Surface Regularity or flatness of screeds is an important aspect that determines the serviceability of a floor. When it is practically impossible to achieve screeds or flooring that are absolutely flat or true to datum, it is important to prevent excessive deviations as it can affect the serviceability of the floor both directly and indirectly.
As specified by the Construction Industry Research and Association (CIRIA), direct impairment includes immediate problems such as:
- Trip hazards to pedestrians
- Excessive sway or wear and tear on wheeled traffic
- Difficulty in installation of specified equipment on the floor
- Ponding in wet areas
- Aesthetic effects at skirting
Indirect impairment includes long term problems such as:
- Quicker degradation of floor due to increased wear
- Impact at high spots
- Lipped Joints
It is therefore important to confine these deviations within reasonable limits so as to ensure adequate serviceability depending on the intended use of the floor.
Surface Regularity or flatness of screeds is generally measured on the basis of deviation from a straightedge laid level on the screed surface. According to British Standards specifications, surface regularity for screeds for normal accuracy floors has been classified as following:
Assessment of surface regularity of screed using a straightedge
As per Annexe C( normative) of BS8204-1:2003 & A1:2009, the surface regularity of a screed for normal accuracy flooring is measured using a 2 m long straightedge laid in contact with the floor surface and resting under its own weight. The deviations of the floor surface from the underside of the straightedge are measured between the points which are in contact with the floor surface, by means of a slip gauge or other suitable accurate measuring device.
For screeds where the straight-edge basis for specification is used, it is advisable that the various interested parties in a contract come to an agreement right at the design stage, regarding the sampling rate for testing, and the procedures to adopt if conformity is not achieved. It is ideal to include the number of positions where the straight edge will be placed to check conformity, as well.