In the Construction industry, the terms level and plumb are used to describe the basic positions of important parts of a construction project. For example, the floor of a building must be level and walls that are perpendicular to the floor are plumb.
A level line or surface is parallel to the surface of still water. In other words it is perfectly horizontal.
A plumb line or surface makes a right angle with a horizontal surface. In other words it is perfectly vertical.
On the construction site you could use a number of different tools and instruments to check or set levels and plumb positions. Many levelling tasks start from a reference point called the datum (plural data).
A datum can be any known point, line or level from which a level line can be transferred to another position. The elevation or height of a datum point can be recorded and then used as a reference during construction.
In this chapter we will talk about:
It might be a good idea at this stage to have another look at the chapter titled Planning Your Work. You’ll find the procedures outlined in that chapter helpful when you are planning and preparing for any task on the construction site. In summary, to plan and prepare for a levelling task you need to:
A spirit level will be a very important part of your tool kit on the construction site. You will use a spirit level to check surfaces, framing members and formwork for level and plumb.
Spirit Levels are generally made from aluminium and are available in lengths up to 1200mm. A handy size for general construction work is 800mm. The photograph below shows a typical aluminium spirit level.
Clear tubes which contain liquid and an air bubble are positioned in the centre and near the ends of the spirit level. The photograph below shows the clear tube in the centre of the spirit level. When the bubble is exactly between the two guide lines the edges of the spirit level are in the horizontal position. In other words, they are level.
When the spirit level is placed in the vertical position the reading is taken from one of the end tubes.
When the bubble is exactly between the two guide lines the edges of the spirit level are plumb.
The accuracy of your work will depend on the accuracy of your spirit level. When you are buying a spirit level, check that it is accurate. You should also check it regularly on the job. To check a spirit level you simply take a reading off a surface, then reverse the spirit level in exactly the same position and take another reading.
The surface you use to check the spirit level doesn’t need to be perfectly level. As long as the reversed readings are exactly the same the spirit level is accurate. For example, if both readings look like the photograph on the right, the spirit level is accurate. Spirit levels should always be checked in both the level and plumb positions.
Some spirit levels have adjustment screws that allow you to move the glass tube a little to give a correct reading. However, many modern spirit levels like the one shown in the photographs don’t have adjustment screws. If a spirit level is not accurate and cannot be adjusted it should not be used.
You can use the following procedure to adjust a spirit level:
The illustration below shows the spirit level resting on its support screws with the end against a stop (nail).
On the construction site you will need to know how to use different levelling devices so you can choose the most suitable one for your task. The photographs which follow illustrate some of the tasks you can perform with a spirit level.
You can use a spirit level for checking, setting and transferring levels over a short distance or where you can place the spirit level on the member to be levelled.
In the first photograph the worker is levelling steel formwork. He’ll pack the form until the spirit level reading is correct and make sure that the form is securely pegged so it doesn’t move when the concrete is placed.
The photograph below shows the construction worker using his spirit level for a different kind of levelling task. Now that he has levelled the form he needs to check its height. The concrete path is to have 20mm fall so rain water will run away from the wall of the house.
The chalk line on the edge of the house floor slab shows the finished level of the concrete path against the house.
The worker is using his spirit level to transfer the level of the chalk line to the front of the path so he can check that the top of the form is 20mm below the level of the chalk line.
How would you check the fall if the spirit level was shorter than the width of the path in the previous example? The simplest way would be to place the spirit level on a straightedge that is long enough to reach across the path.
Lining A Door Opening
This photograph shows a carpenter using a spirit level and straightedge to plumb the jambs in an opening for sliding wardrobe doors.
The jambs must be both straight and plumb. He is using the straightedge to check that the jamb is straight and the spirit level to make sure it is plumb.
He has tacked the jamb in place and will use the chisel he is holding to lever the jamb out to the straightedge.
The carpenter will then pack the back of the jamb so he can nail it off before the architraves are fitted.
The photograph below shows the carpenter levelling and straightening the head of the wardrobe opening to take the tracks for the sliding doors.
If the jambs are perfectly plumb and the head is perfectly level, the corners will be square and the sliding doors will fit neatly in the opening.
Standing Wall Frames
The photograph below shows carpenters standing wall frames on a concrete floor slab.
The carpenter on the left is using a spirit level and straightedge to check that the side wall frame is plumb.
