The Art of Levelling
A judge once asked, "What is truth?" and then pronounced a man innocent before sentencing him to death. Outrageous, but it happened and sentence was carried out.
We have to measure by what is true. If we are not sure about what is true then we are likely to make mistakes. If we are sure about what is true and then fail to act accordingly, we are either dishonest or foolish.
It is the same on the job site when it comes to levelling, we want to know what is true and then to use that knowledge to correct deviations in our work.
Our world changes. The thin crust of earth upon which we build is constantly moving and despite our best endeavours, what we build is affected to some degree by these movements, sometimes violently but certainly and inevitably. Consequently what might have been a true line when a building was constructed may be far from true just a few years later. Ground heave, for instance, is a familiar risk issue in building insurance schedules.
The components we build into structures are not usually of such precision that adjustments are unnecessary as we fit them together. Moreover, the very best bricklayer, joiner and engineer whoever he or she may be, is fallible and must be ready to make corrective adjustments due to marginal but cumulative errors in process of construction. During the building process therefore, we are forced to constantly check and re-check that alignment and levels are correct. There is nothing new in this, it is an age-old challenge, but the quality of our workmanship is under scrutiny today more than ever and those who pay close attention to such details are winners in an increasingly demanding and competitive marketplace.
The purpose of this report is to consider the means we use to measure 'true', the methods we adopt to make adjustments, and to help evaluate traditional and modern means of levelling.
True - The Reference
Still in use are some of the oldest references for vertical and horizontal alignment such as the plumbline and stretchline. As gravity and the straightest line between two points are constant enough physical and geometric references there is little to say against their ongoing use. There are however some practical drawbacks associated with both. The plumbline must be settled into its final hanging position and for a long line, even on a calm day this may take some time - not a cheap commodity in the modern commercial world. Similarly, the stretchline is vulnerable and hazardous on a busy site and can be a nuisance. It is of little use for levelling as no matter how tightly it is drawn, there is an inevitable sag along its length and the longer the line the more pronounced this is.
Line of sight instruments may be necessary tools for the surveyor and site engineer, but are of less practical use to the builder as a handy reference.
Enter the trusty spirit level. There is not a tradesman worth his salt who is without one of these in his toolbag. This ingenious yet simple device is certainly the most widely used instrument for localised levelling. Available in a variety of sizes and configurations, it can be used effectively for two or three-dimensional levelling provided that the manufacturer has produced a good quality instrument in which the vial is precisely set in its housing and properly calibrated. For larger jobs, a long spirit level is more accurate because it 'samples' a greater length. Whether small or large, however, much depends on the uniformity of the surface to be measured. For instance, a warped timber can wreak havoc when even the most accurate levels are applied. There are other limitations too, especially when it comes to levelling longer spans.
Modern technology has given us the laser diode, a high intensity light emitting electronic device used to project a very narrow beam of light. In proportion to the power rating of the device, the beam may be projected over very large distances; indeed lasers have been used to accurately measure the distance of the moon from the earth because the divergence of the light beam is so insignificantly small. High intensity laser light is also used extensively for cleanly cutting and welding metal sheet but on the other hand, can be so preciselY controlled as to perform delicate ophthalmic surgery. Relatively low intensity laser light can be projected over tens of metres and by incorporating an emitting diode into a spirit levelled housing, it provides an unobtrusive yet accurate alignment reference. Notwithstanding this high-tech approach, lasers are now readily available and need not be an expensive option.
Both spirit and laser levels must endure the hardships of life in a particularly tough environment. They are dropped, kicked, trodden on, thrown around and subjected to wide temperature and humidity excursions. The building site is not an ideal place for a precision instrument and builders are not renowned for delicate handling. Still that precision must be carefully preserved. Regular checks for calibration and corrective adjustments are vital. The better products can be readily maintained, service schedules and instructions being provided by the manufacturer. Some require 'return to base' maintenance but the cheaper instruments cannot be recalibrated and so must be replaced. This may be acceptable if replacement costs are low but by the same token, the accuracy of a cheap instrument may be wanting from the outset.
