New flooring is only as good as what it sits on. LVT shows every ridge, laminate clicks and bounces over hollows, tiles lip or crack, and even carpet wears unevenly on a poor base. The question is rarely "screed or no screed". It is whether the floor you have is sound, flat enough and dry enough for the finish you have chosen.
The short answer
You need a new screed if there is no floor yet, if the existing screed is weak, hollow or badly cracked, or if the floor is far out of level.
You need a smoothing compound, not a screed, if the floor is sound but rough, patched or slightly uneven. This is the usual answer before LVT and sheet vinyl.
You may need nothing if the floor already meets the flooring manufacturer's flatness and moisture limits, which is more likely with carpet and floating laminate than with glued LVT or vinyl.
Flatness: what SR1, SR2 and SR3 mean
Short answer: flatness is measured as the biggest gap under a 2 metre straightedge, and BS 8204 sets three classes.
| Class | Maximum gap under a 2m straightedge | Typical use |
|---|---|---|
| SR1 | 3mm | High standard floors, thin sheet finishes |
| SR2 | 5mm | Normal standard for most floors |
| SR3 | 10mm | Utility standard, thick or tolerant finishes |
Swipe the table sideways to see every column.
To check a floor, lay a 2 metre straightedge in several directions across the room, slide a gauge or ruler under it at the lowest point, and note the biggest gap. Also look for small, sharp ridges and trowel marks: a floor can pass a 2 metre test and still show bumps through thin vinyl.
What different floor coverings ask for
Short answer: thin, glued finishes ask the most of the floor; floating and soft finishes are more forgiving.
Flooring manufacturers publish their own subfloor limits, and they override any general rule. These are examples from installation guides we have read:
| Floor covering | Flatness the manufacturer states | Moisture limit the manufacturer states |
|---|---|---|
| Glue down LVT (Karndean, November 2023) | Firm, hard, flat, level and smooth; smoothing compound applied in line with the underlayment maker | Below 75% RH by hygrometer, or no more than 2% CM for cement screeds and 0.5% CM for calcium sulphate screeds |
| Rigid core click LVT (Karndean, January 2024) | Unevenness no more than 5mm below level over 3m in any direction; high spots removed | Below 95% RH by hygrometer, or no more than 3% CM cement and 1.4% CM calcium sulphate |
| Laminate (Quick-Step, July 2026) | Less than 4mm under a 2m straightedge and less than 1mm under 20cm | Cement below 2.5 CM% or 75% RH; calcium sulphate below 0.5 CM% or 75% RH; lower limits over heated floors |
| Carpet | No figure in the sources we read | BS 5325 recommends a hygrometer test; 75% RH is the usual limit for most floor coverings |
| Ceramic and porcelain tiles | Follow the tile adhesive maker | Follow the tile adhesive maker; calcium sulphate screeds need laitance removed and a primer |
Swipe the table sideways to see every column.
Three things are clear from this. Glued LVT and vinyl need the smoothest, driest base, which usually means a thin floor levelling compound to SR1 standard over the screed. Click products are more tolerant of flatness but still have limits. And two products from the same brand can have very different requirements, so always read the guide for the exact product you are buying.
Quick-Step also specifies its own underlay with a vapour barrier on mineral subfloors such as screed, and asks for floor heating to be started at least two weeks before laying.
Moisture: limits and how to test
Short answer: most floor coverings need the base at or below 75% relative humidity, measured with a hygrometer; solid timber needs 65%.
F. Ball, which makes subfloor products, summarises the British Standards this way: BS 8203 (resilient floor coverings), BS 5325 (textile floor coverings) and BS 8201 (wood floor coverings) recommend testing with a hygrometer, and a base "can be considered dry enough to install most floorcoverings" if readings do not exceed 75% RH, or 65% RH for solid timber.
How the test works:
- A hygrometer hood is sealed to the floor, or a probe is placed in a hole drilled into the screed.
- The air trapped under the hood comes into balance with the moisture in the screed.
- The relative humidity reading is compared with the flooring and adhesive manufacturers' limits.
Several readings across the floor give a fairer picture than one, and readings near external walls, doors and thick areas are worth taking. Some manufacturers also accept the calcium carbide (CM) test, which measures moisture by weight; the table above shows that calcium sulphate screeds have much lower CM limits than cement screeds.
