Self-Leveling Concrete and Underlayments: Types, Thickness and Toppings

Self-leveling concrete is a factory-made cement-based mortar that flows and heals under gravity into a flat plane, placed under floor coverings as an underlayment or left exposed as a topping. A bonded cement-based layer is at least 3 mm (0.12 in) thick at any point under BS 8204-7:2003, and data sheets set the upper limit by product, from 10 mm (0.39 in) for Setcrete Latex to 38 mm (1 1/2 in) neat for ARDEX K 15.

In the US the class is tested by ASTM C1708/C1708M-26 and installed by ASTM F2873-24. In the EU a cement-based self-levelling compound is screed material, designated under EN 13813:2002 as CT for cement or CA for calcium sulfate with a compressive and a flexural class. Self-levelers sit beside resin floors, membranes and repair mortars in the wider family of construction chemicals.

This page runs in specification order: identity, mechanism, uses, tested properties, US, EU and UK documents, selection, application and cure by temperature, failures, safety, dated US costs, alternatives and sourcing.

Standards, codes and regulations are checked against their issuing bodies, product values against the named data sheet and prices against the dated source named in the sentence; see our methodology and fact-checking.

The table below summarizes the class in eight attributes, each with its source document or named product.

What Is Self-Leveling Concrete?#

Self-leveling concrete, sold as self-leveling underlayment (SLU), self-leveler or floor leveling compound, is a one-component powder mixed with water, or historically a two-component powder and latex pack, poured or pumped to correct a slab before flooring. The powder is a factory-made hydraulic-cement mortar, so the mix water per bag is fixed on the data sheet rather than chosen on site.

"Self-leveling concrete" is the consumer name, while data sheets and ASTM documents speak of self-leveling underlayments, self-leveling mortars and toppings. An underlayment is a correction layer that a covering hides; a topping is formulated to be walked on. Both form the leveling group of the flooring family, which also holds resin systems such as the epoxy, polyurethane and polyaspartic coatings described under floor coatings for concrete.

Underlayments, toppings and gypsum-based levelers: which products count?#

Four product types count as self-levelers: cement-based underlayments, cement-based interior toppings, exterior toppings and gypsum-based levelers bound by alpha-hemihydrate. The three materials that define the cement-based ternary binder are listed below.

• Calcium aluminate cement: the binder and accelerator that reacts with calcium sulfate to form ettringite.
• Calcium sulfate binders: the sulfate source of the ternary system and the binder of hemihydrate-based levelers.
• Redispersible polymer powders: the polymer that gives bond and flexural strength; the abbreviation RDP also names a flame retardant, so the full name is used here.

Portland cement completes the ternary system, and fillers and a flow agent set the consistency.

Regional names: self-levelling, smoothing compound and latex screed#

Self-levelers carry different names in the UK, Germany and the US, and the table below maps 11 of those names to the same product class.

A UK smoothing compound and a US underlayment are the same class; the UK latex screed is its two-component form, mixed from powder and a latex liquid on site.

How Does a Self-Leveling Underlayment Flow, Set and Dry?#

A self-leveler flows and heals under gravity because a superplasticized, polymer-modified mortar stays fluid for minutes, and it sets and dries fast because calcium aluminate cement reacting with calcium sulfate forms ettringite, which binds the mix water chemically. Water bound in ettringite crystals does not have to evaporate before a covering goes down, which shortens drying. Healing is the ability of a fresh edge to merge with the next batch without a seam, and ASTM C1708/C1708M-26 measures it as healing time next to initial flow and flow retention.

Ettringite formation also compensates shrinkage, and the polymer adds bond to the primed substrate and flexural strength to the thin layer. A self-leveler is not a vapor barrier: slab moisture passes through it to the adhesive and covering above. It is also not a wear surface unless the product is sold as a topping.

Where Is Self-Leveling Concrete Used?#

Self-levelers are used to flatten and smooth sound substrates before resilient flooring, tile, wood or resin floors, and as exposed toppings where the product is formulated as a wear surface. The six substrate groups that accept a primed self-leveler on the data sheets read here are listed below.

• Concrete: structurally sound and at least 28 days old (ARDEX K 15, TDS 2026-04-02).
• Terrazzo: absorbent and non-absorbent terrazzo, primed by porosity.
• Tile: ceramic, quarry and porcelain tile, for example an old tile floor leveled before new tile adhesives and tile installation systems go down; see tile adhesives and tile installation systems.
• Coatings and residues: existing epoxy coatings, such as worn epoxy flooring, and non-water-soluble adhesive residue that has been wet-scraped.
• Metal: steel and other metals except aluminum (ARDEX K 15).
• Wood and gypsum: exterior-grade plywood or OSB with metal lath, and gypsum underlayment that is primed.

