According to British Standards, a departure of up to 15mm from datum is considered satisfactory for large areas of screed intended for normal purposes and can be measured by a 2m standard grid, using standard surveying methods.
But when greater accuracy to datum is essential, for example – small rooms, along the line of partition walls, in the vicinity of door openings, and in areas where specialized equipment are to be placed directly on the floor, laser level dip survey can be used to obtain high levels of precision.
How it’s performed
Deviation of the screed’s level from the initial reference point (master datum) is measured with a dumpy or laser level. The master datum is set with a high precision dumpy level by an engineer and the dip surveys are carried out by either dumpy or laser level by the screed contractor, principal contractor, engineer or floor finisher.
When is the test carried out?
Level survey for deviation from datum should be checked and recorded as soon as possible after the installation of screed. Substrate levels can change due to building settlement and movement.