Self-leveling underlayment floor prep field guide

What self-leveling underlayment is and where it fits

Self-leveling underlayment, SLU, is a flowable cement or gypsum based topping you pour over a slab to flatten and smooth it before the finished flooring goes down. Mixed to the right water and poured out, it flows to a level plane on its own and seeks its own surface, so it fills the lows and feathers off the highs into one flat field. The finished flooring, tile, LVT, VCT, sheet vinyl, engineered wood, or carpet, then sits on that flat plane instead of on a slab that dips, humps, and telegraphs every defect through the floor.

It is a prep layer, not a wear layer. In most jobs nobody walks on the bare underlayment for long. It exists so the floor that goes on top is flat, well bonded, and free of the hollow spots and lippage you get when you glue or float a finish over an uneven slab. The exception is the handful of underlayments rated to be left exposed or polished, but the default underlayment is meant to be covered.

Know which lane you are in before you reach for a bag. An underlayment flattens a floor for a finish. A wear topping is a thicker, harder overlay built to take traffic as the final surface. A patch is a trowel-applied repair for a local low spot or a hole, not a whole-floor pour. And a polished or epoxy floor is a finished surface in its own right, prepped and coated, covered in the resinous floor coating guide. Pour an underlayment where you needed a wear topping and the traffic chews it up. Pour a deep field where a patch and a screed would have done and you have spent the money for nothing.

Reading the slab: flat, sound, and how deep you need to go

Before you order a single bag, read the slab, because the underlayment follows the substrate it sits on and the depth you need drives the material count. Run a long straightedge or a string line across the floor in a grid and mark the highs and the lows. The high points set your minimum depth, since the underlayment has to cover the highest bump by at least its rated minimum thickness, and the lows tell you how much material the field will drink. A floor that looks flat to the eye routinely hides a half inch of swing across a room.

Soundness is the second read. Tap and look for delamination, hollow spots, scaling, and crumbling concrete, because the underlayment bonds to whatever is on top and an unsound surface debonds and takes your pour with it. Concrete showing laitance, scaling, or delamination gets mechanically removed back to solid slab before anything goes down. Old slabs hide a previous coating, a hardener, or a curing compound under the grime, so check for a bond-breaker before you commit.

Cracks and joints get sorted now, not at the pour. A static, non-moving crack gets cleaned and filled so it does not reflect. A moving control or expansion joint has to be honored through the underlayment, covered later, because filling it solid lets the slab crack your pour at the joint line. Map all of it first. The assessment is what tells you the depth, the material, the prep, and the joints before the truck or the mixer shows up.

Do you need a moisture test before self-leveling underlayment?

Yes, whenever finished flooring goes over the underlayment. Slab moisture is the number one bond failure on a floor system, and the underlayment does not stop it. Water and vapor drive up out of a slab that is still drying, and once the underlayment and the flooring seal the top, that vapor builds pressure and lifts the bond. You get debonding, blisters, and a flooring failure that shows up months after the crew has left. There is no underlayment that fixes a slab driving moisture. You test it and you respect the number.

Two ASTM methods carry the work, the same ones used ahead of any flooring. ASTM F2170 reads relative humidity inside the slab with in-situ probes drilled to depth, commonly 40 percent of the slab thickness for a slab drying from one side, and it is the more predictive test because it tells you what the slab interior will do after the top is sealed. ASTM F1869 is the calcium chloride test, measuring the moisture vapor emission rate in pounds per 1000 square feet over 24 hours, and it reads only the surface fraction. For gypsum and other non-standard slabs, ASTM F2659 covers a non-destructive electronic meter as a preliminary read, not a pass or fail.

The limits belong to the manufacturer, not to a rule of thumb. A common F2170 ceiling is 75 percent RH and a common F1869 ceiling is 3 pounds per 1000 square feet, but the number on the data sheet for both the underlayment and the finished flooring is the one the warranty is written against. If the slab reads wet, you install a moisture mitigation system, usually an epoxy moisture barrier rated to the slab's RH, before the underlayment. The epoxy and resinous floor coating guide covers the moisture testing and the mitigation in depth, and the principle is identical here: test before you pour, not after the floor fails.

