Floor Tolerance Classes: All you need to know
All you need to know
Floor Tolerance Classes
Concrete floors can be laid to a range of accuracies depending on the floors immediate or future usage. Floor designers, architects and engineers should assess the requirements of the floor and suggest a specific tolerance finished floors surface should be laid / finished to. It is vital that the tolerance worked to is the correct one for the floors intended usage. Specifying too high a tolerance will result in laying methods and costs that are unnecessary, whilst specifying to low a tolerance may result in a floor that is not fit for purpose.
Floors can be surveyed post installation to check if they have been laid to comply with the tolerances required. It is vital that the installers fully understand the tolerances they are laying to and adjust their laying and finishing methods, plant and labour accordingly as rectifying out of tolerance floors is time consuming and expensive.
Flatness vs Levelness
For many internal ground floors the most suitable tolerances to be worked to are ‘free movement classifications’ (FM) as defined in TR34 by The Concrete Society. A floor is classified as ‘free movement‘ if it can be driven around freely by vehicles such as forklifts, without too many obstacles such as columns or walls. Areas within 1.5 meters of walls, columns or obstacles are excluded from free movement assessment as vehicles cannot freely move in these areas. The properties measured in FM surveys are Levelness & Flatness.
As this exaggerated diagram shows, a floor may be level but not flat, or may be flat but not level.
Measurement of Elevation Heights
How to calculate levelness
Levelness (Property E) is measured by a theodolite or dumpy level or a very accurate laser level, which would measure elevation heights at points on a 3m x 3m grid. The height difference between adjacent points 3m apart is then assessed and the difference in these two heights in millimetres gives the Property E value for the relationship between those two points.
How to calculate flatness
Flatness (Property F) is measured by pulling a profilograph along straight runs in a number of different directions to get a sample average of the ‘waviness’ of the floor. Essentially the profilograph is a trolley with wheels at the front and back separated by 600mm and a ‘jockey’ wheel between these wheels which moves up and down as the floor changes. Instruments accurately measure this change in height and a graph is produced showing how much of the floor is in tolerance.
Profilograph measures ‘waviness’. The height difference over 600mm is calculated. Multiple runs are selected to give an accurate average of the whole floor.
Understanding the difference
"Surface Regularity" or SR Tolerances
Choosing the right class
The SR assessment process
Some types of floor, unsuitable for FM classes are best assessed by ‘Surface Regularity‘ which are classed as having SR values. They are measured by putting a 2m straight edge, without feet, directly on the floor under its own weight and seeing how much of a wedge can be fitted between straight edge and the floor. This type of class is mainly concerned with flatness. It should be agreed in how many places the floor will be surveyed at before floors are poured as there are infinite positions the floor can be surveyed at.
A 2 m long Straight Edge and Graduated Wedge used to Assess SR Values.
Suitable floor types
FM or SR assessment?
Raised upper floors and external floors laid to falls may not lend themselves well to the FM tolerances. This is because upper floors are typically subject to movement from steels and decking bending and it is not appropriate to try to assess the level of a floor laid on a moving and deflecting structure. Floors to falls are also not suitable for level assessment by FM classes as they are not level (horizontal) by design. Ground floors with many internal walls and
columns, recesses and features that would restrict free vehicular movement may also not be suitable for free movement classes.
There are tolerances for the type of finishes on concrete floors as specified in BS EN 13670;2009 and elaborated on in ‘National Structural Concrete Specification (NSCS) 4th Edition’, and follow as below.These are generally measured using straight edge and wedges as Surface Regularity is above.
Free Movement Classifications
|Floor Class||Origins||Typical Floor Use||Additional||Typical Type of Laying Method|
|FM1||TR34 4th Edition||Where very high levels of flatness and levelness are required, sometimes called a ‘superflat floor’. Used where forklift trucks are lifting over 13 meters high, also where high accuracy scientific equipment is used.||All points should be within +/- 15mm from Datum||Long, narrow eg. 5m strip construction with very accurate shuttering or permanent joints|
|FM2||TR34 4th Edition||Where high levels of flatness and levelness are required. Used where forklift trucks are lifting between 8 and 13 meters high. Most warehouses are now laid to this.||All points should be within +/- 15mm from Datum||Laser Screed or Screed Rail construction with very accurate shuttering or permanent joints|
|FM3||TR34 4th Edition||Where high levels of flatness and levelness are required. Used where forklift trucks are lifting up to 8 meters high. Also used where retail or manufacturing floors are recieving either no covering or a a very thin layer covering||All points should be within +/- 15mm from Datum||Laser Screed or Screed Rail construction with very accurate shuttering or permanent joints|
|FM4||TR34 4th Edition||Where low levels of flatness and levelness are required . Used where forklift trucks are lifting less than 4 meters high. Used where floor is to take additional layers of screed, insulation, tiles ect. Also for workshops and manufacturing floors||All points should be within +/- 15mm from Datum||Traditional Handlay method is sufficient, usually checked with laser level during pour|
|FM2 Special||TR34 3rd Edition||Where high levels of flatness and levelness are required initially, but where areas of the floor may be converted to having even higher level tolerance areas later in its life.||All points should be within +/- 15mm from Datum||Laser Screed or Screed Rail construction with very accurate shuttering or permanent joints|