+---
+title: Cementitious Self-Leveling Underlayment
+category: Architectural / Finishes
+toc_depth: 3
+description: >
+ When to use: Portland-cement-based, polymer-modified self-leveling floor toppings applied over concrete, wood, or existing resilient substrates to produce a flat, smooth, level surface in preparation for finish flooring; new construction and renovation in commercial, institutional, multifamily, and light industrial work where tolerances finer than FF 25 (3/16 in. in 10 ft) are required; bonded and unbonded pours from feather-edge to 3 in., primer selection, substrate preparation, moisture mitigation, and encapsulation of in-floor radiant heating elements.
+ Not intended for: finish resilient flooring such as LVT, VCT, and sheet vinyl ([[sync/resilient-flooring]]); carpet ([[sync/carpet]]); ceramic and porcelain tile and bonded mortar beds ([[sync/ceramic-tile]]); epoxy or urethane resinous flooring topcoats ([[sync/epoxy-flooring]]); gypsum/calcium-sulfate self-leveling underlayments (a distinct binder category, explicitly excluded here); structural concrete topping slabs and composite deck toppings; and standalone substrate grinding or shot-blasting.
+---
+
+# Scope {toc}
+
+## This standard covers the supply and installation of Portland-cement-based, polymer-modified self-leveling underlayment (SLU) applied over concrete, wood, or existing resilient substrates to achieve a flat, smooth, level surface ready to receive finish flooring. {note}
+
+## Self-leveling underlayment is a flowable, polymer-modified cementitious mortar that seeks its own level under gravity, correcting local depressions, slab curl, trowel ridges, and out-of-flat conditions that would telegraph through thin or large-format finish flooring. It is a preparation layer, not a wear surface or a finish in its own right. {note}
+
+## The work shall include primer, the underlayment, moisture testing of the substrate, substrate preparation to the required surface profile, and any topical moisture mitigation, primer, and edge or joint treatment necessary to deliver a sound, bonded, flat surface to the finish-flooring installer. {note}
+
+## SLU is the appropriate remedy when measured substrate flatness is coarser than the finish flooring can tolerate. Resilient and large-format finishes commonly require FF 35 or better, which existing slabs and renovation substrates rarely meet without correction. {note}
+
+## Underlayment shall be Portland-cement-based; gypsum-based and calcium-sulfate-based self-leveling underlayments shall not be substituted for the work of this standard.
+
+## Gypsum (calcium-sulfate) underlayments are a separate product category with different binder chemistry, lower compressive-strength ceilings, and wet-area and ceramic-tile exclusions; they are deliberately out of scope so that a wet-area or high-load substitution cannot slip in. {note}
+
+## Finish resilient flooring (LVT, VCT, sheet vinyl) is specified in [[sync/resilient-flooring]]; carpet over pad or direct-glue in [[sync/carpet]]; ceramic and porcelain tile and bonded thick-set mortar beds in [[sync/ceramic-tile]]; and epoxy and urethane resinous flooring topcoats in [[sync/epoxy-flooring]]. SLU may precede an epoxy topcoat, but the topcoat itself is governed by that standard. {note}
+
+## The following work is outside the scope of this standard: {note}
+
+- Structural concrete topping slabs, composite deck toppings, and other thicker structural assemblies governed by concrete design codes.
+- Standalone surface remediation by grinding or shot-blasting, performed as substrate preparation equipment work rather than as a poured underlayment, except where invoked here as a profile-preparation step.
+
+# Referenced Standards {toc}
+
+## Materials, testing, and installation shall comply with the latest adopted edition of each of the following unless a specific edition is cited.
+
+## Where referenced standards conflict, the more stringent requirement shall govern unless the Engineer of Record directs otherwise in writing.
