the second picture, then I can bring them into a square by checking the diagonals and adjusting accordingly...this wouldn't happen in the first scenario. Considering that the initial measurement out from the wall is often just with a tape and a stake, is it a good idea to bear that in mind?

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If you set out the end lines independantly using 3:4:5 then they will be square to the wall you're setting them out from, assuming that wall to be straight and not curved in the first place. Oh... and that you haven't made a mistake in calculating the 3:4:5 or are using a "stretchy" tape measure, of course!

It rather makes your problem moot. :shrug:

Google 'square rhombus' and find me a reference (other than something that is over 2000 years old) and I'll believe you :p

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

DJ
Super Moderator
Woodworking Australia's
WOODWORK FORUMS

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

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

Quote:

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Originally Posted by 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

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Aw, spoilsport!:rolleyes:

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

Quote:

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Originally Posted by 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

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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?

Quote:

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Originally Posted by 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

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Good to see someone answered the question:2tsup:

though you beet me to it:(

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.

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

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.

Quote:

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Originally Posted by 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.

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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.

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.

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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

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... :wink: 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. :U

Still, Ivan is right: to each his own.

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 :wink: