Who this guide is for
- Owners told their floor needs levelling before a covering can be laid
- Anyone weighing a wet trade against a dry one late in a refurbishment programme
- People whose floor has to contain pipes, cables or a heating layer within its depth
- Readers who have found several floor base reference pages and cannot tell them apart
- Anyone planning a wet room where a fall has to be formed in the floor
The four questions that actually choose
Before any product is named, four things decide the shortlist. How far out is the existing base, and is the deviation a general unevenness or a real hollow? Does anything have to be buried in the floor? Does a fall have to be formed towards a drain? And can the programme accommodate a wet layer that has to dry before a covering goes over it?
Almost every disappointment in this area comes from answering the fourth question last. A pour that is placed quickly does not dry quickly, and the material chosen for programme reasons is frequently the one whose waiting period nobody had allowed for.
- How much correction: general unevenness, or hollows a thin layer cannot fill
- What has to be buried: pipes, cables, heating elements, or nothing
- Whether a fall is needed: a levelling material seeks level by nature
- Whether drying fits the programme: the layer that waits longest is usually this one
Building up versus correcting
A hand-laid cement screed is laid to a thickness, compacted and worked level with a straightedge, and it is that thickness that lets it bury pipes, form falls and take up substantial differences in the deck. A flowing screed is pumped and finds its own level with no compaction, which is why large areas can be placed in one operation.
A levelling compound works the other way: it is a thin correction over a base that is already broadly right, it flows into hollows under its own weight, and it cannot be relied on to create a fall. A thin lining sheet fills nothing at all — it lies on the high points and takes its plane from whatever it is fixed to.
The dry options, and what they are really buying
A dry screed board is bedded on levelling granulate or insulation, glued along rebated edges and often screwed, so it can be levelled as it is laid and introduces no water to the building. The attraction is that nothing downstream waits for drying, and that attraction is easily lost if the rest of the sequence has not been planned around it.
A thin underlayment board solves a different problem again: it gives a thin resilient covering a continuous, stable plane to sit on, fixed down across its whole face rather than only at the edges. It does not level a floor, and that distinction is constantly confused on site.
Weight, height and what the structure is being asked to carry
Every one of these adds depth and most of them add weight. Whether a floor can carry a screed is a structural matter for a qualified professional, and it is the question most often reached last on a timber floor where a wet layer is proposed. It is a structural question rather than a finishing one.
Height is the constraint that catches people even when weight does not. Even a low-build layer is still a rise, and door undercuts, stair bottoms and thresholds have to absorb it somewhere. That arithmetic belongs at the start of the conversation, not after the material has been chosen.
The chemistry question nobody asks in time
A sulfate-bound flowing screed and a cement screed are different substrates chemically, and that difference decides which adhesives, primers and levelling compounds may go over them. Those answers should be obtained in writing before the covering is chosen rather than after the screed is down.
A flowing screed also leaves a weak surface skin as fine material rises and dries. It looks like a sound floor, and covering it over without removing and preparing the surface takes the new covering with it when it fails. That preparation is a separate operation with its own slot in the programme.
Compatibility runs upwards as well as downwards
Each of these materials is a base for a covering that has its own requirements. A covering manufacturer usually states what base it will accept, what flatness it requires and what happens to the guarantee if a different arrangement is used. Knowing that before the base is specified is far easier than discovering it afterwards.
It runs downwards too. Smoothing a floor does not fix what is under it: movement, deflection or moisture in the base comes back through, and a compound made for concrete may not tolerate the movement of a timber deck.
Who decides what
Whether the structure can carry the proposed build-up is determined by a qualified engineer. Whether the base is dry enough for a covering is a specialist judgement that no page can set. Which primers, adhesives and compounds are accepted comes from the product data sheets, and the flooring installer's requirements are better known before the base is specified than after.
Before choosing how to correct a floor base
- 1Establish whether the deviation is general unevenness or genuine hollows
- 2Ask whether anything has to be buried within the depth of the floor
- 3Confirm whether a fall towards a drain is required anywhere on this floor
- 4Ask a qualified professional whether the structure can carry the proposed build-up
- 5Add up the total height added and check it against doors, thresholds and stairs
- 6Ask the covering manufacturer what base and flatness its product requires
- 7Establish whether the programme can absorb a drying period, honestly
- 8For a sulfate-bound screed, obtain adhesive and primer compatibility in writing
- 9Ask who removes the surface skin from a flowing screed, and when
- 10Confirm what primer the chosen compound requires for this substrate
- 11Establish whether movement or moisture in the existing base has been investigated
- 12Record which material was used, so the next contractor is not guessing
Common mistakes to avoid
- Treating a levelling compound as a thin screed and expecting it to build up depth
- Expecting a self-levelling material to form a fall, which it seeks not to do
- Choosing a pumped screed for speed and then squeezing the drying it does not shorten
- Covering the weak surface skin of a flowing screed instead of removing it
- Assuming adhesive compatibility from experience with cement screeds
- Fixing a lining sheet over a deck with real hollows and expecting it to level them
- Deciding on the floor base before anyone has added up the finished height
When to involve a professional
- Whether a floor can carry a screed or a board build-up is a structural determination made by a qualified engineer
- Drying assessment before a floor covering is laid is a specialist judgement, and the measurement belongs to the flooring specialist
- The covering manufacturer states what base, flatness and preparation its product requires, and that statement governs
- Which primers, adhesives and levelling compounds a given substrate accepts comes from the product documentation, in writing
- Wet-area floors, where falls and drainage interact with waterproofing, are designed by a qualified professional
Frequently asked questions
Questions readers ask about this topic
Is a levelling compound just a thin screed?
No, and the library is explicit about it. A screed is laid to a thickness, compacted and worked level by hand, which is what lets it bury pipes and form falls. A compound is a flowable correction poured over a base that is already broadly right, and it finds its own level.
Can a levelling compound create a fall to a drain?
It seeks level by nature, so it cannot be relied on to create a slope. A wet-room floor where water has to reach a drain is a different conversation involving a graded former or a screed laid to falls, and the whole arrangement is designed by a qualified professional.
Why choose a dry board floor instead of a screed?
Its whole reason for existing is to do the work without water, so nothing downstream waits for drying. That advantage is real and it is easily lost if the rest of the sequence has not been planned around it, and the boards follow their bearing layer exactly rather than correcting it.
What is the difference between a dry screed board and a lining sheet?
A dry screed board is a load-spreading plate bedded on granulate or insulation and levelled as it is laid, so it can answer a genuinely uneven deck. A thin lining sheet is fixed down over a deck whose plane it simply inherits, and it fills nothing.
How do I know when the base is dry enough?
By assessment rather than by time or appearance, and the judgement belongs to the flooring specialist rather than the programme. For a sulfate-bound screed in particular, drying before an impermeable covering is laid is verified by testing, and the record of that assessment is worth keeping.
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