Above the leveler, the finished floor decides the grade: coverings and resin coatings go over an underlayment, while a topping stays exposed or takes a sealer or coating.

Which Properties Define a Self-Leveling Compound's Performance?#

Six properties describe a self-leveler, three fresh (flow, flow retention, healing time) and three hardened (compressive strength, setting time, flexural strength), all tested by ASTM C1708/C1708M-26 in the laboratory. Data sheets add dry density, shrinkage and the Robinson floor test (ASTM C627) as further evidence of service class. The table below sets named product values next to the test method each maker cites; ASTM C1708/C1708M-26 states that its results are not intended to represent field performance.

Named product values; not requirements.

Standard requirement or product value: reading self-leveler data sheets#

No US standard sets a minimum strength for a self-leveler: ASTM C1708/C1708M-26 gives test methods, so every strength on a data sheet is one product's value. ARDEX K 15 (TDS 2026-04-02) reports 37.9 MPa (5,500 psi) at 28 days by ASTM C109/mod with air cure, while LevelQuik RS reports more than 29.6 MPa (4,300 psi) by ASTM C1708, so the two figures come from different tests and are not ranked. In the EU, EN 13813:2002 asks the maker to declare a compressive class from C5 to C80 and a flexural class from F1 to F50 (N/mm²), so two products declared CT-C30-F7 meet the same class thresholds.

Which Standards Cover Self-Leveling Underlayments in the US, EU and UK?#

Self-levelers answer to four documents, not one specification: the US tests them by ASTM C1708/C1708M-26 and installs them by ASTM F2873-24, the EU designates them EN 13813:2002 screed material, and BS 8204-7:2003 sets UK thickness. The table below sets the documents side by side with their edition, type and status.

ASTM C1708, F2873, F3191 and F710: what each US document covers#

Four ASTM documents govern self-leveler testing and installation in the US, and each covers a different step, as listed below.

• ASTM C1708/C1708M-26: laboratory tests of fresh and hardened properties; scope section 1.3 warns that wet cementitious mixtures are caustic and can cause chemical burns. Editions -25, -23, -19 and older are superseded.
• ASTM F2873-24: installation, including priming over wood, concrete and bonded existing floors to receive resilient flooring; it does not address structural adequacy.
• ASTM F3191-26: classes a substrate as porous or non-porous by water absorption, which decides the primer.
• ASTM F710-26: preparing concrete floors to receive resilient flooring.

EN 13813 CT and CA designations for leveling compounds#

EN 13813:2002 treats a cement-based leveling compound as screed material with the binder code CT (CA for calcium sulfate), and compressive (C) and flexural (F) classes are mandatory, written as a designation such as CT-C20-F4. A polymer-modified product may add a bond class, as in CT-C40-F10-B1,5.

Shrinkage is tested by EN 13872 where the layer is under 10 mm (0.39 in), and consistency by EN 12706 where the flow exceeds 300 mm (11.8 in). EN 13813:2002 covers internal use, excludes structural screeds and assigns AVCP system 4 to products that need no reaction-to-fire testing, with systems 3 and 1 for the footnoted fire classes. The marking carries the designation, quantity, date and shelf life, batch and maximum thickness. Regulation (EU) 2024/3110 applies from 2026-01-08, and harmonised standards cited under Regulation (EU) No 305/2011 keep applying until a product family moves under a 2024/3110 act, so CE marking continues on this edition; the full class lists are explained under EN 13813 screed materials and floor screeds.

BS 8204-7 minimum thickness for self-smoothing screeds#

BS 8204-7:2003 requires a cement-based bonded self-smoothing screed to be at least 3 mm (0.12 in) thick at any point. The code names 6 mm (0.24 in) for industrial use and at least 8 mm (0.31 in) nominal to level a slab. Published on 2003-03-12 and confirmed in December 2011, it also covers pumpable calcium sulfate screeds, which start at 25 mm (0.98 in) bonded.

How do you check a self-leveler claim on a data sheet or declaration of performance?#

Four checks confirm a self-leveler claim, whichever brand is asked about, and they are listed below.

1. Read the test method next to each value: ASTM C109/mod and ASTM C1708 strengths come from different specimens.
2. Read the temperature next to each time: times without a temperature cannot be compared.
3. Read the EU designation: a CT-Cxx-Fx designation on the declaration of performance is the declared class under the EU Construction Products Regulation.
4. Confirm the edition: ASTM C1708/C1708M-26 and EN 13813:2002 are current; C1708-19 is not.

