Self-Leveling Underlayment

What it is, and what it is not

Underlayment is a smoothing layer. You mix it thin, pour it out, help it along with a gauge rake, and it finds its own level. Flooring goes over the top. That is the whole job.

It is not a slab, not a wear surface, and not structural. It will not fix a floor that moves, because a leveler bonded to a deflecting subfloor cracks along with it, which is why several manufacturers specify a deflection limit of L/360 before they will stand behind the installation. It also will not fix moisture. A slab pushing vapor up needs a moisture mitigation system under the leveler, not a thicker pour. And it will not raise a slab that has sunk: a floor that is flat but sitting too low has a soil problem under it, which is concrete slab lifting territory rather than underlayment.

Two related products get calculated as levelers and are not:

• Ardex Feather Finish is a trowel applied finish underlayment in a 10 lb bag. Ardex tells you to apply it with the flat side of a steel trowel. Its published skim coat coverage is 100 to 300 square feet per bag, a range so wide it tells you the product is not being sized by volume at all.
• Mapei Planitop 330 Fast is described by Mapei as a rendering, patching and leveling mortar. Trowel applied, used on walls as well as floors, 1/8 inch to 1-1/4 inch in a coat.

If your floor needs a skim of a sixteenth of an inch in a few places, a patch product is cheaper and faster. If it needs a consistent eighth of an inch or more across an area, that is underlayment. If it needs two inches across a garage, look at a bonded concrete topping instead and price it with the concrete pad calculator.

Coverage per bag: a 55 percent spread

Bagged concrete is sold by cubic feet of yield. Underlayment is sold by square feet at a stated thickness, almost always a quarter inch, and only Quikrete publishes a cubic foot yield alongside it. So comparing products means comparing coverage, and the range is much wider than most people expect.

Between identical 50 lb bags at the same quarter inch depth, coverage runs from 21 to 32.5 square feet. Ardex Liquid BackerBoard is a lightweight formulation, which is where the extra area comes from, and on a per pound basis the field spans 0.42 to 0.65 square feet.

What that costs you in practice. A 200 square foot room at a quarter inch takes 10 bags of Quikrete No. 1249 or Sika Level-125, and 7 bags of Ardex Liquid BackerBoard. Three bags of difference on a small room, which is larger than the ten percent waste allowance most people add. Compare the price per bag between those two and the cheaper bag is often the more expensive floor.

Two cautions on the table. Sakrete and Rapid Set publish ranges rather than numbers, 25 to 30 and 24 to 30 square feet, so order against the low end: a user who buys against the optimistic end of a manufacturer's own range is 20 percent short with a floor already wet. And Henry publishes only the eighth inch figure, so the quarter inch equivalent of roughly 21.5 square feet is arithmetic, not a Henry number.

Bag sizes are not standard either

Fifty pounds is the most common bag and it is only about half the field, so a 50 lb assumption quietly breaks the math.

• 55 lb: Ardex K 15 (25 kg) and Sika Level-325 (24.9 kg). Two different 55 pounds, since the metric figures differ.
• 40 lb: Henry 555 LevelPro.
• 35 lb plus a gallon of latex liquid: Ardex K 60 ARDITEX, sold as a two component unit rather than a bag.
• 25 lb: Mapei Novoplan 2 Plus offers a small bag alongside its 50 lb.
• 10 lb: Ardex Feather Finish, which is a patch product, not a leveler.

One oddity worth knowing before you order in quantity: Laticrete prints "55LB (22.7KG)" on both the NXT Level Plus data sheet and the product page, and those figures contradict each other, because 22.7 kg is 50 lb. If your order size makes the difference matter, confirm it with Laticrete rather than with a retail listing.

Primer, and what fails without it

Primer is required by essentially every product here, and the data sheet language is more absolute than data sheets usually get. Mapei writes "always use a MAPEI primer". Sakrete writes that all surfaces require priming with Bonder and Fortifier. CTS writes to apply its primer to all substrate surfaces. CUSTOM writes "prime all surfaces". Laticrete lists four permitted primers and adds the word "only".

Laticrete's TDS 230 is the one document in the set that states the consequences outright: insufficient drying or poor film formation "will result in pinholes and poor bond strength and may cause self-leveling products to debond". Three failure modes, all of them expensive once flooring is down. The same guide gives a drying window, and a primer film that dries in under 30 minutes or takes more than 24 hours has failed in both directions.

Why the dilution changes with the substrate. Primer does two unrelated jobs. On absorbent concrete it blocks suction, so the slab cannot pull mix water out of the leveler before it hydrates, which is why very thirsty slabs get a double prime or a thin first coat followed by a stronger second. On non-absorbent surfaces, tile or epoxy or old cutback residue, there is no suction at all and the primer is there to give the leveler something to key into, so you switch products entirely: Ardex P 82, Sika Level-02 EZ, or a Laticrete primer with sand broadcast into it.

The direction reverses on wood. Wood takes primer undiluted, at full strength. Ardex says do not dilute P 51 over wood, Henry says 554 goes on 1 to 1 with water for concrete and undiluted for wood. People routinely get this backwards.

Three real exemptions exist across the whole field, and they are worth money:

• Ardex K 60 ARDITEX needs no primer for most applications, because the latex liquid component does the bonding job. Only non-Ardex epoxy substrates need P 82.
• Laticrete NXT Level Plus needs no primer over STRATA_MAT or STRATA_HEAT Mat.
• Mapei Ultraplan Easy needs no priming over Mapeheat Membrane, Mapeguard UM and similar crack isolation and uncoupling membranes.

Everywhere else, primer is a second purchase with its own coverage rate, and it is the single most skipped step in a self-leveling job.

Over plywood and wood subfloors

Most levelers allow wood, none allow it unconditionally, and two common products do not allow it at all.

Not approved over wood: Ardex K 15 lists concrete, terrazzo, metal, tile, epoxy and adhesive residue, and no wood anywhere, which is why Ardex routes wood work to Liquid BackerBoard or K 60. Rapid Set Concrete Leveler specifies clean, sound concrete. Mapei Ultraplan 1 Plus does not list plywood either and is limited to dry interior areas.

Approved with conditions: LevelQuik RS, Mapei Novoplan 2 Plus and Ultraplan Easy, Ardex Liquid BackerBoard, Ardex K 60, Laticrete NXT Level and NXT Level Plus, Sika Level-125 and Level-325, Henry 555, Sakrete, and Quikrete No. 1249. The conditions are where the detail lives:

• Plywood spec. The common baseline is 3/4 inch tongue and groove, APA rated Type 1, exterior exposure plywood, or an OSB equivalent. Sika is the strictest: two layers, 1-1/4 inch minimum total.
• Deflection. L/360 recurs across Sika, Mapei and Laticrete. Mapei also caps joist spacing at 19.2 inches on center.
• Metal lath. Quikrete requires expanded metal lath for attachment over wood floor systems. Sakrete states metal lath is required for all wood installations. Laticrete requires lath fastened every 6 inches, except with NXT Level Plus, which is that product's main reason to exist.
• Existing hardwood. Mapei wants nailed down wood flooring covered with at least one layer of 5/8 inch plywood, glued and screwed. Ardex allows solid, mechanically fastened hardwood directly.
• Interior only. Laticrete marks exterior glue plywood and OSB as interior use only. None of these products is an exterior surface.

Other approved substrates across the field are broadly the same list: concrete cured 28 days or more and meeting ASTM F710, existing ceramic, porcelain and quarry tile, cementitious and epoxy terrazzo, epoxy coatings, VCT, cement backer board, gypsum underlayments, steel and non-aluminum metal for some Ardex products, uncoupling membranes, and the manufacturer's own moisture mitigation systems.

Maximum pour depth, and the aggregate question

Maximum single lift spans nearly five times across these products, and the worst trap is inside one brand rather than between brands.

The Quikrete trap. Standard No. 1249 caps at five eighths of an inch and its data sheet says anything deeper goes on in layers no greater than five eighths each. The Fast-Setting version, sold under the same 1249 number with different suffixes, goes to an inch and a half in one pour. Same product name, nearly three times the depth. Read the suffix.

Aggregate extension is a different material. Ardex K 15, Rapid Set, Henry 555 and both Sika products only reach 3 to 5 inches by adding pea gravel or aggregate, and at that point the published coverage per bag no longer applies, because you are placing something else. Sika even raises its minimum thickness for extended mix from an eighth of an inch to a quarter, which confirms it behaves differently. Past roughly two inches, price a thin bonded concrete topping as an alternative using the slab calculator, because bagged underlayment at that depth gets expensive quickly.

