String line and Plumb Bob

On this page.
Using a string line. | Using a line with an offset | Plumb Bobs | Transferring a vertical point or edge. | A Digression into medieval building techniques
The interior of salisbury cathedral
String Line: Interior of Salisbury Cathedral. Built entirely with simple hand tools.

If you have come here without seeing my hand tools page then check out the intro to string lines first.

So, you have had a good look at the photo above and you are probably wondering what it has it got to do with the subject? . Why take up so much space anyway?

I was coming to the end of a long job a while ago, and the owner brought inside his latest toy. A small battery operated laser level, complete with tripod, "cheap at $350". It could project a red beam of light either horizontally or vertically onto the surfaces of his walls, when set up indoors.

I asked John what he was going to use it for, expecting something esoteric to say the least. "Levelling my kitchen cupboards" he said.

To the best of my knowledge his kitchen cupboards look no different after using the laser than they would if he had used a spirit level (which he already had). With his spirit level he would have had no set-up time, no dodging the tripod himself or with his power leads, no changing from horizontal to vertical, no charging batteries.

So what I am trying to say here is that when you are looking at all the wonderful new gizmos in the hardware store, resist the urge to buy, and consider if you can do it with something simpler and cheaper.

The methods on this page have been used for centuries. They didn't have laser levels and computerised theodolites when they built the Gothic cathedrals in Europe like the magnificent example shown on this page, built nine hundred years ago.

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String Line

A line is described by two points, the start point and the end point, and the shortest distance between two points is a straight line.

A carpenter or a mason uses a string line between two points (nails or line blocks etc.) and he pulls the line tight to get it the shortest distance, to make it straight, with no sag.

A carpenter can get accurate readings on a string line, by using three small packers, all the same thickness, say 20mm. He stretches his line between two end points that are fixed first. Then he puts a packer under each end to give the line clearance.

string line on floor joists
String line -String line between two floor joists.
  • In the sketch above, he has fixed two floor joists 'A' and 'B' and put a 20mm packer under his string line at each end.
  • Then he uses his third packer as a gauge to check the height of the rest of the joists. At 'C' he can see that the joist is too high.
  • This is a basic method, used on walls, ceilings, formwork etc. If you don't use the packers, there is a danger of each joist lifting the line a small amount and you finish up with a curve.
  • In a long run of string you may get sag in the line, pick a point about the centre and pack it up until when you sight along the line from one end it looks straight.
  • A good string line should have a fair amount of stretch in it.

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Using a line with an offset

string line on wall
String line - Offset line around obstacles.

  • In the sketch above I show another method of using the line with an offset.
  • I want to mark out, on my profile (to the left), a building line for a new extension, that is exactly a straight continuation of the main house wall.
  • I just pick a distance from the house that gets me clear of the various obstructions. In this case it is 145mm.
  • I make sure the string line is offset the same distance from the house,and then I put a nail or a screw into my profile at the continuation of that line.
  • I then measure back the offset distance to get a point exactly in line with the wall.
  • I bang a nail into a brick joint or a peg touching the wall, and string a line, shown dashed, to fix the true building line.

One a similar vein, sometimes when you have to get machinery into a job, rather than pull out the profiles that are in his way, first set up some offset or recovery profiles. Say you set the recovery profile 6M away from the real one, then it is a case of a simple 6M measurement offset to get back to the original set out, rather than starting from scratch again, squaring etc.

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Plumb Bob, Plumb levels

When a builder refers to something as being plumb, or out of plumb he is not talking about anything to do with the trade of a plumber. Plumb derives from the Latin word for lead, plumbum. (pb.)and the practice of finding true vertical by hanging a lead weight.
Plumber derives from the same source, because plumbers were the people who worked with lead. Lead roofing and lead water pipes etc.

brickies plumb level.
String line - Home made brickie's or mason's plumb level

I can remember making one of these plumb levels for a brickie when I was about sixteen. (He must have been an old die hard not into the new stuff). It is just a straight piece of timber with a hole cut in the bottom where a brass plumb bob swings. Usually about 1200 long (4 ft.)

To use it you rock it slowly towards you until the bob swings free and back slowly until it rests again. Do this a couple of times and it steadies the swing and gives a true reading quite quickly.

I am not saying let's start using these again, no way. But what I am saying is that sometimes old methods are quite adequate for one off jobs. I have made up and used timber plumb levels on a few occasions.

When I was fixing column formwork it was common for one guy to climb say three metres up a form, nail a stick of wood on the top, hang a plumb bob off the stick and measure into the ply from the line and say "183". His mate would measure the bottom and say something like "197". "that's er.......14 this way". Then invariably "Hang on......, not that way, the other way" and he'd back off and then push the opposite prop until they had it right.

sketch of an old mason's level
String line - Using a plumb line to get horizontal levels.

I made up a plumb level with a couple of bits of timber screwed to it (shown as dotted lines in sketch), that made it a lot easier, no climbing. The reason for all the messing about with plumb bobs etc. was that the formwork was never exact on the outside where you could use a spirit level , so a spirit level would give a wrong reading. There were odd sizes of column clamps, rough sawn or bent timber, etc.

