Do I need to prime screed before tiling?
Often — but not as a universal rule. Whether a screed is primed before tiling, and with what, depends on the screed type and condition, how dry it is, and the tile adhesive and primer system that has been selected. The screed itself is only part of the answer.
Based in Gerrards Cross, Buckinghamshire.
The short version.
- Priming before tiling is common, but it is a system decision, not an automatic step. The tile adhesive manufacturer's requirements govern it.
- Sand and cement screeds and calcium sulphate / anhydrite screeds are prepared differently and must not be treated as the same substrate.
- Any screed must be sound, clean, free of laitance and loose material, and dry enough for the system before tiling starts.
- General-purpose PVA is not normally recommended as a tiling primer. Use the primer the adhesive system specifies.
- Walkable is not the same as ready to tile. Drying to a floor covering criterion takes considerably longer — see screed drying times.
This guide is about preparing and handing over a screed correctly for the follow-on tiling trade. It is written for main contractors, developers, flooring and tiling contractors and specifiers who need the interface between screed and finish to work first time.
Sand and cement screed before tiling.
A traditional cementitious screed is chemically compatible with a cement-based tile adhesive, so the questions are about condition, moisture and suction rather than material reaction.
Soundness
The screed must be structurally sound and well bonded to its base where a bonded build-up was specified. Hollow, cracked, friable or debonded areas should be identified and dealt with before tiling, not tiled over.
Cleanliness
Curing compounds, plaster splashes, paint, oil, dust and site debris all interfere with adhesion. The floor should be mechanically prepared where required and then vacuumed, not simply brushed.
Curing and drying
A cementitious screed needs to cure and then dry to the moisture criterion set by the adhesive and floor finish system. Thickness, screed type and site conditions all affect how long that takes, so no single figure applies to every floor.
Suction and priming
A porous screed can draw water from the adhesive too quickly. Some cement-based adhesives are formulated to be used on a prepared cementitious screed without a primer; others require one. The adhesive data sheet decides, and dilution and coverage are product-specific.
More on the material itself: sand and cement screed.
Calcium sulphate / anhydrite screed before tiling.
Most pumped liquid screeds in the UK are calcium sulphate based. They perform very well, but they are a different substrate from a cementitious screed and the preparation before a cement-based tile adhesive is more demanding.
- Identify the screed and the system. Confirm the screed product and the adhesive and primer system that will be used. Manufacturers such as ARDEX, Instarmac / UltraTile, BAL and Mapei each publish their own preparation and priming requirements, and those are the instructions that apply.
- Confirm the moisture criterion. The screed must be dry to the criterion the chosen system sets, measured by the method it specifies. Some system guidance uses a 75% relative humidity threshold as its criterion; that is an example of a manufacturer-system requirement, not a universal British Standard figure.
- Remove laitance mechanically. Abrade or otherwise mechanically remove surface laitance and any loose or weak material so that sound screed is exposed.
- Clean and vacuum thoroughly. Mechanical preparation generates fine dust, which will prevent a primer bonding if it is left in place.
- Apply the specified primer or barrier. A primer or barrier appropriate to the system is used before a cement-based adhesive. ARDEX's published system guidance for anhydrite screeds, for example, uses ARDEX P 51 within their system — an illustration of how these products are specified, not a universal product instruction.
- Check adhesive compatibility. Where the adhesive is cement based, the primer or barrier separates it from the calcium sulphate screed. In the presence of moisture, calcium sulphate and cement can react at the interface to form expansive compounds — the ettringite reaction — which can weaken the bond. Following the system's preparation and priming requirements is what prevents that.
More on the material: liquid screed, and a side-by-side comparison in sand cement vs liquid screed.
Why laitance has to be removed.
Laitance is the weak layer of fine binder and fines that can rise to the surface as a screed is placed, compacted or finished. It looks like part of the floor, but it is not part of the sound body of the screed.
Anything bonded onto laitance is bonded onto a weak layer. The adhesive can be perfectly sound, the tiles can be laid perfectly well, and the floor can still fail — because the failure plane is the laitance, not the adhesive. That is why manufacturer guidance for calcium sulphate screeds consistently calls for mechanical removal rather than chemical treatment or a heavier coat of primer.
The practical sequence is: mechanically abrade, remove the debris, vacuum thoroughly, then prime. Skipping the vacuum stage is one of the most common reasons an otherwise correct preparation still fails.
What does primer actually do?
A primer is not a glue and not a waterproofer. BAL describe priming as controlling suction, consolidating suitable surfaces, assisting adhesion and creating a barrier where materials could otherwise react. Those four roles explain why the right primer differs from floor to floor.
| Role | What it means on site |
|---|---|
| Control suction | A porous screed can pull water out of a cement-based adhesive too quickly, weakening the bond. A primer moderates that absorption so the adhesive can cure properly. |
| Consolidate the surface | Where a surface is sound but slightly dusty after preparation, an appropriate primer can bind residual fines. It is not a substitute for removing weak or friable material. |
| Improve the system interface | Primers are formulated as part of an adhesive system. Using the primer the system specifies keeps the interface between screed and adhesive within the conditions the manufacturer has tested. |
| Isolate incompatible materials | On calcium sulphate screeds a primer or barrier is used to separate the screed from a cement-based adhesive, because the two materials can react in the presence of moisture at the interface. |
Because primers are formulated as part of a system, dilution rates, coverage and waiting times are product-specific. Mapei's current Primer G literature, for example, sets out different treatment for gypsum-based and cementitious substrates within the same product range. No universal dilution ratio can be published here, and any source that offers one should be treated with caution.
Why PVA is usually the wrong primer for tiling.
General-purpose PVA is cheap, widely available and long-established on site, which is exactly why it still gets used as a tiling primer. Current UK manufacturer guidance advises against it.
BAL state that PVA-based primers are not normally recommended for tiling, because moisture can re-soften or emulsify the dried film — and moisture is present in a cement-based adhesive by definition. If the film softens at the interface, the bond it was supposed to help can be the thing that fails. Instarmac give the same advice and direct users to purpose-designed primers chosen to suit the adhesive system.
The practical rule: use the primer the selected adhesive system specifies, and do not substitute a general-purpose bonding agent for it unless that system's own literature explicitly permits it.
What changes if there is underfloor heating?
Underfloor heating adds a commissioning sequence between laying the screed and tiling it, and that sequence has to be planned into the programme rather than compressed at the end of it.
- The screed must cure undisturbed before any heat is applied, for the period the screed manufacturer or specification requires.
- Commissioning — the controlled heat-up and cool-down cycle — is carried out in line with the screed, underfloor heating and adhesive manufacturers' guidance. There is no single fixed timeline that applies to every floor.
- Drying and moisture testing follow the system's criterion, and the heating is normally turned off for a defined period before measurement and before tiling, as the relevant guidance directs.
- Adhesives and, where relevant, decoupling or movement provision should be suitable for a heated floor. That is an adhesive and tiling specification decision.
More on the build-up and sequencing: screed over underfloor heating.
A practical pre-tiling checklist.
System-agnostic and deliberately conservative. It does not replace the adhesive manufacturer's instructions or the project specification; it is the sequence that keeps those instructions from being missed.
- Confirm the screed type and, where it exists, the specification: traditional sand and cement, calcium sulphate / anhydrite, or a proprietary system.
- Confirm the tile adhesive and primer system that will actually be used, and read its preparation requirements before touching the floor.
- Confirm the screed is dry enough for that system, using the measurement method and criterion the system requires — not a rule of thumb.
- Check the screed is sound: no hollow, cracked, friable or debonded areas, and no movement underfoot.
- Mechanically remove surface laitance and any weak or loose material where the system requires it.
- Remove contaminants — curing compounds, plaster, paint, dust, oil, adhesive residues and site debris.
- Vacuum thoroughly. A visually clean floor is often still dusty, and dust prevents a primer bonding properly.
- Check surface regularity and level against the tiling specification, and agree any levelling work and who is doing it.