Minimums matter too. Most products want at least an eighth of an inch over the high point of the floor. Mapei Ultraplan 1 Plus and Ultraplan Easy go down to a sixteenth, and LevelQuik RS and Quikrete feather to nothing. Under an eighth of an inch on average you are skim coating, and the trowel products are the right answer.

Walk-on and flooring times, including the gaps

The field clusters tightly at 2 to 3 hours for foot traffic, and the exceptions are the useful part.

• 2 to 3 hours: Ardex K 15, Ardex Liquid BackerBoard, Ardex K 60, all three Mapei levelers, Laticrete NXT Level and NXT Level Plus, Sika Level-125, Henry 555.
• 2 to 4 hours: LevelQuik RS, Rapid Set Concrete Leveler.
• 4 hours: Sika Level-325, notably slower than its sibling.
• 6 hours: Quikrete No. 1249 Fast-Setting, which also asks 24 to 48 hours before finished flooring.
• Not published: standard Quikrete No. 1249 gives a setting time of 10 hours or more and no foot traffic figure at all. Sakrete gives initial set at 40 to 70 minutes and final set at 1 to 1.5 hours, which is not the same thing as walkable, plus tile at 24 hours and sheet goods at 3 days.

Covering material is the other clock, and it runs longer than the walk-on time in every case. LevelQuik RS allows ceramic tile after 4 hours but moisture sensitive coverings after 16. Mapei allows tile and stone at 3 hours but rules out rolling dynamic loads for 3 days. Sakrete wants 3 days for sheet goods. If your flooring is vinyl, sheet goods or anything with a moisture sensitive adhesive, plan the job around that number rather than the walk-on figure. The same principle applies to concrete generally, covered in our concrete cure time guide.

Naming confusion worth clearing up

Four things come up constantly in searches, and three of them are simply not real.

• LevelQuik RS is not Quikrete No. 1249. LevelQuik is made by CUSTOM Building Products, which sits under The QUIKRETE Companies corporately, so they are siblings rather than the same thing. Separate data sheets, separate primers, 23 to 24 versus 21 square feet of coverage, an inch and a half versus five eighths of maximum depth. If you want ordinary Quikrete bagged concrete instead, that is the Quikrete calculator.
• There is no E5 catalyst in the Laticrete NXT system. The NXT levelers are powder only, and NXT Level's data sheet says never to mix it with cement or admixtures. E5 Nano Silica is a concrete admixture from an unrelated company, used for internal curing and moisture control in slabs.
• There is no SikaLevel-315. Sika's US line is retail SikaLevel plus professional Sika Level-125 and Level-325. The retail product and Level-125 publish identical figures, which suggests one formulation under two names.
• Ardex K 60 ARDITEX and Ardex Liquid BackerBoard are two products. K 60 is the 35 lb powder plus a gallon of latex, needs no primer on most substrates, and caps at half an inch. Liquid BackerBoard is the 50 lb high coverage powder that needs primer and goes to an inch and a quarter.

Choosing, in one page

• Plywood or hardwood subfloor: Laticrete NXT Level Plus if you want to skip metal lath, Ardex Liquid BackerBoard or K 60 if you are in the Ardex system, Mapei Novoplan 2 Plus if you want the clearest published joist and deflection rules. Rule out Ardex K 15 and Rapid Set.
• Deep fill in one pour: Laticrete NXT Level at 3 inches neat, or Sika Level-325 at 3 neat and 4 extended. Rule out standard Quikrete No. 1249.
• Most floor per bag: Ardex Liquid BackerBoard at 32.5 square feet, then Ardex K 15 at 30.
• Back in service fast: Quikrete Fast-Setting takes tile early but wants 6 hours for foot traffic, while the 2 to 3 hour group is quicker to walk on. Read both numbers, not just the one on the front of the bag.
• Small patch or skim: not underlayment at all. Use a trowel applied patch such as Ardex Feather Finish and stop calculating volume.

Once you have a product, put your floor area, average depth and that product into the self-leveling concrete calculator and it will use the published coverage rather than a generic figure. For the work that comes before the pour, grinding, patching and deciding whether the floor is worth leveling at all, see how to level a concrete floor, and for a basement slab in particular, basement concrete floors. If the floor is sound but ugly rather than uneven, concrete floor resurfacing is the cheaper route.