+
+| Standard | Title |
+|----------|-------|
+| ASTM C1708/C1708M-25 | Test Methods for Self-Leveling Mortars Containing Hydraulic Cements |
+| ASTM C109/C109M | Test Method for Compressive Strength of Hydraulic Cement Mortars (2 in. Cube Specimens) |
+| ASTM C1583/C1583M-20 | Tensile Strength of Concrete Surfaces and Bond Strength of Repair and Overlay Materials (Pull-Off Method) |
+| ASTM F1869 | Measuring Moisture Vapor Emission Rate of Concrete Subfloor Using Anhydrous Calcium Chloride |
+| ASTM F2170 | Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes |
+| ASTM C150/C150M | Standard Specification for Portland Cement |
+| ASTM C1315 | Liquid Membrane-Forming Compounds Having Special Properties for Curing and Sealing Concrete |
+| ASTM F710 | Preparing Concrete Floors to Receive Resilient Flooring |
+| ASTM E1155 | Determining FF Floor Flatness and FL Floor Levelness Numbers |
+| ACI 302.1R | Guide to Concrete Floor and Slab Construction |
+| ICRI 310.2R | Selecting and Specifying Concrete Surface Preparation for Sealers, Coatings, Polymer Overlays, and Concrete Repair |
+
+# Submittals {toc}
+
+## Action submittals establish that the proposed product, primer, and accessories are an approved, system-matched set before any material reaches the site. {note}
+
+## The Contractor shall submit the following action submittals for review:
+
+- Product data sheets for the underlayment, the primer, and any moisture-mitigation product, from a single manufacturer's matched system.
+- Manufacturer's published technical data confirming compressive strength, flexural strength, flow, set time, foot-traffic time, and flooring-ready time.
+- Mix-design and water-ratio range for each product, with the rapid-set vs. standard-set classification identified.
+- A statement of the maximum single-pour thickness and any aggregate-extension method proposed for deep fills.
+- Compatibility confirmation between the proposed SLU system and the scheduled finish flooring, signed by the underlayment manufacturer.
+
+```datasheet
+label: Action submittals required
+type: checkbox
+options:
+ - Underlayment product data
+ - Primer product data
+ - Moisture-mitigation product data
+ - Manufacturer technical/strength data
+ - Mix design and water-ratio range
+ - Maximum pour thickness / aggregate method
+ - Finish-flooring compatibility statement
+default:
+ - Underlayment product data
+ - Primer product data
+ - Manufacturer technical/strength data
+ - Finish-flooring compatibility statement
+```
+
+## Informational submittals document the field conditions and qualifications that govern whether the underlayment may proceed. {note}
+
+## The Contractor shall submit the following informational submittals:
+
+- Substrate moisture test reports per ASTM F2170 and ASTM F1869, identifying which method governs acceptance.
+- Substrate pH test results.
+- Documentation of the achieved concrete surface profile (CSP) per ICRI 310.2R.
+- Installer qualification records demonstrating prior cementitious SLU experience.
+- Manufacturer field-technician inspection report, where the project requires manufacturer presence at the first pour.
+
+```datasheet
+label: Informational submittals required
+type: checkbox
+options:
+ - ASTM F2170 RH test report
+ - ASTM F1869 MVER test report
+ - Substrate pH test results
+ - CSP documentation per ICRI 310.2R
+ - Installer qualification records
+ - Manufacturer field inspection report
+default:
+ - ASTM F2170 RH test report
+ - ASTM F1869 MVER test report
+ - Substrate pH test results
+ - CSP documentation per ICRI 310.2R
+```
+
+## Closeout submittals transfer the as-installed record and any warranty obligations to the Owner. {note}
+
+## The Contractor shall submit the following closeout submittals:
+
+- Field bond pull-off test results per ASTM C1583.
+- As-installed record of pour locations, thicknesses, and any joints saw-cut or tooled into the underlayment.
+- Manufacturer and installer warranties.
+
+```datasheet
+label: Closeout submittals required
+type: checkbox
+options:
+ - ASTM C1583 bond test results
+ - As-installed pour/thickness record
+ - Manufacturer warranty
+ - Installer warranty
+default:
+ - ASTM C1583 bond test results
+ - As-installed pour/thickness record
+ - Manufacturer warranty
+```
+
+# Quality Assurance {toc}
+
+## The single-manufacturer, system-matched rule is the central quality requirement for SLU. Primers and underlayments are formulated and tested together; mixing a primer from one manufacturer with an underlayment from another voids the published bond data and is a leading cause of delamination. {note}
+
+## The primer, underlayment, and any moisture-mitigation product shall be supplied by a single manufacturer as a matched system.
+
+## A primer system shall be specified for every application; specifying SLU without naming its primer is incomplete and shall not be permitted.
+
+## The installer shall have demonstrated prior experience placing cementitious self-leveling underlayment on projects of comparable scope.
+
+## A pre-installation conference shall be held with the Contractor, the finish-flooring installer, and, where radiant heating is present, the heating-system installer, to coordinate moisture acceptance, joint treatment, and the radiant-heat off/on schedule.
+
+## A test pour or mock-up shall be placed where the project requires verification of color, flow, finish, and flooring-ready time before full-scale work.
+
+## Moisture testing is the single most important acceptance gate. The most common cause of SLU and finish-flooring failure is placing material over a substrate that is still releasing moisture above the limit the finish system can tolerate. {note}
+
+## Both ASTM F2170 in-situ relative humidity testing and ASTM F1869 calcium-chloride moisture-vapor-emission testing shall be performed on concrete substrates.
+
+## The project shall state which moisture test method governs acceptance.
+
+## ASTM F2170 in-situ relative humidity is generally preferred over ASTM F1869 for predicting post-installation moisture behavior because the probe measures deep-slab humidity rather than only surface emission. {note}
+
+## Substrate pH shall be tested before priming; a pH below 7 indicates contamination and shall be investigated and corrected before proceeding.
+
+```datasheet
+label: Governing concrete moisture test method
+type: radio
+options:
+ - ASTM F2170 in-situ RH governs
+ - ASTM F1869 calcium-chloride MVER governs
+ - Both required, more stringent governs
+default: ASTM F2170 in-situ RH governs
+```
+
+# Environmental and Service Conditions {toc}
+
+## SLU is sensitive to temperature, humidity, and ventilation during placement and cure. Material placed below the minimum temperature in an unheated building is a recurring cold-weather failure mode. {note}
+
+## Air and substrate temperature shall be maintained within the manufacturer's published range for the duration of placement and the initial cure.
+
+## The substrate temperature shall not be allowed to fall below the minimum within 24 hr after placement.
+
+## Ambient relative humidity shall be maintained within the manufacturer's published range.
+
+## High RH slows set and low RH accelerates set and may cause surface cracking. {note}
+
+## Direct drafts, forced heating discharge, and direct sunlight shall be controlled across freshly placed underlayment to prevent uneven set and plastic cracking.
+
+```datasheet
+label: Minimum air and substrate temperature during placement and cure
+type: range
+unit: °F
+min: 45
+max: 60
+step: 5
+default: 50
+```
+
+```datasheet
+label: Maximum air and substrate temperature during placement and cure
+type: range
+unit: °F
+min: 80
+max: 95
+step: 5
+default: 85
+```
+
+```datasheet
+label: Ambient relative humidity range during placement
+type: range
+unit: "%"
+min: 30
+max: 70
+step: 5
+default: 50
+```
+
+# Substrate Preparation {toc}
+
+## Bond is created or destroyed at the substrate interface. Laitance, curing compounds, adhesives, sealers, paint, and other bond breakers must be removed, and the surface must be opened to a defined profile, before any primer is applied. {note}
+
+## The concrete surface profile shall be prepared to ICRI 310.2R CSP 1 through CSP 3 by mechanical means such as shot-blasting or scarifying as required by the product.
+
+## Laitance, bond breakers, curing compounds, sealers, paint, adhesive residue, and other contaminants shall be fully removed before priming.
+
+## Curing compounds complying with ASTM C1315, and similar membrane-forming compounds on existing slabs, inhibit bond and shall be mechanically removed.
+
+## Existing control, construction, and movement joints shall be honored and shall not be bridged blindly with a continuous pour; SLU placed across a moving joint will crack at the joint. {note}
+
+## Active control and movement joints shall be carried up through the underlayment by saw-cutting or tooling the joint directly above the substrate joint after cure.
+
+## Cracks and large voids shall be repaired and dormant cracks treated per manufacturer instructions before the leveling pour.
+
+## Wood subfloors require additional restraint and crack-resistance measures because they deflect and move more than concrete; a fiber-reinforced product and a mechanically fastened reinforcement may be required. {note}
+
+## Over wood subfloors, the underlayment, reinforcement, and primer shall be a system rated by the manufacturer for wood substrates.
+
+```datasheet
+label: Substrate type
+type: radio
+options:
+ - Concrete slab-on-grade
+ - Concrete over metal deck
+ - Wood subfloor (plywood or OSB)
+ - Existing resilient or tile surface
+default: Concrete slab-on-grade
+```
+
+```datasheet
+label: Required concrete surface profile (ICRI 310.2R)
+type: select
+options:
+ - CSP 1
+ - CSP 2
+ - CSP 3
+default: CSP 2
+```
+
+```datasheet
+label: Acceptable substrate pH range before priming
+type: range
+min: 7
+max: 11
+step: 1
+default: 9
+```
+
+# Moisture Mitigation {toc}
+
+## Where substrate moisture exceeds the limit for an unmitigated pour, a topical moisture-vapor barrier or reducer is applied to the prepared substrate before the underlayment. The decision is driven by the measured MVER or RH, not assumed. {note}
+
+## Concrete substrate moisture shall be within the limits below before the underlayment is placed, or a manufacturer-matched moisture-mitigation product shall be applied.
+
+## The moisture-mitigation product, where used, shall be part of the same manufacturer's matched system as the primer and underlayment.
+
+## Rapid-set products are not moisture-neutral; their accelerated set does not eliminate residual slab moisture beneath the underlayment. {note}
+
+## The underlayment surface shall still be moisture-tested against the finish-flooring manufacturer's limits before the finish flooring is installed.
+
+```datasheet
+label: ASTM F1869 MVER limit before underlayment (no mitigation)
+type: range
+unit: lb/1000 ft²/24 hr
+min: 3
+max: 5
+step: 0.5
+default: 3
+```
+
+```datasheet
+label: ASTM F2170 in-situ RH limit before underlayment
+type: range
+unit: "%"
+min: 80
+max: 90
+step: 1
+default: 80
+```
+
+```datasheet
+label: Topical moisture mitigation
+type: radio
+options:
+ - None required (substrate within limits)
+ - Moisture-vapor reducer primer
+ - Epoxy moisture-vapor barrier
+default: None required (substrate within limits)
+```
+
+# Primer {toc}
+
+## The primer bonds the underlayment to the substrate, controls porosity so the slab does not pull water out of the fresh mix, and suppresses pinholing and outgassing. Primer type is matched to the substrate condition and moisture result. {note}
+
+## The primer shall be the type specified by the underlayment manufacturer for the measured substrate condition.
+
+## A latex bonding primer shall be used over sound, dry concrete within moisture limits.
+
+## An epoxy primer shall be used where the slab is porous, dusty, or moisture-compromised within the system's published tolerance.
+
+## The underlayment shall be placed within the primer's open window; if the primer dries beyond the manufacturer's reuse limit, it shall be re-primed.
+
+## Primer dry time and open window vary by primer type and ambient conditions; a primer placed too wet or left open too long both defeat the bond. {note}
+
+```datasheet
+label: Primer type
+type: radio
+options:
+ - Latex bonding primer
+ - Epoxy primer (porous/moisture-compromised slab)
+ - Manufacturer no-primer system
+default: Latex bonding primer
+```
+
+```datasheet
+label: Maximum elapsed time between priming and underlayment placement
+type: range
+unit: hr
+min: 4
+max: 24
+step: 2
+default: 8
+```
+
+# Material Properties {toc}
+
+## The compressive strength of the underlayment shall be matched to the load and the finish system above it. A product acceptable under carpet is undersized under large-format tile or a warehouse epoxy topping. {note}
+
+## Compressive strength shall be selected for the service load and finish system and shall be verified per ASTM C109 at 28 days.
+
+## The standard commercial default is 3,500 psi at 28 days; light residential and low-traffic areas may use 2,000 psi; heavy-commercial and light-industrial floors and large-format tile substrates shall use 4,500 psi or higher.
+
+## Flow, flow retention, healing time, compressive strength, flexural strength, and setting time shall be established per ASTM C1708.
+
+## Over-watering is the top cause of surface dusting and reduced strength. {note}
+
+## Field water additions beyond the manufacturer's published range shall not be permitted.
+
+```datasheet
+label: Compressive strength class at 28 days (ASTM C109)
+type: radio
+options:
+ - 2000 psi (residential / light, low traffic)
+ - 3500 psi (standard commercial)
+ - 4500 psi (heavy commercial / light industrial)
+default: 3500 psi (standard commercial)
+```
+
+```datasheet
+label: Product set classification
+type: radio
+options:
+ - Rapid-set (foot traffic 2-4 hr)
+ - Standard-set (flooring-ready 24 hr)
+default: Rapid-set (foot traffic 2-4 hr)
+```
+
+```datasheet
+label: Fiber reinforcement
+type: radio
+options:
+ - None (standard concrete substrate)
+ - Fiber-reinforced (wood subfloor / crack resistance)
+default: None (standard concrete substrate)
+```
+
+# Application Thickness and Flatness {toc}
+
+## SLU thickness ranges from a feather-edge repair fill to a deep single pour. Specifying a minimum without a maximum invites oversized pours that increase drying shrinkage and cracking risk, especially in renovation work with extreme voids. {note}
+
+## The underlayment shall be placed within the specified thickness range; both a minimum and a maximum single-pour thickness shall be stated.
+
+## Where required depth exceeds the product's single-pour maximum, the manufacturer's aggregate-extension method or successive bonded pours shall be used.
+
+## The feather-edge minimum and the deep-fill maximum are product-specific; the default specification range is 1/8 in. to 1 in., with feather-edge fills to 1/16 in. permitted for local repairs. {note}
+
+## The finished underlayment surface shall meet the specified flatness tolerance, verified with a 10 ft straightedge or per ASTM E1155.
+
+## Sensitive finishes such as LVT and large-format tile require FF 35 or better; FF 25 (3/16 in. in 10 ft) is the minimum acceptable result of the leveling work. {note}
+
+```datasheet
+label: Minimum single-pour thickness
+type: range
+unit: in
+min: 0.0625
+max: 0.25
+step: 0.0625
+default: 0.125
+```
+
+```datasheet
+label: Maximum single-pour thickness
+type: range
+unit: in
+min: 0.75
+max: 3
+step: 0.25
+default: 1
+```
+
+```datasheet
+label: Required finished flatness tolerance
+type: radio
+options:
+ - FF 25 (3/16 in. in 10 ft) minimum
+ - FF 35 (large-format tile / LVT)
+ - FF 50 (highly sensitive finishes)
+default: FF 35 (large-format tile / LVT)
+```
+
+# Radiant Heating Encapsulation {toc}
+
+## Where SLU encapsulates hydronic tubing or electric heating mats, the underlayment provides cover and thermal transfer over the elements. Coordination with the heating-system installer is mandatory because the system schedule constrains the pour. {note}
+
+## The underlayment shall provide a minimum cover of 3/4 in. (19 mm) over the top of any hydronic tube or electric heating element.
+
+## The radiant heating system shall be off for 48 hr before placement and shall not be energized for 72 hr after placement.
+
+## The heating-element layout is shown on the drawings; coordinate cover depth against the as-installed element elevation. [[drawing: radiant element layout]]
+
+## Because the radiant-heat spec usually lives in a separate division, the off/on schedule has no enforcement unless this underlayment work cross-references it; the pre-installation conference is where that coordination is fixed. {note}
+
+```datasheet
+label: Minimum underlayment cover over radiant element
+type: range
+unit: in
+min: 0.75
+max: 1.5
+step: 0.25
+default: 0.75
+```
+
+```datasheet
+label: Radiant heating present
+type: radio
+options:
+ - None
+ - Hydronic tubing
+ - Electric heating mat
+default: None
+```
+
+# Installation {toc}
+
+## Placement is continuous and fast; the material flows and self-levels within a short working window, so mixing, pumping, and spreading must be staged before the pour begins. {note}
+
+## The underlayment shall be mixed and placed strictly in accordance with the manufacturer's published procedure, water ratio, and working time.
+
+## Material shall be spread and gauged to the specified thickness and worked only as needed to assist flow; over-working a self-leveling mortar disrupts the surface.
+
+## Pours shall be planned so that wet edges are kept alive and cold joints within a continuous floor area are avoided.
+
+## Movement and control joints carried up from the substrate shall be saw-cut or tooled into the cured underlayment directly above the substrate joint.
+
+## The cured surface shall be sound, free of dusting, hollow areas, and surface defects, and shall be protected from traffic and trades until it reaches foot-traffic strength.
+
+## Foot traffic shall not be permitted until the underlayment reaches the manufacturer's published foot-traffic time.
+
+## Finish flooring shall not be installed until the underlayment reaches the manufacturer's flooring-ready time and the surface passes the finish-flooring manufacturer's moisture limit.
+
+```datasheet
+label: Foot-traffic time
+type: range
+unit: hr
+min: 2
+max: 12
+step: 1
+default: 4
+```
+
+```datasheet
+label: Flooring-ready time
+type: range
+unit: hr
+min: 4
+max: 24
+step: 2
+default: 4
+```
+
+# Field Quality Control {toc}
+
+## Field acceptance turns on bond. A direct-tension pull-off test confirms that the underlayment is mechanically anchored to the substrate and is the field counterpart to the manufacturer's factory bond data. {note}
+
+## Bond strength shall be verified by direct-tension pull-off test per ASTM C1583, with a minimum of 200 psi (1.4 MPa) at 28 days between the underlayment and the substrate.
+
+## Hollow or drummy areas detected by sounding shall be removed and re-placed.
+
+## The finished flatness shall be verified against the specified tolerance and documented before finish-flooring installation.
+
+```datasheet
+label: Minimum bond pull-off strength (ASTM C1583, 28 day)
+type: range
+unit: psi
+min: 150
+max: 300
+step: 25
+default: 200
+```
+
+# Delivery, Storage, and Handling {toc}
+
+## Cementitious underlayment and its primers are moisture- and temperature-sensitive and have a defined shelf life; material that has absorbed moisture or frozen will not perform. {note}
+
+## Material shall be delivered in the manufacturer's original, sealed, labeled packaging.
+
+## Material shall be stored off the floor, under cover, in a dry, conditioned space within the manufacturer's temperature range, and used within its shelf life.
+
+## Material that is lumped, partially set, frozen, or beyond its shelf life shall be rejected.
+
+# Warranty {toc}
+
+## A coordinated warranty covers both the material and its installation so that a bond or finish failure does not fall into a gap between trades. {note}
+
+## The manufacturer shall warrant the underlayment system, and the installer shall warrant the installation, for the periods specified by the project.
+
+```datasheet
+label: Installer warranty period
+type: range
+unit: year
+min: 1
+max: 5
+step: 1
+default: 1
+```