Departure from Datum
Departure from datum simply refers to minor deviations in a screed surface. According to British Standards, a deviation of up to 15mm from is considered satisfactory for large areas of screed intended for normal purposes, and can be measured by a two-metre standard grid, using standard surveying methods.
But when lower tolerances are essential, such as small rooms, along the line of partition walls, in the vicinity of door openings, and in areas where specialised equipment needs to go directly on the floor, you can use a laser-level dip survey to achieve high levels of precision.
When is it done?
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.
How is it done?
A screed’s deviation 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 either by the dumpy or laser level by the screed contractor, principal contractor, engineer or floor finisher.
Wearing screeds, widely known by the former name of granolithic screeds, are high-density toppings suitable...