How do you prep a slab for self-leveling underlayment?

You prep mechanically and you prep clean, because the underlayment needs to key into sound, open concrete, not sit on a sealed, dusty, or contaminated surface. The slab has to be free of dirt, oil, grease, wax, paint, curing compound, sealer, old adhesive, drywall mud, and any loose cementitious junk, all of which act as a bond-breaker. A slick steel-troweled finish, a sealed slab, or concrete reading below roughly 72 psi, about 0.5 MPa, surface tensile gets mechanically profiled by shot blasting or scarifying so there is a surface the underlayment can grab.

Shot blasting is the workhorse for open floor. It fires steel media that strips contaminants and profiles in one pass and leaves no dust ground into the surface. Diamond grinding handles edges, columns, and tight rooms, but it drives fines into the pores, so you have to vacuum and clean before you prime or you have built your own bond-breaker. Acid etching has no place here. It does not reach a real profile, leaves a residue and salts you have to neutralize, and does nothing to a contaminated or hard-troweled slab.

Not every slab needs aggressive profiling. A clean, sound, porous broom-finished slab can take some underlayments after a thorough cleaning and the primer, which is exactly what the primer is partly for. But the moment the slab is sealed, troweled slick, coated, or contaminated, the answer is mechanical. The curing guide covers why a membrane-forming curing compound and a bonded topping fight each other, and the fix is the same as for any coating: grind or blast back to clean, sound concrete, then verify the surface is open and dust-free before the primer comes out.

Do you need to prime before self-leveling underlayment?

Yes, and skipping the primer is the classic underlayment failure. The primer does three jobs that the pour cannot do for itself, and on most products skipping it voids the warranty. It seals the slab so the porous concrete does not pull the water out of the mix and starve the cure, which leaves you a weak, dusty, poorly bonded layer. It controls the slab's porosity so air does not outgas up through the fresh material and leave a field of pinholes. And it promotes the bond between the slab and the underlayment.

Pinholes are the visible symptom of a skipped or thin primer. As the underlayment flows over a porous or warm slab, air pushes up out of the concrete and the substrate breathes through the wet material, popping holes in the surface that you cannot fix after it sets. On a slab that will outgas, you prime to seal it, and on a very porous slab you run two coats, the first to soak in and seal, the second to build the film, with dry time between coats per the data sheet.

Match the primer to the substrate and follow the coats and dry time on the bag. A standard acrylic or resin primer suits a normal porous concrete slab. A non-porous, dense, or sealed surface, or a metal, terrazzo, or glazed substrate, needs a bonding primer or an epoxy primer broadcast with sand to give the underlayment something to grip. Let each coat dry to the stated condition, usually tacky to dry rather than wet, before the next coat or the pour. Prime wet and pour onto it and you have trapped the primer instead of bonding through it.

Mixing and the water ratio that controls everything

The water ratio is the single number that makes or breaks an underlayment, and the bag states a narrow window for a reason. You mix to the exact water on the bag, measured, every batch, not eyeballed by how it looks in the barrel. Over-water it because it pours easier and you have wrecked it: the strength drops, shrinkage climbs, the mix segregates as the heavy aggregate settles and the fines and water rise, and you get cracking, hollow debonded areas, and a chalky white film or swirl on the surface. That swirl visible right at the pour is the tell that the batch was wet.

Under-water it and the material will not flow, sets too fast, and leaves ridges and trowel marks that defeat the point of a self-leveler. The window is narrow on both sides. Mix with a high-shear paddle on a drill or a proper barrel mixer at the speed the manufacturer states, mix to a uniform, lump-free slurry, and keep every batch identical, because a floor poured from inconsistent batches cures at different rates and flatness and color wander across the field.

On a small floor you mix by the bag and pour by hand. On a large floor you pump, and the discipline is the same but tighter. A pump meters water continuously, so a drift in the water setting runs through the whole pour before anyone sees it on the floor, which is why crews calibrate the pump and check the flow with a flow ring or patty test against the manufacturer's target. The mistake that costs the most is adding water to a batch that is going off to buy working time. You are not buying time, you are pouring a weak floor.

