Below you will find questions of commonly asked slip-related questions. For further information about SlipAlert or the science of slipping then please contact us.

Questions about SlipAlert...

Slipping is a known major cause of injury in the UK and most developed countries. In the UK, the HSE and Local Authority Environmental Health Officers are realising that slip prevention is the key to reducing many different types of accident. Many accidents previously recorded as, for example, ‘put hand in machine’ or ‘became impaled on a knife’, are frequently demonstrated to have been initiated by slipping.

Under the Management of Health & Safety at Work Regulations an employer has a duty to assess all the factors which might affect the risk of injury to his employees. This very much includes the risk of slipping. It is thus an essential part of any professionally conducted risk assessment. You cannot assess the risk of slipping unless you can properly assess the slip resistance of the floor.

SlipAlert was designed by Dr Malcolm Bailey who was the Chairman of both the British Standards Committee and was the Head of the UK delegation to the European Standards Committee dealing with slipping and the measurement of slip resistance. In tests against the TRL Pendulum, SlipAlert has been proved to be a very acceptable substitute.

SlipAlert is the only machine which has been specifically designed to reproduce the characteristics of the hydro dynamic film created when a pedestrian slips. It correlates with the Pendulum, which has proven over many years experience to provide a reliable assessment of the slip resistance of flooring. The Pendulum has become the ‘Gold Standard’ of slip resistance testing in the UK.

SlipAlert is designed for quick and easy floor measurement. It is significantly easier to use than the Pendulum. The Pendulum is a very accurate scientific instrument and should be used by experienced professionals. To use the Pendulum, one needs to be an experienced operator and follow a strict protocol in order to obtain reliable results.

SlipAlert is significantly quicker to use than the Pendulum, ideal for measuring floors and testing large areas quickly and easily.

SlipAlert was tested at the Health & Safety Laboratory at Buxton. A paper giving their findings was read at the UK Ergonomics Society Conference at Hatfield in April 2005. It has also been tested by Richard Bowman at CSIRO in Australia and he too has published his findings. The results of all testing has been combined to show a 95% correlation with the Pendulum. 

Once you have carried out an initial assessment with SlipAlert you will know which of your floors might be classed as a potential problem and need to be looked at to see how they could be made safer. Floors can change their slip resistance due to two main factors: wear and cleaning (or lack of effective cleaning). Wear effects normally take place over months or even years, so monitoring may well be on a 3 or 6 monthly basis. However, cleaning effectiveness can change far more rapidly. In certain situations, such as food production industries with heavy contamination levels, it may be wise to check floors weekly or even daily.

No. It really is simple to operate. We provide a short set of instructions, and within minutes of removing SlipAlert from its carrycase, you will be able to start testing your floors.

We can provide three different slidersdepending on your requirements.

Four S slider

This is the same slider as used as a Standard on the Pendulum. If you are really concerned about getting a reading as near as possible to that which the Pendulum would give in a British Standard test, then this slider is the one to use. However, it does wear rapidly and needs to be correctly prepared before use and between test locations.

Durable SlipAlert slider (standard with every SlipAlert)

This slider is provided with the machine. It correlates well with the Standard Pendulum Test and it is far more durable. The correlation with the Pendulum test is not quite as precise as with the Four S slider, but it remains a very good measure of slipperiness. For most purposes it is perfectly satisfactory and does not need to be replaced as often as the Four S slider. In many respects it is more akin to a normal shoe heel than Four S.

TRL slider

This slider is used to simulate someone walking in soft rubber soles/heels like those found on trainers. It also simulates the bare foot situation so is useful for swimming pool surrounds, changing rooms and bathrooms.

In its normal mode of operation, SlipAlert should only be used on a horizontal surface. SlipAlert can be used on a sloping surface by measuring the angle of the incline with a simple mobile phone app and making pre-calculated adjustments to the operation of the machine. This allows you to judge whether that surface on that slope is safe using the same criteria as in a level/horizontal situation.

Questions about Physics

When you walk, you need friction between your shoes and the ground to give you the ability to move forwards. Without friction you would not be able to remain standing for very long, let alone walk. If at some stage the amount of friction that the ground-shoe contact can provide is less than you need, then you will slip over. It is thus important to know how much friction a particular surface will provide.

Work at the Building Research Establishment in the late 1950s discovered that in straight walking the average dynamic coefficient of friction required by the population as a whole was around 0.19. However, it did vary substantially from person to person, and they calculated that 1 person in 1 million would require a coefficient as high as 0.36.

If you include turning when walking, this 1 in 1 million requirement goes up to around 0.39. For this reason the value of 0.4 is usually regarded as the criterion for safety.

If a body of mass W rests on a plane horizontal surface and needs a force F to move it horizontally across the surface in a sliding mode then the coefficient of friction is numerically equal to F/W. It is usually given the symbol µ.

However, there are two coefficients:

i) The limiting coefficient of static friction. This is based on the force required to start the body moving.

ii) The dynamic coefficient of friction. This is based on the force required to keep the body moving at constant velocity across the surface.

With many combinations of materials, µ static > µ dynamic, but for rubber and plastics in particular, µ dynamic may increase with velocity and become greater than µ static.

There are essentially two components of friction that create the horizontal/lateral forces that occur in sliding.

The first is in interaction between the peaks and troughs inherent in the micro roughness and indeed macro roughness of the two sliding surfaces. These peaks are sometimes referred to as asperities. It is a form of interlocking of the surfaces which requires a particular level of force to overcome it.

The second component of friction is a molecular adhesion between the two surfaces. Whilst not a major component in very rough surfaces, it becomes the major component when the surfaces are smooth, and in particular when one surface is rubber or plastic. With a smooth surface such as glass and a very slightly damp, soft rubber (the slight dampness appears to enhance the adhesion) one can achieve far higher values of µ than can be obtained with even the roughest surfaces.

The coefficient of friction is dependent on both the material of the surface and that of the body which is sliding over it. One might argue that if both these were the same material then one would get a unique value for that material. Unfortunately, this is not generally true since it can depend on the degree of roughness of each of the surfaces.

With many hard, non-resilient materials, such as metals, ceramics and timber, µ is independent of the contact pressure over a relatively wide range. This unfortunately does not extend to any system where one or both surfaces are rubber or plastic. Depending on the hardness of the rubber/plastic one can get some degree of independence of contact pressure, but it is over a much restricted range of pressures.

Not only does the presence of a contaminant such as water or oil affect the value of µ, but also the manner in which it affects µ is very dependent upon factors such as the contact pressure, the size and shape of the contact area, the velocity of movement and the viscosity of the liquid. This makes the measurement of µ in wet conditions much more complex. The failure to understand this complexity has resulted in different models of slip test machine giving totally different test results for the same surface.

Dry contaminants such as dust, powders, etc., can well affect µ by various different mechanisms. These contaminants can act like tiny spheres and have a ball-bearing effect. They can effectively destroy the adhesion factor in the creation of friction forces. They can fill in the troughs of the roughness, thus changing the nature of the surface. It all depends on the dry contaminant itself and the nature of the two sliding surfaces.

Slip resistance is the dynamic coefficient of friction of a walking surface as determined in relation to a given contaminant and a given shoe heel material, as would be experienced by a slipping pedestrian. It should always be quoted in relation to the contaminant, e.g. water wet or dry/clean. If no heel material is quoted, it is generally assumed in the UK to be in relation to a standardised rubber known as Four S.

Slips generally take place either when the heel of the ‘front’ foot lands or as the sole of the ‘rear’ foot pushes off. The latter rarely leads to a fall, it is merely an annoyance to the pedestrian. It frequently happens when pushing a heavily loaded trolley in a supermarket.

The most dangerous slip occurs as the heel lands on the floor. At this precise moment, the foot/leg needs restraint to stop it moving forwards as it lands and if it does not find it, the foot/leg slips forwards uncontrollably. Indeed, unless the pedestrian is able to somehow stop the slip by stumbling, or by some other means, once the foot has moved about 100mm or even less, the geometry of the body (the angle of the legs) becomes such that more and more restraint is required. The slipping system thus becomes very rapidly out of control.

The ‘classic’ slip is for the leading leg to shoot forward, the rear leg buckles at the knee and that foot also shoots forward due to the increase in lateral loading on the foot caused by the forward bending of the knee. The body then falls vertically to the ground with the person usually landing on their bottom or the base of their spine. Usually they land almost exactly over the spot where their foot originally slipped.

Questions on Measurement

The most usual way to measure slip resistance is to measure the force required to move a slider horizontally across the surface. By knowing the weight of the slider, or the downwards force with which it is held in contact with the surface, the value of µ or slip resistance is numerically F/V where F is the measured horizontal force and is the vertical force.

Most machines, but not all, measure the horizontal force in some physical direct manner such as with a spring balance or by electrical means (e.g. a strain gauge bridge or a cantilever) or by other electronic means, some of which are quite complex. Some machines measure this force by indirect means, typically by seeing how it retards the slider assembly from an initial specific velocity. Both the UK’s TRL Pendulum and SlipAlert use this principle. Another group of machines either statically or dynamically apply a force to the slider at an angle to the vertical (α) which is gradually increased until slipping occurs. The vertical and horizontal components of the force arF cos α and F sin α, respectively, and hence µ=tan α. Like the Pendulum and SlipAlert, these machines have the advantage of not needing to measure the forces directly.