The carpenter on the right is about to nail the temporary brace that will hold the wall frame in the plumb position.
Before leaving this page, think of what you've just been reading, and test yourself with these questions.
[[ mc /f ][ How do you check that your spirit level is accurate? ][ * Place it on a level table and check the bubble position. ][ Compare two readings with the level turned around for the second. ][ * Compare the spirit level with another spirit level. ][ * Send it back to the manufacturer for recalibration. ][ Comparing two readings of the same surface, in opposite directions, will allow you to verify the accuracy of your spirit level. ]]
[[ so /f ][ Place these steps for adjusting a spirit level in the correct order: ][ Support the level at each end. ][ Adjust one support until the bubble indicates level. ][ Reverse the level level and note the off-centre distance of the bubble. ][ Adjust the level until the bubble is half-way back to centre. ][ Re-adjust a support to again centre the bubble. ][ Reverse the level again and check that the bubble now gives a level reading. ][ Repeat the procedure again if the bubble is still not truly centred. ][ Support at each end => Adjust one support until level indicated => Reverse the level and note the off-centre distance => Adjust until the bubble is half-way back => Re-adjust supports to centre the bubble => Reverse and check that the bubble is now level => Repeat the procedure again. ]]
The photograph below shows a laser level mounted on a dome head tripod.
Laser levels, sometimes called electronic levels, send out a signal in a full circle (360°) from the laser head.
The signal travels in a horizontal plane. This means that any point where the signal can be detected must be level with the laser head.
The signal is detected by a box called the receiver. The receiver is attached to a staff so that it can be moved up and down until it is in the horizontal plane of the laser signal. When the laser signal is found by the receiver it makes a loud beeping sound. Other types of laser levels emit a visible coloured light to show the level.
The photograph below shows a construction worker securing the laser level to a dome head tripod.
When you are setting up a laser level, make sure you place the tripod in a working location where it won’t be in the way.
Plant the tripod legs firmly in the ground and spread them wide enough so the level will be stable.
Even though the laser signal is self-levelling you still need to get the instrument as level as possible before you turn on the signal.
The photograph on the previous page shows the circle bubble that is used to level the instrument.
This is like the bubble in a spirit level except that the bubble has to be inside a circle instead of between two lines when the instrument is level.
You can move the instrument around on the dome head of the tripod to get it reasonably level before you tighten the set screw to clamp the level on the tripod.
If your tripod doesn’t have a dome head make sure its top platform is reasonably level when you set the legs of the tripod firmly in the ground.
The photograph below shows the three adjustment points, often called the trivet stage, that are used to centre the circle bubble and level the instrument.
Make sure you look at the circular level from directly above as you adjust the trivet stage (to avoid parallax error).
Ask your teacher about the problem of parallax error in measurement.
The photograph on the right shows the construction worker turning the adjustment screws of the trivet stage to level the instrument.
When you have completed these simple procedures the laser level is set up and ready to be switched on.
Powering Up The Laser
Laser levels are battery powered. Some have rechargeable batteries, others have batteries that need to be replaced. The receiver usually has a small 9 volt battery.
The photograph below shows the power button being pressed to start the laser signal.
The laser level is now sending out a continuous signal in a horizontal plane. The plane of the signal is levelled electronically by the laser level itself.
The photograph below shows the construction worker taking a reading with the laser level.
Make sure you always hold the receiver staff in the plumb position when you are taking a reading. Check the circle bubble which is usually on the receiver or it may be attached to the staff.
When the receiver beeps, very carefully lock the receiver onto the staff. In this example the position of the receiver on the staff measures the height of the laser signal above the surface of the concrete.
The surface of the existing concrete becomes the datum or reference point for any levels required for the job.
The photograph on the right shows receiver and staff. Staff graduations are in units of 10mm. The numbers represent 100s of millimetres.
The worker has shot the level so he can prepare the site and erect the formwork for a concrete slab at the back of the house.
The photograph below shows the level of the existing concrete, or datum, being transferred to another point.
When the receiver beeps, the bottom end of the staff is level with the surface of the concrete (datum) which can be seen in the left of the picture. The worker marked this point then drove a nail into the retaining wall plank.
In the photograph the staff is resting on the nail while the worker checks the level. If the nail is in the correct position the receiver will beep.
