ALL YOU NEED TO KNOW
Floor Tolerances Explained...
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.
Image: A floor may be level but not flat, or may be flat but not level
Video: Example of measuring elevation heights on site in Doncaster
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.
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.
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.