The easiest and simplest check is to reverse the instrument. For instance, place your spirit level on a surface - any flat surface, it does not need to be level. Note the bubble position and then turn the instrument end to end through 180° and check that the bubble is giving the same reading. If it is not, it is not worth using because any inaccuracy in the level will be multiplied many times in your work and the resultant costs to you will make the cost of recalibration or replacement look insignificant. Your work can only be as good as your reference level, so recalibrate regularly or replace it if necessary.
Similarly, there are easy ways of checking a laser, setting level in one direction should be verifiable from the opposite direction. However, since these instruments take different forms, you need to refer to the manufacturer's recommendations.
No matter which type of level you like, look for accuracy, ruggedness and serviceability. Aluminium box-beam spirit levels with milled acrylic vials and accuracy better than 1mm/m (or 0.05°) are a good option especially if from a reputable manufacturer offering after sales service. Most laser levels incorporate levelling vials for setting up and some can be used in either mode. Some instruments even have digital displays so you don't have to rely on your own judgement when centring the bubble.
One more word about laser levels. It is now common for several beams to be projected from the same instrument being factory set at right angles to each other so as to set level, square and plumb at the same time, a very useful additional feature. What is more, you can rotate this combination so as to set square in any plane.
Getting it Right - Levelling Methods
Having chosen and set our reference, we must decide how we will achieve the adjustments required. In fact it is prudent to include a system of levelling at the planning stage, as it may be difficult or impossible to employ after a component has been positioned.
Certain items such as kitchen appliances come complete with level adjusters in the form of screw down legs, very heavy structures require mechanical or hydraulic jacks, but for most standard building, flooring and roofing members, other means must be sought. Since levelling involves either raising or lowering, the easier option once a timber, for instance, is in place is to raise one end rather than lower the other. This involves ‘packing’ with a suitable material of the required thickness. This packing must endure a high concentration of forces. The levelled member may well be designed to evenly distribute load but the applied packing material used to level it can be subjected to localised stresses far beyond its capacity to bear.
The decision taken at this stage is an important one and will have a long term bearing upon the integrity of the structure not to say the reputation of the builder. The temptation is to look around for scraps of waste material with the mind set on its dimensions rather than its suitability as material. Let's examine the types of waste to hand at the building site.
Pieces of brick, stone, tile or slate. These are certainly durable materials however; they do not readily lend themselves to preparation for the packing job in hand. Cutting to size is not easy and can leave an irregular surface causing high stress points on the underside of a component. Load variation and vibration can then cause abrasion, weakening and even failure. If the component we are levelling is timber, it may need to be nailed into position through the packing so rendering these materials unsuitable.
In its soft state mortar overcomes the problems associated with sizing, but must be set hard before it can be used under load and is still less suitable for nailing.
Wood is probably the most widely used material for packing, but understand this; woods vary widely in density and are far from ideal for our purpose. The level they are used to set can quickly change as a result of compression, shrinkage or even rot. Nailing can cause splitting and cracking. Then there is the time consuming preparation. Some builders and joiners actually set aside time to prepare wooden wedges for a day's work. Wood may come cheep but time doesn't.
UPVC off cuts are another alternative but again involve time forpreparation.
Cigarette packets, folded paper, WHAT?!
"But we've always done it that way!" I hear someone say, "Why change?" To him I answer, put yourself in the place of the purchaser. How comforted would he be to know that bits of largely unsuitable materials were used to level his floor or position his windows? If you were purchasing a car what would you think of the manufacturer who used a cable tie instead of a hose clip?, or filled the battery with tap water instead of distilled water?, or used a rag instead of an oil-cap? You might not know about it at the time of the purchase but the deficiency would show up sooner or later, and how is the manufacturers image and long term credibility served by bodging in this way?
