Resilient Flooring

Self-Leveling Underlayment: When and How

Profile, primer, water, depth and cure time for self-leveling underlayment, including metal lath over wood subfloors.

Updated · 2 min read

A self-leveling underlayment (SLU) fixes waves and widespread low areas that patching can't handle. It flows out flat on its own, but only if the substrate is prepared, primed and mixed exactly as the manufacturer specifies. Most SLU failures trace back to prep, primer or mix.

When to use it

Reach for an SLU when the problem is bigger than a skim coat can fix.

  • The floor fails a flatness check over large areas. See Subfloor flatness standards.
  • Low spots are too deep for a patch or skim coat.
  • You need a smooth, uniform surface for thin resilient flooring.

Prepare and prime

SLUs need a clean, open surface to bond to.

  • Custom LevelQuik RS calls for smooth concrete to be roughened to ICRI CSP 3.[3] See ICRI CSP profiles.
  • Remove anything that blocks bonding: old adhesive, curing compounds, sealers, dust.
  • Prime every surface, and double-prime gypsum.[3] ARDEX K 15 uses P 51 mixed 1:1 on concrete and P 82 on wood and non-absorbent surfaces.[2]
  • Dam off drains, doorways and gaps so the material stays where you want it.

Over wood subfloors

Wood needs reinforcement, and manufacturers specify slightly different lath.

  • ARDEX K 15 calls for galvanized diamond metal lath (3.4 lb) stapled every 6 inches over at least 3/4" tongue-and-groove plywood.[2]
  • Custom LevelQuik RS calls for 2.5 lb/yd² metal lath fastened every 6 to 8 inches.[3]
  • Use the lath and minimum thickness in the product you're pouring.

Mix and pour

Water and mixing are the two things you control most directly.

ARDEX K 15[1][2] Custom LevelQuik RS[3]
Water per bag 7 quarts 5.25 to 5.75 quarts per 50 lb
Mixing Per data sheet 1/2" drill, minimum 650 RPM, 2 minutes
Depth Up to 1 1/2" neat; 5" with aggregate Featheredge to 1.5" in one pour
Minimum Data sheet: average minimum thickness 1/4". CSI spec: 1/8" over the high point Featheredge

The ARDEX data sheet and spec section list different minimums; confirm with ARDEX for your job.[1][2]

  1. Pre-measure water for every bag before you start so the batches stay consistent.
  2. Mix to the time and speed on the bag.
  3. Pour in a continuous ribbon, keeping a wet edge.
  4. Spread with a gauge rake set to your depth, then knock down ridges with a smoother. For ARDEX, use a smoother and cleated shoes with non-metallic spikes.[1]

Cure and temperature

SLUs are fast, but finished flooring still has to wait.

  • ARDEX K 15: tile after 6 hours; all other floor coverings after 16 hours at 70°F.[2]
  • LevelQuik RS: walkable in 2 to 4 hours; moisture-sensitive flooring after 16 hours or more.[3]
  • Temperature: ARDEX requires at least 50°F, held for 48 hours after.[1] Custom lists 50 to 90°F.[3]

Common mistakes

Most SLU problems start before the first bag is mixed.

  • Skipping primer. Pinholes, poor bond and fast drying follow.
  • Adding water for flow. It weakens the surface and can cause dusting and cracks.
  • Stopping mid-pour. A cold joint shows up as a ridge.
  • Pouring on a cold slab. Stay inside the manufacturer's temperature range.

Tools and products we carry

These are the leveler, primer and tools for pouring.

References

  1. ARDEX K 15 technical data sheet
  2. ARDEX K 15 specification section
  3. Custom Building Products LevelQuik RS (TDS-108)

Linked PDF documents are archived copies saved by PreFloor Tools so they stay available if the publisher moves or removes them. Check the publisher's website for the latest revision.

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