Substrates & Preparation
Substrate Surveying and Setting Level Datums Before Screeding
A quick survey of the existing substrate and an agreed level datum let a liquid screed pour achieve a flat, level finish even where the base below varies.
Need help establishing levels before your screed is ordered?
We carry out a level check as part of planning most pours, so depth and material quantities are based on the real substrate across Exeter and the South West.
Or call our landline on 01392 237700
Liquid screed is prized for its self-levelling properties, but achieving a genuinely flat and level finished floor still starts with knowing where 'level' actually is on a given site. A substrate survey — checking the high and low spots across the base — combined with an agreed datum line lets the installer plan screed depth so the finished floor comes out level even where the substrate underneath doesn't.
This step matters more on refurbishment projects and larger floor plates, where an existing substrate is more likely to have variation, than on a small, freshly cast new-build slab. A slab poured to a tight tolerance needs far less checking than an older solid floor that's settled or been patched over the years.
Establishing the survey and datum early also feeds into other trades' planning, since door heights, staircase rises and kitchen unit levels are all set against the same expected finished floor level that the screed is being asked to deliver.
Why the highest point on the substrate matters
Because liquid screed self-levels to a horizontal plane, the minimum practical screed thickness across a floor is generally governed by the highest point of the substrate beneath it — the screed needs enough depth over that high point to meet the manufacturer's minimum. A level survey identifies this before the pour, so the required screed depth, and the volume of material needed, can be worked out accurately rather than estimated.
On an uneven existing substrate this can mean noticeably more material is needed over low spots to keep the top surface level, which is worth understanding at the quotation stage rather than as a surprise on pour day.
Setting out from a fixed datum
A datum — a fixed, known reference height — lets everyone on site work from the same point when talking about finished floor levels, whether that's the screed team setting laser levels, the door supplier ordering doors, or the kitchen fitter planning unit heights. Agreeing this datum early, and making sure it's clearly marked on site, avoids conflicting assumptions about where the finished floor will actually sit.
Recording and sharing survey results
A level survey is most useful when its results are captured somewhere the wider site team can access, rather than existing only as a mental note from the day it was carried out. A simple spot-height sketch or marked-up plan, showing the high and low points and the agreed datum, lets other trades check their own work against the same reference without needing to repeat the survey themselves.
This is particularly useful on projects with a gap of weeks or months between the survey and the actual pour, where relying on memory alone increases the risk of a detail being missed or misremembered by the time the pump arrives.
Checking tolerances against other trades' requirements
The finished floor level agreed from the survey and datum needs to work for more than just the screed — door undercuts, staircase first-riser heights, and kitchen unit levels are all set against the same expected finish. A tolerance check at the survey stage, comparing the planned screed depth against these other requirements, catches conflicts while there's still time to adjust the specification.
Leaving this check until after the pour risks discovering that a door has been hung too low, or a staircase riser is now the wrong height, which is a far more disruptive fix than adjusting the plan before material goes down.
Surveying & Level Datums — the points that decide it
- A quick level survey identifies the highest point on the substrate, which usually sets the minimum screed depth needed elsewhere on the floor
- A datum line or level, often taken from a fixed reference point such as a damp-proof course or finished floor level on drawings, is agreed before the pour
- Screed rails or laser levels are set from the agreed datum so the pump operator can maintain consistent depth and fall across the floor
- Variation across an existing substrate can mean the screed is thicker in some areas than others, which is worth understanding ahead of ordering material
- Doorway thresholds and any changes in finished floor level between rooms should be identified during the survey, not discovered during the pour
- Where falls to a drain or gully are required, these are built into the level survey and datum rather than left to the substrate's existing profile
- A tolerance check against the drawn finished floor level flags any conflict with door heights, staircase rises or kitchen unit levels before material is ordered
- Survey findings are best recorded on a simple plan or spot-height sketch that the whole site team can refer back to, rather than kept only in one person's notes
Need help establishing levels before your screed is ordered?
We carry out a level check as part of planning most pours, so depth and material quantities are based on the real substrate across Exeter and the South West.
Or call our landline on 01392 237700
Surveying & Level Datums — your questions
In short
A quick survey of the existing substrate and an agreed level datum let a liquid screed pour achieve a flat, level finish even where the base below varies.
Need help establishing levels before your screed is ordered?
We carry out a level check as part of planning most pours, so depth and material quantities are based on the real substrate across Exeter and the South West.