The simple answer is no. In dry conditions the differences tend to be relatively small, but significant differences can be found in wet readings.

The reason for this was discovered about 15 years ago by the UK’s Health & Safety Laboratory and the designer of SlipAlert, who were both working independently on the problem

The simple answer is that the film of water that becomes trapped between the slider and the surface being measured produces an uplift on the slider and reduces the effective friction between the slider and the surface.

Understanding precisely how a film of water produces uplift and how it is going to affect the machine is much more difficult. This is because the uplift depends on several factors, all of which need to be taken into account when designing the machine. What the machine designer has to do is to ensure that the combination of those factors on his machine produces exactly the same proportional uplift as a pedestrian experiences when their heel slips across the same surface.

As far as we are aware, the only machine that has been designed to react to the hydrodynamic film in the same way as a pedestrian’s heel reacts on the same surface is SlipAlert.

The TRL Pendulum was not designed to do so, but by pure chance does react correctly. It is for this reason that measurements using the Pendulum have been found from many years experience to correlate with actual slipping accidents.

As one would logically expect, SlipAlert therefore correlates with the Pendulum.

Unfortunately, because the matter is so complex, the answer is no. The problem is that two surfaces tested by machine A may seem identical, but the Pendulum and SlipAlert will indicate that they are quite different. Conversely, two surfaces that give very different readings on machine A may in fact have very similar slipping properties. There is simply no easy means of unravelling the complexity of the way that the uplift is produced and how it affects the final reading to make one machine correlate with another by use of a simple factor.

The slider on the machine is designed to replicate the heel of the pedestrian. If you are investigating an accident, it is helpful to use a material that is similar to the heel on the shoe worn by the pedestrian who slipped. In simply monitoring a floor or testing it to see how it is likely to behave in use there are two schools of thought.

One school of thought suggests using a standardised rubber such as Four S. This means that your results can be compared directly with someone else’s. Whilst this can be very useful in terms of standardisation, there are three problems with Four S. The first is that it does not wear well and needs to be reprepared and replaced quite frequently. The second is that in the wet it does need to be very carefully prepared to ensure it is quite smooth, otherwise one gets an occasional significantly different reading. The third is that it represents a relatively good heel in terms of its performance. This means that one cannot tell whether the floor will be satisfactory against the whole range of heel materials that are considered to be acceptable in respect of their frictional qualities. Ideally, the standard heel should be at the lower end of the range rather than towards the top end; as yet no one has put forward such a ‘standard’ slider material.

The second school of thought is to test the floor using a range of heel materials. This can be quite illuminating, but because there are only two rubbers that have been standardised it can lead to disagreements.

The overall roughness of the surface certainly has an effect on friction in both the wet and dry states. If one really understands the way that it does so, particularly in the partially lubricated wet situation, it will become apparent that measurements of roughness are not likely to lead directly to the slip resistance of the surface or be such that one could reliably make specific judgements about the surface using that data alone.

When the concept of the hydrodynamic film was first put forward, the measurement of roughness seemed a logical step forward that might assist in the measurement of slip resistance. However, if one goes back to first principles and starts manipulating the mathematical equation that relates to lubrication and at the same time considers how friction is itself developed, one begins to realise that although a roughness measurement of the surface might give a not unreasonable indication of the wet slip resistance, equally it might give a totally incorrect indication. Dr Malcolm Bailey has tested several hundred surfaces and found this to be true. The latest version of the UK Slip Resistance Group Guidelines gives the clear warning:

‘It should be noted that the micro roughness of one surface may have the same numerical value as measured as that of another surface but be quite different in profile, as illustrated by the difference in profiles shown in Figure 1.

For this reason, roughness readings should not be used in isolation but linked with other salient information, such as Pendulum readings from the particular material being tested or specified.’


‘It is imperative that roughness measurements should not be relied upon of themselves to judge the likely slip resistance of the floor. Some reliable means of directly measuring the slip resistance in wet conditions should always be used in conjunction with roughness measurements.’

The ramp, for those who have never seen one, is a platform on which a floor specimen is mounted and upon which a test subject walks in a downhill direction. The ramp angle is steadily increased until either the subject feels unsafe or slips. Whilst it may seem to be a very reasonable way of testing floors, it must be understood that the stepping method used (of necessity) does not correspond particularly well to normal walking with ‘heel strike’ and ‘push off’. Although the subject is facing downhill at all times he walks in short half steps, usually four half steps down, then four half steps up (walking backwards).

There are four ‘protocols’ currently used:

The German DIN protocol uses oil as the lubricant and the test subject wears boots with rubber heavily profiled soles.