A colleague of mine recently came across a firm of replacement window installers whose main business was fitting UPVC frames, which presumably had been sold for their maintenance benefits, and durability compared with wood. The fitters explained how they aligned and anchored the new frames by using pieces of the old wooden frames that they had removed. My friend was left wondering whether the wood that was considered no longer suitable as a frame and was already showing signs of weakness, was any more suitable in its new role. The installation of window and doorframes requires particular attention for safety and security. Why risk danger for the sake of inadequate materials?
To jeopardise quality and safety is not sound economic sense. To cost-cut may in fact be a short-cut to disaster for you, your business, your product and your customer. Is it really worth the risk when suitable materials and components are now available?
Consider a moulded system of wedges of material consistent in strength, stability and long term security, which can be nailed through without risk, are moisture proof and so cannot rot or shrink, each one needing no preparation but ready for use and at low cost. Consider as two such wedges are folded together and then driven in opposition - gradually increasing in combined thickness, presenting two parallel surfaces for even load distribution and interlocking so as not to slip apart. If you can be confident that they are environmentally friendly even when burned, you have an economical, yet convenient and technically excellent solution for levelling.
An alternative approach is to use sets of moulded shims or packers of similar material. These are thin, parallel sided (blocks or slides) of various thicknesses which are combined to achieve a required overall packing thickness. Fine if you are able to lift the component to be levelled sufficiently to insert them, otherwise the two-wedge system is hard to beat.
Often an area of flooring or the span of a beam is such that it is needs to be supported at several points along its length. Naturally, the greater the load it carries, the closer together should these supports be. In this case you should know the load bearing capability of the packing you propose to use so you can calculate the correct pitch between supports. More reason to avoid bits of waste!
Keeping it Right - Permanence
Apart from the many internal levelling jobs, there are structures that will remain outside and exposed to the weather. Here the packing material has to be especially durable. In recent years for instance, decking has become popular for use in gardens and on flat roofs and patios. Good quality decking panels are made of treated wood so it may seem reasonable to level them with wooden packing. However, wood tends to swell when wet and shrink and crack when dried. The daily temperature cycle accelerates those processes and all of them are constantly in play outdoors. If you want your well levelled decking to stay that way, you need packing that is dimensionally stable. A carefully laid stone or brick plinth could be the answer but remember that subsequent levelling will almost certainly be needed anyway as you lay the decking. Since the decking itself may succumb to the weathering processes, further adjustments might be needed later, so packing that is adjustable is a definite bonus in this case.
Foot bridges and other raised walkways exposed to the elements are in equal or greater need of stable packing if levelling is required.
Unfortunately, the ravages of time affect us all and the buildings we erect. Ground movement, storm damage, rot, infestation, wear and tare and eventually the time comes to do repairs. Many companies now specialise in building refurbishment and remedial treatment and no wonder when conservation is such a hot potato politically as well as domestically. These days people are seeking out old properties for their charm and character. Moreover, preservation is high on the agenda of council planning offices.So this is a real growth industry.
Remedial work often involves levelling and requires materials of long term durability enabling these specialist firms to confidently guarantee their work.
Many old buildings simply cannot be levelled to true horizontal without major reconstruction. As long as the authorities after surveying, consider the building to be structurally safe, they may not insistthat every component be level. It is now more a question of alignment in keeping with the geometry that prevails. Here the spirit level is not so suitable a reference (though the laser level might well be) and the whole alignment may be a matter of judgement for the builder. Adjustability is therefore even more desirable particularly before final fixing. This calls for adjustable packing that can be easily ‘tweaked’ but will not slip out before he sets the final position. Then when it is set, he wants it to stay that way in the long term.
Level then, is measured against a standard reference, as is length and weight, and is only as true as the reference itself.
Packing for levelling is not the matter of least concern to the professional builder but vitally important componentry. It must be strong, stable, durable, reliable, bearing a key and permanent role in the integral relation between the parts comprising the whole structure.
In fact, levelling is a science not an art.
J G Stacey, B Eng. FInstSMM.