How Do You Choose a Self-Leveling Underlayment or Topping?#

Choose by what goes on top and below: an underlayment under tile, resilient or wood flooring, a topping as the wear surface, and another class where depth, a wood frame or slab moisture rules a cement leveler out. The table below pairs six situations with the product that fits and the named limit behind it.

The applicator follows from the job size: small residential pours are bag-applied, while commercial pumped installs go to trained contractors. Ardex states that its underlayments are intended for professional contractors and are not sold through home centers.

When is a self-leveler the wrong product?#

Five situations call for another product class, because a leveler corrects flatness and nothing beneath it, as listed below.

• A sunken or settled slab: the slab is raised by polyurethane foam slab lifting, not filled over.
• Moving joints: joints are kept open and filled with joint fillers for industrial floors; "self-leveling caulk" is a sealant, not a leveler.
• Cracks only: isolated cracks take concrete crack fillers before or instead of a full layer.
• High slab moisture: RH above the covering maker's limit calls for moisture mitigation systems.
• Structural spalls and section loss: these go to concrete repair products and are rebuilt before any leveling.

How Is Self-Leveling Concrete Applied? Preparation, Thickness, Coverage and Cure#

A self-leveler bonds only as well as its substrate allows, so the specification fixes five things before the pour: mechanical preparation, porosity and primer, slab moisture, thickness, and the temperature window through drying. Each of the five has its own subsection below, and the product values in them come from named data sheets.

Substrate preparation, porosity and primer selection#

Preparation is mechanical, by shot blasting or similar methods, and removes bond breakers such as curing compounds, sealers and old adhesives; acid etching, adhesive removers, solvents and sweeping compounds are excluded. The prepared slab needs a minimum tensile strength set by each data sheet, and the concrete surface profile each system requires (table below) is read against the ICRI chips described under concrete surface profile (CSP 1 to 10).

ASTM F3191-26 decides the primer by water absorption: a porous slab takes a diluted primer that seals the pores against pinholes, and a non-porous surface takes a primer that bonds to a closed face. Moving joints are honored, carried up through the leveler. The three primer paths are listed below, with one maker's ratios in the table after them.

• Porous concrete: an acrylic primer diluted with water; dilution by substrate is compared under primers for self-leveling underlayments.
• Extremely absorbent concrete: two primer coats, the first more dilute than the second.
• Non-porous surfaces: an epoxy-type primer, which can limit the leveler depth above it.

The table below lists substrate and primer requirements as stated by named products.

Slab moisture and vapor retarder checks before the pour#

Slab moisture is measured in place by ASTM F2170-19a relative humidity, and the limit comes from the flooring or adhesive maker, not from the test method. The RH reading is compared with the covering's limit and with the leveler's own moisture-control trigger, shown in the table below.

Where the reading exceeds the limit, a moisture mitigation membrane goes under the leveler. ASTM F3010-24 covers two-component membrane-forming systems and ASTM F3513-24 one-component fluid-applied systems, both at 0.10 perm or less by ASTM E96 Procedure B. The F2170-19a procedure and the ASTM F1869-23 calcium chloride alternative in concrete moisture testing are described under concrete moisture testing.

Self-leveler thickness and coverage per bag#

Self-leveler thickness and coverage come from each data sheet: ARDEX K 15 covers 2.79 m² per 24.9 kg bag at 6.4 mm (30 ft² per 55 lb bag at 1/4 in), theoretical, with a 6.4 mm (1/4 in) average minimum. Bags are counted on the average depth across the area, not on the minimum over the highest point, because the low spots take most of the material. BS 8204-7:2003 sets 3 mm (0.12 in) at any point as the UK floor for a bonded cement layer, while product minimums sit at or above it. The table below gives coverage per bag at stated depths from five data sheets, with each maker's basis.

Per-bag coverage as stated by each maker; not comparable across bases.

Leveling over wood subfloors: minimum depth and lath#

A cement self-leveler goes over a wood subfloor only with metal lath and at the minimum depth its data sheet sets for the subfloor thickness and joist spacing. ASTM F2873-24 covers priming over wood, while the depth and lath rules come from the product. Deflection is the risk: a wood floor that bends under load cracks or debonds a thin rigid layer, which is why the minimums rise as the subfloor gets thinner. The table below lists the minimums of one named product.