- Apply the primer specified for the chosen adhesive system, exactly as its data sheet directs, and allow the stated waiting time.
- Record the handover: screed type, dates, moisture readings taken, preparation carried out and any outstanding items.
Note that tiling readiness is not the same as being walkable. A screed can take foot traffic long before it has dried enough for a floor finish — see screed drying times for why the two are routinely confused on site.
Who is responsible for the floor before tiling?
Most pre-tiling disputes are really handover disputes. The split below is the common arrangement, but it is project-specific and should be confirmed in writing.
The screeding contractor
Installs the screed to the agreed specification, depth and regularity, protects it during curing where that is in scope, and hands it over in the agreed condition with the relevant information about screed type and dates.
The tiling or flooring contractor
Selects the adhesive system, confirms the substrate is acceptable for it, carries out final preparation and priming to the system's requirements, and takes responsibility for the tiled finish.
The specifier and manufacturer
The specification sets the required screed system, tolerances and finish. Manufacturers set the preparation, moisture criteria and priming requirements for their own products, and those documents are the technical authority. BS 5385, the UK tiling code, was revised in 2024 and provides the wider context.
Where Screeding.com sits
We install and hand over the screed. This guide exists so the floor we leave behind is understood correctly by the follow-on trade — screed type, condition, drying and what still has to happen before tiling begins.
Common questions.
Do I need to prime screed before tiling?
Often, but not as a universal rule. Whether a primer is needed, and which primer, depends on the screed type, its condition and moisture state, and the tile adhesive system chosen. Cement-based adhesive manufacturers publish preparation and priming requirements for their own systems, and those instructions take precedence over any general advice.
Do sand and cement screeds always need priming before tiling?
No. Some cement-based adhesives are designed to be applied to a sound, clean, suitably dry cementitious screed without a primer, while others require priming to control suction or consolidate the surface. The correct answer for a specific floor comes from the adhesive manufacturer's data sheet for the system being used.
Do anhydrite or calcium sulphate screeds need to be primed?
Preparation is more involved. Manufacturer guidance for calcium sulphate screeds generally requires that the screed is confirmed dry to the system's criterion, that surface laitance and loose material are mechanically removed, that the floor is thoroughly cleaned and vacuumed, and that the primer specified by the chosen system is applied before a cement-based tile adhesive.
Can I use PVA as a primer before tiling?
General-purpose PVA is not normally recommended for tiling. BAL and Instarmac both advise against it because the film can re-soften or emulsify in the presence of moisture, which can contribute to debonding. Purpose-designed tiling primers selected for the adhesive system should be used instead, unless that system's own literature specifically says otherwise.
How dry does a screed need to be before tiling?
There is no single universal figure. Cementitious and calcium sulphate screeds have different criteria, and manufacturers set thresholds for their own systems — for example, some calcium sulphate system guidance uses a 75% relative humidity criterion. Drying depends on screed type, thickness, environment and the system specified. See our guide to screed drying times.
What is laitance and why does it have to be removed?
Laitance is the weak, fine layer of binder and fines that can rise to the surface as a screed is placed and finished. It is not part of the structural body of the screed, so anything bonded to it is bonded to a weak layer. Mechanically abrading and then vacuuming it away exposes sound screed for the primer and adhesive to bond to.
Does underfloor heating change the preparation before tiling?
Yes. The screed must be cured and dried, and the heating commissioned, in line with the screed, underfloor heating and adhesive manufacturers' guidance before tiling. There is no fixed heat-up timeline that applies to every floor, and the commissioning sequence should be planned into the programme rather than compressed at the end.
Who is responsible for preparing the floor before tiling?
It varies by project and contract. The screeding contractor is responsible for installing the screed to the agreed specification and handing it over in the agreed condition. Final preparation, priming and adhesive selection normally sit with the tiling or flooring contractor working to the specification and the manufacturer's system requirements. The responsibility split should be confirmed in writing before handover.
Planning a floor that has to be tiled?
Run an indicative estimate for traditional sand-and-cement work, or send drawings and specifications for a commercial quotation.