A new screed takes time to reach these limits. Our working rule is about one day per millimetre for the first 40mm, then slower. Our guide on how long screed takes to dry covers the factors.
If the floor is still too wet
A surface damp proof membrane can let flooring go down over a base that is not dry enough. F. Ball's Stopgap F77, a two part epoxy membrane, is stated for use up to 98% RH, or up to 90% RH over underfloor heating screeds, with hygrometer readings taken in accordance with Appendix A of BS 8203 first. It adds cost and must suit the screed and the flooring, so it is a decision to plan, not a last minute fix.
Screeding over an existing floor
Short answer: often you can level over what is there, if it is sound and well bonded. If it is not, it comes up first.
| Existing floor | Usual approach |
|---|---|
| Sound concrete or screed, slightly uneven | Prepare, prime and apply a smoothing compound |
| Concrete with paint, adhesive or laitance | Grind or shot blast, then prime and smooth. See concrete floor grinding |
| Calcium sulphate (anhydrite) screed | Remove laitance, check moisture, prime, then smooth |
| Firmly bonded quarry or ceramic tiles | Clean, replace loose tiles, then a suitable two part or latex screed |
| Timber floorboards | Fix down, overboard with flooring grade plywood, then a compound made for flexible bases |
| Weak, hollow or cracked screed | Repair or replace. See screed repairs |
| No floor, or a big change in level | A new screed. See floor screeding |
Swipe the table sideways to see every column.
Karndean's glue down guide makes the same points: concrete and sand and cement screeds should be mechanically abraded to remove laitance, paint, plaster or adhesive; calcium sulphate screeds need the screed maker's advice on primers and compounds, with all laitance removed; and plywood should be fixed and its joints smoothed with a suitable compound.
If you are not sure whether to use a compound or a screed, our guide to latex, self levelling compound or a new screed walks through the decision. All of this starts with floor preparation.
Why screeds crack, and whether it matters for flooring
Short answer: most cracks come from shrinkage, thin sections, poor compaction or drying too fast. Fine, stable cracks can be filled; cracks that move need repair before flooring.
Common causes:
- Drying shrinkage. Cement screeds shrink as they dry. Without curing and movement joints, fine cracks form.
- Too thin. A screed below its minimum depth for its construction has too little strength. The Mineral Products Association's data sheet gives a minimum of 25mm bonded, 50mm unbonded and 75mm floating, with 65mm normally satisfactory in housing.
- Poor compaction in sand and cement screeds, leaving voids that crack under load.
- Curling. The MPA notes that curling and hollowness are likely at joints, cracks and perimeters of unbonded screeds; fully bonded screeds carry the least risk.
- Drying too fast or heating too early. Direct heat, or underfloor heating turned up before the screed is ready, can crack and curl it.
- No curing. The MPA advises covering cement screeds to retain moisture for at least five days.
- Movement from below, such as joints in the slab or building that were not carried through the screed.
Thin, glued finishes such as vinyl and LVT will show cracks and ridges, and tiles can crack over a moving crack. Before any flooring goes down, cracks should be assessed: stable shrinkage cracks can usually be filled, while moving or widespread cracking points to a screed that needs repair or replacing.
A simple pre flooring checklist
- Sound? No hollow sounds, no powdery surface, no loose areas.
- Flat? Checked with a 2 metre straightedge against the flooring maker's limit.
- Smooth? No sharp ridges, trowel marks or debris.
- Dry? Hygrometer readings within the flooring and adhesive limits.
- Clean and primed? Dust, laitance and contamination removed, with the right primer for the base.
- Cracks dealt with? Stable cracks filled, moving cracks repaired.
Get your floor ready for its finish
Tell us which flooring is going down and we will advise whether the floor needs a screed, a compound or nothing at all. Send us the floor area in m², the depth you have to work with, what is going on top and the site postcode. We call back the same day and give you a fixed written price within 24 hours. Ask for a quotation or call 07466 650936.
Sources
- BSI: BS 8204-1, concrete bases and cementitious levelling screeds
- Mineral Products Association: Mortar data sheet 22, Screeds
- Karndean: glue down installation guidelines, UK, November 2023
- Karndean: rigid core installation guidelines, UK, January 2024
- Quick-Step: laminate installation, with installation guide version 07.2026
- F. Ball: digital hygrometer and BS 8203, BS 5325 and BS 8201 guidance
- F. Ball: Stopgap F77 surface damp proof membrane