The laser level is quick and easy to set up. However, its main advantage over other levelling instruments is that it can be operated by one person. Other instruments such as the automatic level require one person to sight the level and another person to hold the staff.
Before leaving this page, think of what you've just been reading, and test yourself with these questions.
[[ so /f ][ Place these steps for setting up a laser level into the proper order: ][ Choose a location that won't be in the way. ][ Plant the tripod legs firmly in the ground. ][ Mount the instrument on top of the tripod. ][ Level the instrument by adjusting the trivet stage. ][ Power up the laser transmitter. ][ Choose a location => Plant the tripod legs firmly => Mount the instrument => Level the instrument => Power up the transmitter. ]]
[[ mc /f ][ What is the main advantage of using a laser level? ][ * It's the most accurate way to find levels. ][ * It's quick and easy to set up. ][ * You can always see the laser light. ][ It can be operated by one person. ][ It's main advantage is that it's a one-man operation. ]]
The automatic level is a type of optical level, which means you look through a telescope at a magnified view of the staff. The photographs show the main parts and adjustments of a typical automatic level.
Procedures for setting up the tripod, securing the instrument to the tripod and levelling it with the circle bubble are the same as for the laser level. Make sure a tripod leg won’t be in your way when you are sighting.
When you have finished mounting the automatic level on the tripod you will need to:
When you look into the eyepiece of the level you will see cross hairs like the illustration on the right.
The automatic level contains a pendulum device that allows the line of sight to level itself automatically. When you look in the eyepiece, the line of sight is at the intersection of the cross hairs.
When the automatic level is lined up correctly and focussed on the staff, the image you see through the eyepiece should look like the illustration on the right.
The vertical cross hair is lined up in the centre of the staff and the horizontal cross hair gives the reading on the staff.
Imagine you have set up an automatic level to transfer the level of the existing concrete as in the example on pages 181 and 182. The tripod should be positioned so the sighting distance is about the same to both points. This helps to reduce errors.
When you shoot the level of the concrete (datum), make sure your helper has the staff plumb. Let’s say the reading on the staff is 820mm. Double check the reading and write it down. Your helper can now move the staff to the new position.
The staff should be placed on something solid like the top of the retaining wall. Sight the level on the staff and check the level’s circle bubble. Make sure the staff is plum and then take the reading.
Let’s say your reading is 750mm. double check the reading and write it down. The difference between the two readings is 70mm (820 − 750 = 70). This means that the level of the concrete (datum) is 70mm below the top of the retaining wall at that point.
Measure down 70mm from the top of the retaining wall and clearly mark the transferred level. This point should be level with the datum.
It’s a good idea to drive a nail in at that point, place the staff on the nail and shoot the level again to make sure it’s the same as the datum.
You might sometimes find that older types of optical levels are still in use on the construction site. Older types such as the tilting level don’t automatically level the line of sight. Basic setup using the circle bubble is the same as for the automatic level, but you have to level the line of sight manually using a small tubular spirit level for every level you shoot.
If you are handling the staff when levels are being shot, make sure you:
Before leaving this page, think of what you've just been reading, and test yourself with these questions.
[[ so /f ][ Place these steps for using an automatic optical level in their correct order: ][ Your assistant holds the staff plumb on the surface height you want to transfer. ][ Rotate the telescope towards the staff. ][ Look through the eyepiece. ][ Adjust the focus to get a clear shot of the staff. ][ Note the reading from the crosshairs. ][ Your assistant moves the staff to the new location. ][ Take a second reading from the staff. ][ The difference between the readings provides the height info for the new location. ][ Hold the staff plumb at location 1 => Rotate the telescope towards the staff => Look through the eyepiece => Adjust the focus => Note the reading => Hold the staff at location 2 => Take a second reading => The difference provides the height info. ]]
The accuracy of levelling instruments should be checked regularly. Optical levels generally have adjustment screws for the circle bubble and the line of sight. You will need to follow the manufacturer’s procedure very carefully if you have to adjust a levelling instrument. Laser levels generally have to be adjusted by an authorised service agent.
You can check the accuracy of levelling instruments by doing a two peg test. The diagrams on the next page show that you take readings at two different locations (pegs) shooting from two different positions and then compare heights.
The illustration below shows a level set up between two pegs. The reading at peg A is 0·910 and at peg B 0·750. The difference between the heights of the pegs is 0·160.