Temperature limits, working time and drying before floor coverings#

ARDEX K 15 needs a substrate of at least 10 °C (50 °F), radiant heat off from 48 hours before to 48 hours after pouring, and 6 hours before tile at 21.1 °C (70 °F). Other self-levelers set different windows, so the table below keys every working, walkable and covering time to its temperature and product. Moisture-sensitive coverings wait longest, because they react to the water still leaving the layer.

How Are Self-Leveling Floors Inspected, and Why Do They Fail?#

Self-leveler failures trace back to substrate, mix or timing: pinholes on unprimed porous slabs, delamination over bond breakers, weak foamy surfaces from overwatering, and covering failures over a wet or uncured layer. Pinholes form when air leaves an unsealed porous slab into the fresh mortar, and delamination follows when the leveler bonds to a curing compound, sealer or adhesive residue instead of to concrete. Ardex lists yellowish foam, sand settling and a weak surface as the signs of overwatering. Inspection also uses a straightedge for flatness, next to the checks in the table below.

Cracks telegraph through a leveler from moving joints and active cracks in the slab, because a thin rigid layer cannot bridge movement. Wood subfloors that deflect crack or debond the layer, and a cement leveler poured unprimed onto a calcium sulfate screed forms expansive ettringite at the interface. Cracks are therefore treated before leveling, as covered under concrete floor and slab crack repair. The table below links each of the seven failure modes to its cause and the check that catches it.

What Health, Safety and VOC Rules Apply to Self-Levelers?#

Cement-based self-levelers are alkaline when wet, and ASTM C1708/C1708M-26 warns that wet cementitious mixtures can cause chemical burns; dust while mixing and grinding, and the primers under the leveler, carry their own rules. The table below collects the classifications and rules by substance and instrument.

Cement hazards and chromium (VI)#

Portland cement (CAS 65997-15-1) has no Annex VI entry; ECHA C&L notified classification: H315, H317, H318, H335; see the product SDS. Calcium aluminate cement (CAS 65997-16-2): No Annex VI entry; no hazard statement in a majority of ECHA C&L notifications (H315, H318 42.5 % each; H319 38.3 %); classification is product-specific, see the product SDS. REACH Annex XVII entry 47 limits soluble chromium (VI) to 2 mg/kg (0.0002 %) of the total dry weight of the cement when hydrated, and packs with a reducing agent show the packing date and storage period, after which the agent loses effect. OSHA 29 CFR 1926.1126 excludes portland cement exposures in paragraph (a)(3), so cement dermatitis is handled through hazard communication, PPE and the SDS.

Respirable crystalline silica and dust during preparation and mixing#

Shot blasting and grinding the slab release respirable crystalline silica, regulated in US construction by OSHA 29 CFR 1926.1153 with a PEL of 50 µg/m³ and an action level of 25 µg/m³ (8-hour TWA). Table 1 of the standard requires floor grinders with integrated water delivery or a dust collector of at least 99 % filter efficiency, and the standard bars dry sweeping and compressed air for cleaning. The EU binding limit is 0.1 mg/m³ under Directive (EU) 2017/2398. Mixing raises dust as well, and Ardex specifies HEPA dust extraction while mixing ARDEX K 15.

Primer VOC limits and diisocyanate training#

Ardex states 0 g/L VOC for the ARDEX K 15 powder; VOC rules apply to the primers and coatings used with it. South Coast AQMD Rule 1113 (amended 2016-02-05) limits Primers, Sealers, and Undercoaters to 100 g/L (0.83 lb/gal) and Floor Coatings over a topping to 50 g/L (0.42 lb/gal). Rule 1168 (amended 2022-11-04) sets 250 g/L (2.09 lb/gal) for All Other Adhesive Primers, and it applies only where the primer is used under an adhesive. Outside the South Coast, the EPA AIM rule (40 CFR 59 subpart D) limits primers and undercoaters to 350 g/L (2.92 lb/gal) for coatings manufactured on or after 1999-09-13.

In the EU, Directive 2004/42/EC Phase II (since 2010-01-01) limits binding primers, category (h), to 30 g/L (0.25 lb/gal) water-based and 750 g/L (6.26 lb/gal) solvent-based, and primers, category (g), to 30 and 350 g/L (0.25 and 2.92 lb/gal), with Annex I scoping category (g) to wood, walls and ceilings. Polyurethane primers with diisocyanates at 0.1 % by weight or more require training before industrial or professional use since 2023-08-24, renewed at least every five years under REACH Annex XVII entry 74. The remaining categories are compared under VOC limits for construction chemicals.

How Much Does Self-Leveling Concrete Cost?#

Installed self-leveling costs USD 37.67 to 69.97 per m² (USD 3.50 to 6.50 per ft²) for a 6 mm (1/4 in) overlay in the US, per a Cary, Illinois contractor price page dated 2025-07-07. A US cost calculator page dated 2026 gives USD 32.29 to 75.35 per m² (USD 3 to 7 per ft²) installed, including primer, preparation and labor. Material only, a US moisture-meter maker's guide updated 2026-10-01 lists USD 30 to 60 per 22.7 kg (50 lb) bag (USD 1.32 to 2.65 per kg), with 1.9 to 2.8 m² (20 to 30 ft²) per bag at 6 mm (1/4 in). The two installed ranges measure different scopes and are not averaged; UK and EU jobs are priced by quote on area, depth, preparation and primer. The table below lists the three dated US ranges.

Installed and material prices from US contractor and cost-guide pages.

What Are the Alternatives to Self-Leveling Underlayments?#

Five product classes compete with a cement self-leveler, and each wins where depth, a wood frame, slab moisture, a repair or an exposed finish rules it out. A gypsum underlayment wins over a wood frame because hemihydrate rehydrates to gypsum, and its mass and non-combustibility add fire resistance and sound attenuation. An under-slab vapor retarder and a decorative microcement work below or instead of a leveler in narrower cases. The table below compares the seven options by binder, use and governing limit.

Self-Leveler Grades, Packaging and Sourcing#

Self-levelers are sold as powders in 20 kg (44.1 lb), 22.68 kg (50 lb) and 25 kg (55 lb) bags, in underlayment, topping and exterior grades, and buyers compare test method, strength, flow, working time, covering times and primer. A two-component latex leveler arrives as powder plus a latex liquid. The packing date matters, because chromium (VI) reducing agents lose effect after the storage period. Data sheets and maker lists are compared under construction chemicals manufacturers and suppliers. The five items a buyer checks on every offer are listed below.

1. Confirm the test basis: ASTM C1708 values in the US or an EN 13813:2002 CT or CA designation in the EU.
2. Read strength with its method: 28-day compressive and flexural strength with the test named.
3. Compare flow and healing time: fresh properties decide crew size on a pumped pour.
4. Check covering times with their temperature: tile, sensitive coverings and coatings.
5. Request the documents: data sheet, SDS, EU declaration of performance, primer system, packing date and storage period.

Where Does a Self-Leveler Fit in a Floor System?#

A self-leveler is the flatness layer of a floor build-up, on a prepared, primed and dry substrate and below the covering, adhesive or coating, so its defects start beneath it or show above it. The layer beneath sets bond, porosity and moisture, and the layer above sets the covering time and the moisture limit the leveler must meet.

Screed drying accelerators and similar mix additions belong to concrete additives, not covered here.

Self-leveling concrete FAQs#

Can self-leveling concrete be the finished floor?#

Only when the product is sold as a topping: underlayments are not wear surfaces, while toppings take a sealer, polish or coating at the times listed in the cure table.

Sealer types for an exposed topping are compared under concrete sealers.

Can self-leveling concrete be used outdoors?#

Only an exterior-grade topping qualifies: EN 13813:2002 covers internal screeds, and one exterior topping (ARDEX K 301) reaches 29.6 MPa (4,300 psi) at 28 days.

Interior underlayments are not made for frost and weather.

Can you tile directly on self-leveling compound?#

Yes, after the product's tile time: ARDEX K 15 takes moisture-insensitive tile after 6 hours and Fortane SL-HF ceramic after 2 to 3 hours, both at 21.1 °C (70 °F).

Moisture-sensitive coverings wait longer.

Is self-leveling compound the same as a floor screed?#

No: a self-leveler is a thin smoothing layer, while a floor screed is a thicker leveling layer, and a pumpable calcium sulfate screed is at least 25 mm (0.98 in) bonded under BS 8204-7:2003.

Screed types and thicknesses are compared under floor screeds.

What happens if a self-leveler gets too much water?#

Too much water leaves yellowish foam, settled sand and a weak surface, so the water per bag is measured and flow is checked with a ring or cup at the pump.

Is self-leveling compound waterproof?#

No: a self-leveler is not a vapor barrier, so slab moisture is controlled below it.

How many bags of self-leveler does a floor need?#

Bag count equals the floor area divided by the product's stated coverage per bag at the planned average depth, rounded up.

The self-leveling compound calculator works this out from area, depth and the chosen data sheet.