I once also made up something similar for concrete walls that were narrower at the top than the bottom.

At the risk of being called an old fart, here is a sketch of a mason's level that would have been standard equipment on any of the old cathedrals. It is the same as above with a cross piece added. Simple technology used for creating incredibly complex work.

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Transferring a vertical point or edge.

plumb bob using
String line - Finding a vertical point with a plumb bob.

Let's say that I have a first floor landing that I want to plumb down to the floor below so that I can start measuring up and setting out a flight of stairs.

  • I drop my plumb bob from the top and as it touches the floor below I lift it up and down a couple of times, from touching the floor to swinging free. This steadies the swing.
  • Then when it has settled I slowly let it down into contact and let it lie on it's side.
  • A well designed bob may roll at this stage, but it will only roll about the tip, (usually a steel point).
  • When I come down from above I mark the floor where the tip rests.
  • I could repeat the process and get a straight line on the floor that is exactly under the edge above.
  • One man operation and it can be done over a reasonable height.

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Another Old Fart Digression

the stone spire of salisbury cathedral
String line - The stone spire of Salisbury cathedral.

I visited Salisbury Cathedral, many years ago and stood directly under the centre of the cathedral's spire. It is of partly hollow stone construction, and towered over me for 404 ft.

It was one of the tallest buildings in the world at the time it was built.

I couldn't see the insides of the spire, there are beautiful arched ceiling overhead, but I was blown away by the incredible upward reaching space around me, with the slender columns transforming into Gothic arched stone ceilings.

What made what I was looking up at almost inconceivable, was that I knew that the tower and spire was added in 1330, a generation after the main building was finished. All 6400 tonnes of it sitting on four stone columns. These sat on, by modern standards, poor foundations.

In 1668 Sir Christopher Wren surveyed the spire and found that it had taken on a twenty nine and a half inch lean. He designed stone bracing between the support columns and a series of iron tie rods etc. to fix it.

  • So how did he know that the spire was leaning?
  • You're in front of me eh? He used a plumb bob.
  • Directly under the centre of the spire, set into the stone floor paving is a small brass datum mark.
  • This mark would have been placed there nearly seven hundred years ago by the master masons.
  • At each stage of the construction as the scaffolding went higher a plumb bob would have been suspended, centred on that datum.
  • All the measurements for each stage of work would have been related to that vertical line.
  • When the cap stone of the spire was set in place, it would have had an iron ring bolt set into it exactly above the datum.
  • The maintenance staff would hang further plumb bobs from it over the centuries, to check that all was OK with the structure.
  • So when Wren checked with a plumb bob he found as I said that the centre of the spire was 29 1/2 inches away from the original datum.
  • He set another brass datum into the stone floor, while he was doing his remedial work.
  • Three hundred and thirty nine years later, modern instruments prove that old Chris and his boys had stopped the lean. "Aye, he knew his onions did that lad".

In the case of the spire of Salisbury, the masons were fully aware of the problems presented by the structure they were going to build.
had to build the the stone skin very thin, to keep the weight as low as possible. They solved the problem of stability by building into the structure a series of timber struts and braces to give the slender structure stiffness to resist wind forces.

  • The struts and braces were held in position by the mason's scaffolding as the spire reached higher.
  • When the capstone was placed in position a series of iron bars was suspended from it and they were attached to the timber struts and braces.
  • As the scaffolding was removed the capstone above took the weight of these struts and braces and held them in position.
  • Just as important though, their weight added a firm hold down force to the capstone and upper masonry.
  • A bit like a modern photographer, who hangs a bag of sand from under the tip of his tripod to steady his camera.

All this was done by a workforce that apart from one or two, was totally illiterate. In a time before universities and professors of engineering. In a time before, dare I say it, $350 laser levels.

the interior bracing of salisbury spire
String Line: The timber bracing of the interior of Salisbury Cathedral spire.

So looking at this shot you may be prompted to ask how the heck they got the centre point with that lot in the way. Easy peasy, the rope at the side gives a clue. It is fixed to a pulley up near the top that allows the rope to hang between the obstacles.

  • Earlier in the page we saw a sketch that shoed how to offset a horizontal string line, and you do the same here by offsetting the top of the plumb-line by a distance (x) that will safely clear any obstructions. Then to find the true vertical line again apply that same offset (x), in the reverse direction at the bottom, to show the true centre line
  • We do this all the time when plumbing up wall formwork. We don't trust that the forms and clamps are accurate enough to use a spirit level, so we drop an offset plumb line down the outside to get clear of the extra gear and just measure ply to ply.
  • In the case above and also when plumbing columns it is necessary to do it at least twice withe separate lines, and best at right angles to each other.

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Please Note! The information on this site is offered as a guide only!  When we are talking about areas where building regulations or safety regulations could exist,the information here could be wrong for your area.  It could be out of date!  Regulations breed faster than rabbits!
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Copyright © Bill Bradley 2007-2012. All rights reserved.
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