(0·910 − 0·750 = 0·160)
If the instrument is accurate, the difference of readings shot from another position will still be 0·160 even though the heights are different. If the instrument is not accurate the difference between the new readings could be more or less than 0·160.
The illustration above shows that the difference between the readings taken from position 2 is still 0·160.
1·010 − 0·850 = 0·160
This means that the level is accurate. If the difference of the readings taken from position 2 was not 0·160 the level would need to be adjusted and tested again. The illustration below shows a typical setup for a two peg test.
Before leaving this page, think of what you've just been reading, and test yourself with these questions.
[[ so /f ][ Place these steps for checking the accuracy of your levelling instrument in order: ][ Drive two pegs into the ground at different heights. ][ Place the levelling instrument between the pegs. ][ Determine the height difference between the two pegs. ][ Move the levelling instrument to a significantly different location. ][ Determine the height difference a second time. ][ If the measurements differ, the instrument may need to be recalibrated. ][ Drive two pegs => Place the instrument between the pegs => Determine the height difference => Move the levelling instrument => Determine the difference again => If measurements differ ... ]]
The water level is the simplest of all levelling devices and is the only one that doesn’t need adjustment from time to time. It consists of a length of clear plastic tube almost filled with water.
When you use a water level you need a helper to hold the other end of the tube. Make sure the tube is free of air bubbles and hold the ends upright at the points where you want to mark the levels.
The water will find its own level at each end of the tube. You can adjust the height by raising or lowering the ends of the tube until the water level is at the height you want, or simply mark a level and measure to the required height.
An Example
Imagine you have been given the job of erecting a pair of gates between a fence post and a brick wall. While preparing for the task you determine that you’ll need to transfer the level of the gate hinges from the post to the wall.
You find that your straightedge isn’t long enough so you can’t use a spirit level and the boss’s laser level isn’t on the site. So what do you do? You found a length of plastic tube in the shed and set up the water level shown in the illustration below.
The line level is a small levelling device that you can hook over a taut string line. The line level must be in the centre of the length of the string line so your reading can be as accurate as possible.
Never use a line level where really accurate checking or recording of levels is required. It should only be used for approximate levels.
A common example is checking the level of string lines on a site setout to make sure measurements are made accurately.
A plumb bob is a weight made from brass or steel which generally weighs about 0·5kg. Plumb bobs come in a variety of shapes. A typical shape is shown in the illustration on the right.
When the plumb bob is suspended on a string line, the string line is plumb. The plumb bob and string line can be used when you need to plumb a point down to the ground.
For example, a tripod sometimes needs to be set up exactly over a survey peg in the ground.
A plumb bob can be used as shown in the illustration on the left. This could be necessary in a site survey when measurements have to be taken from the centre of the peg.
You can drive a nail into the centre of the peg and use a tape measure to make your measurements.
Look back at page 53 for information about the types of tape measures you would use for site measurement.
Before leaving this page, think of what you've just been reading, and test yourself with these questions.
[[ mc /f ][ What is the principle behind the use of a water level? ][ Water levels at both ends of a tube will always be at the same height. ][ * Water can flow around corners and over hills. ][ * Water is colourless, odourless and environmentally friendly. ][ * Differences in air pressure guarantee that the water is level. ][ The surface height of a contiguous body of water is the same everywhere because of the uniformity of local air pressure. ]]
[[ mc /f ][ What is meant when we say that a string is "plumb"? ][ The string is vertical. ][ * The string is straight. ][ * The string is very taut. ][ * The string is laid out horizontal. ][ "Plumb" means that something is vertical. ]]
When you have learned to use a levelling device for a particular construction task, you can apply the same skills to other tasks. For example, if you used a spirit level to check the fall of concrete formwork (see page 177) you could use the same method to check the fall of drainage pipes like the ones in the photograph on the right.
Also a laser level or automatic level could be used to measure the difference in height, or the fall, over the full length of the drainage pipes.
The photographs show how profiles (offsets) and pegs are used for marking the corners and setting out the shape of a building. You would apply basic levelling methods when erecting the profiles.
Always remember your work isn’t finished when the levelling or construction task is completed. It is your responsibility to clean up your work area.
The work area must be cleared so that materials, tools and equipment won’t be in the way or cause a safety hazard: