Koster SL FLOW - Fast Setting, Thin Layer Self-Leveling Underlayment - 50 lb Bag - SL 281 022

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Fast setting, thin layer, cement based self-leveling underlayment.


KOSTER SL Flow is a high quality, fast setting, thin layer application underlayment that is non-shrinking and provides a smooth, level surface ready to receive flooring systems. It may be applied onto a variety of substrates such as the KOSTER VAP I® 2000 coatings, concrete, etc. KOSTER SL Flow accepts all major floor coverings and is compatible with most adhesives.

Technical Data

Compressive Strength: >5500 psi at 28 days
Thickness: min. 1/16 in - 1 in
Working time: Approx. 25-30 min at 70 Degrees F
Initial Set: Approx 25-35 mins
Walkable: Approx. 3-4 hr
Install Flooring: Approx. 16 hr


Fields of Application

KOSTER SL Flow is designed for thin layer levelling unevenness of plain concrete surfaces as well as surfaces covered with KOSTER SL Primer and KOSTER VAP I® 2000 moisture control coatings. It is intended for interior use.


Substrates to receive KOSTER SL Flow must be sound, clean, and free of curing compounds, oil, grease, paint, loose particles, or any contaminant that may weaken bond. Do not acid etch substrates. Do not clean concrete substrate with chemicals. May be applied over traces of adhesive residue. Do not bridge movement joints with KOSTER SL Flow. Environmental temperature must be 60 - 90°F. For use directly on concrete: Concrete moisture must be less than 90% RH.

Test concrete moisture according to ASTM F2170. If concrete moisture exceeds 90% RH, install KOSTER VAP I® 2000 moisture control coating before installing KOSTER SL Flow. For direct application to concrete, the concrete surface must be absorptive. Dense, hard-troweled concrete, or concrete with residual curing compounds must be mechanically abraded to minimum ICRI CSP3 until water is easily absorbed. Clean the surface thoroughly by sweeping and vacuuming to remove dust. Prime the cleaned concrete substrate using KOSTER SL Primer. Apply a thin, even coat of primer at 400 - 800 sq ft/gal. Thick application may cause bonding problems. Allow to dry to a translucent green film, usually 30 min to 1 hr at 70°F and 50% RH. High humidity or low temperature will extend drying time. Keep the primed surface clean until the underlayment is installed. Install KOSTER SL Flow within 48 hours after primer application.

For use on KOSTER VAP I® 2000 systems: Prime clean, cured KOSTER VAP I® 2000 coating with undiluted KOSTER VAP I® 06 Primer. Apply a thin, even coat at 500-600 sq ft/gal using a short-nap roller. Ensure thin, uniform, complete coverage. Thick applications may cause bonding problems. Allow to dry to a transparent blue film, usually 1 - 2 hr at 70°F and 50% RH. High humidity or low temperature will extend drying time. DO NOT wear metal spike shoes when walking over cured KOSTER VAP I® 2000 coating to avoid damage to the coating.


Mix two bags of KOSTER SL Flow at a time. Pour 12-13 quarts (11.35-12.3L) of cool, clean potable water into a clean mixing drum. Add KOSTER SL Flow while mixing using an electrical mixer (600-700 rpm) with a helical mixing paddle. After all powder has been added into the mixing water, mix for 2-3 minutes. The resulting consistency must be lump free. Do not retemper.

Pour the mixed KOSTER SL Flow on the floor. Spread using a hand trowel, a smoother, and/or a magic trowel. Wear cleated shoes (not metal spikes) to avoid damage to cured KOSTER VAP I 2000 moisture control coating. A minimum thickness of 1/8-in should be maintained in wide open areas. Protect from freezing for 5-7 days. Do not use fans to blow air directly on fresh KOSTER SL Flow in an attempt to force dry the product.

Approx. 1/8 in-50 sqft/bag

Clean mixing equipment thoroughly with water and stiff scrub brush to avoid getting hardened product into subsequent batches. Keep mixing bucket clean.

50 lb bag (SL 281 022)

Store in a cool, dry place. When stored properly in original, unopened bag and pallet, shelf life is 9 months.

Consult Safety Data Sheet. Contains portland cement and silica sand; avoid contact with eyes or prolonged contact with skin. Wear personal protective equipment including gloves and safety eyewear. Do not take internally. Eliminate exposure and avoid breathing dust; contains free silica. In case of eye contact, flush immediately with clean water and consult a physician. In case of skin contact, wash immediately with soap and water.

The information contained in this technical data sheet is based on the results of our research and on our practical experience in the field. All given test data are average values which have been obtained under defined

conditions. The installer is responsible for the correct application taking into consideration the specific conditions of the construction site and the final results of the construction process. This may require adjustments

to the recommendations given here for standard cases. Specifications made by our employees or representatives which deviate from the specifications contained in any Company literature may not be relied upon in

the absence of written confirmation from the Company. The installer must comply with all testing, technical requirement, guidelines, and industry customs at all times. The terms, conditions, and limitations contained

in the written warranty for the product controls over the specifications contained herein. This guideline has been technically revised; all previous versions are invalid.