The UK (HSL) protocol uses plain water as the lubricant and normal type shoes with Four S rubber soles and heels.

The UK (HSL) protocol no. 2 again uses plain water, but the subject is barefoot.

The fourth protocol is the CEN (European Standards) barefoot ramp test, which again uses bare feet, but a wetting agent is put in the water. Because a fixed amount of wetting agent is used, the results of this test vary from laboratory to laboratory depending on the hardness of the local water.

The German ramp test correlates with no known portable device and while it may be suitable for industrial situations, many consider it has limited value in other situations. The results are declared as ‘R’ numbers. These start at R9 and go up to R13 (there is no R1 to R8). R9 is the lowest classification and indicates a floor with minimal oil wet slip resistance.

The other protocols either declare a maximum ramp angle or convert it to the coefficient of friction using the tangent of the angle (tan α), i.e. 15 degrees would give µ as 0.27.

No usable correlation exists between the ramp test and SlipAlert/Pendulum Results, this is reflected by theoretical analysis.

Slip consequences

The majority of people who slip over do indeed land on their back or on their bottom on the floor after a slip. They may land on their side if they twist during the fall.

On a very slippery floor it is possible for the foot or feet to slip sideways so that the person lands sideways. It is very rare for a person to fall forwards, although one cannot say that it never or cannot happen. A forward fall is usually symptomatic of a trip.

People usually land on their back because the critical phase in walking is when the heel of the leading foot hits the ground and needs restraint to prevent it moving forwards. If that restraint cannot be provided by friction, the heel will slide forwards and the person will slip.

The net effect is that the person’s legs accelerate forwards relative to the upper part of the body, which results in the bottom part of the torso (i.e. the hips) moving forwards slightly faster than the shoulders. This causes the upper part of the body to be inclined backwards as it falls due to the lack of support from the feet.

The most common injury is to the lower back and in particular to the coccyx at the bottom of the spinal cord. Wrists and arms sometimes get injured because the person tries to save themselves, and may try to prevent their head from hitting the ground by using their arms to absorb the shock.

Older people, especially ladies, are particularly vulnerable due to having more brittle bones. Hence, fractured hips and femurs can easily result from such a fall.

Recent work by the UK Health & Safety Laboratory has discovered that a huge number of industrial accidents were in fact initiated by a simple slip. However, due to the way accidents are reported, this significant fact is often omitted from the report or only mentioned in passing. Thus, the main cause of the injury, for instance ‘stabbed themselves with a knife’ (when they fell on the floor) or ‘caught their hand in machinery’ (when they put it out to save themselves) or ‘scalded by hot oil’ (when they grasped and pulled over a deep fat fryer as they were falling) are the ones which feature in the statistics.

Unfortunately, slips are reported in the same category as trips and falls. It is thus not possible to be precise about the numbers involved, but it is believed that slips represent by far the majority of accidents reported in that category. Slips, trips and falls account for 33% of all reported major workplace injuries, 20% of over 3 day injuries, and 50% of all reported accidents to members of the public. A recent estimate suggests the cost to industry is over £500 million each year.

The UK National Health Service statistics suggest that result of slips is several times more costly in terms of bed occupancy than, for instance, road accidents, and costs the NHS many £millions each year to treat the victims.

As an employer, you have a duty under the Workplace Regulations Section 12 to ensure all access routes (any part of a floor over which people are likely to walk) are safe, which includes not being slippery.

Under the Management of Health & Safety at Work Regulations you may well find that you will need to assess whether your floors constitute a risk. You are required to take reasonable steps, e.g. by reading HSE guidelines, the trade press, etc., to familiarise yourself with the hazards and risks in your employees’ work (see Approved Code of Practice para 9). Slipping is a possible hazard affecting all employers. How do your floors stand up? (excuse the pun!)

As the owner of a building to which the public has access, there is also a legal duty of care. In recent months, Environmental Health Officers and HSE Inspectors have become far more active in the field of slipping and are investigating reported slipping accidents and even checking floors (with SlipAlert) that they think might be a problem.

There are several costs associated with an accident. Whilst the main cost of any compensation claim by the victim may well be covered by your insurance, the fact of that claim will almost certainly be reflected in future premiums – insurance companies are not fairy godmothers!


The most costly item is likely to be your management time and that of your staff in dealing with the claim. There will be time taken in making statements, discussing the legal aspects of the claim with solicitors, and attending Court if it goes that far. It will add up to several days work – days when you could be doing something far more profitable for your business.

The answer is sometimes ‘yes’ and sometimes ‘no’. There are ways of overcoming the situation, even if the Local Authority has issued an improvement order. You do, however, need specialist advice for the particular circumstances.