All about the Datum Level

I’m Andy Parkin, Managing Director of the Multi-Award Winning Speed Screed. Today I’m here to give you a little more insight into the Datum Level; the Datum Level is a crucial part of any construction project. Let’s get started.

What is a Datum Level?

A Datum Level is an arbitrary horizontal plane on a construction project used as a reference point for all other vertical dimensions to be measured. It is used to highlight differentials between vertical heights of floor levels along with differing levels of two separate points on the site.

It is often the case that the Datum Level is given a 0.000m designation (though this doesn’t have to be the ground level outside; the ground floor could be 0.650m with a first floor at 4.650m for example. A basement would be below the Datum Level so would be shown as a minus number, -2.950m for example.

It is not uncommon, however, for the minus sign to be missed or misrepresented (as a dash for example) so often a temporary benchmark (TBM: a fixed point) is used so all over levels (even subterraneous) are represented by a positive value. Looking at the example above, the value assigned to the Datum Level (0.000m) would be assigned to the basement and the levels above would be adjusted accordingly.

Setting out

Datum levels give points of reference to enable the vertical setting out of the building by the site engineer; they are marked clearly on all site drawings and the levels are articulated in metres in terms of a unit of measure. They are all described to three decimal places and because it is the building being set out the numbers should all be positive.

Often and particularly with larger-scale developments, the Ordnance Survey (OS) datum is used as the Datum Level for the project. There are several advantages to doing this:

• Work on the site can be related to other buildings and hard landscaping features like sewers or manholes.
• You can take into account local gradients.
• If you use the OS datum it is immediately compatible with the OD grid.

It is worth noting that the OS datum is the height above sea level and pertains to a specific datum point at Newlyn which is in Cornwall. If you are using the OS datum it must be clearly demarcated on your drawings.

Datum Level and Screed

British standards allow for a maximum departure from Datum of 15mm. For 85% of projects SR (Surface Regularity) 2 is specified and is plus or minus 5mm over a 2-meter straight edge.

So there you have it; a full update on Datum Levels and why they are so critical to every building project. They are literally the reference point and starting point for every type of building work and are absolutely crucial in getting things right the first time. For more information, please contact us. We love to help!