Pouring: flow, the gauge rake, and the spiked roller

Once it is mixed, the clock is running and the underlayment is doing most of the leveling itself, so the crew's job is to place it evenly and release the air before it sets. You pour it out in ribbons and spread it to the target depth with a gauge rake set to the thickness you want, then knock it flat with a smoother. The gauge rake meters the depth, the flow does the leveling. You work the material to the dams and the edges and keep it moving so nothing sits and starts to skin in the middle of an open field.

Then you back-roll it with a spiked roller. The spikes break the surface tension and let the air entrained in mixing and the air coming off the slab escape, so the cured surface comes up smooth instead of pocked with bubbles and pinholes. You roll right after spreading, while the material is still fully fluid, in both directions, and you wear spiked shoes to walk the wet field without leaving tracks.

Working time is short, commonly around 15 to 20 minutes at 70 degrees F, 21 degrees C, and heat shortens it hard. That window governs the whole choreography. You plan the pour so you keep a wet edge, with each batch flowing into the live edge of the last before it gels, because material that has started to set will not blend into fresh and leaves a cold-joint ridge in the floor. Plan the start point, the bay sequence, and the exit before you mix the first batch, mix only what the crew can place and roll inside the window, and never paint yourself into a corner over wet material.

Depth: minimum, maximum, featheredge, and deep fill

Every underlayment has a depth range, and both ends of it are real limits, not suggestions. Poured below the rated minimum the layer is too thin to hold together and it cracks, crumbles, and debonds, especially over the high spots where the underlayment runs thinnest. Poured past the rated maximum in one lift it cracks from its own shrinkage and curls. The bag states the range, commonly something like a featheredge up to about 1 to 1.5 inches neat in a single lift, and you stay inside it for the product you are pouring.

Featheredge and deep fill are the two ends you handle differently. At a featheredge, where the pour tapers to nothing against a high spot or a transition, the material thins below its minimum and gets fragile, so some products call for a featheredge additive or a feather-finish patch at the taper rather than running the self-leveler to zero. For a deep fill past the neat maximum, you extend the mix with the aggregate the manufacturer specifies, usually a clean pea gravel, which lets you place more depth without the shrinkage of a deep neat pour, or you build it in lifts with the primer or scratch coat the data sheet requires between them.

The depth decision ties straight back to the slab read. The high points set the minimum you have to carry across the whole floor, and a floor with a lot of swing either takes a lot of neat material or gets brought up with an aggregate-extended deep fill first and a smoothing neat coat on top. Do the math on depth before you order, because a floor that needs more depth than you planned is a floor you run short on halfway through the pour, with no way to stop and keep a wet edge.

Dams, doorways, drains, and containment

Self-leveling material is a liquid until it sets, and a liquid finds every gap, so containment is part of the prep, not an afterthought you scramble for mid-pour. Anywhere the pour ends, you build a dam, and anywhere it could escape, you seal it. The perimeter against a wall, the edge at a doorway, the open side of a partial pour, the gaps along a bottom plate, a sole plate, or a slab edge, and any penetration all get dammed or sealed before the first batch is mixed.

Doorways and thresholds are the usual leak. The underlayment wants to run out the door and down the hall, so you set a screed strip, a dam board, or a foam dam at the threshold to hold the material at the line you want and give a clean transition for the flooring. Plate lines and slab cracks get sealed, often with a tape, a caulk, or expandable foam, so the liquid does not drain under a wall and disappear into the next room or the wall cavity. On a wood subfloor the seam and perimeter sealing matters more, because the gaps are everywhere.

Drains are their own problem and they cut both ways. Where the floor has to drain, you protect the drain so the pour does not fill it but you still hold the slope to it, which a true self-leveler fights because it wants a level plane, not a slope. A floor that has to pitch to a drain is usually not a self-leveler job in that zone, or it is a sloped underlayment or a hand-screeded mortar there with the self-leveler held back. Plan the drains and the slopes before you decide the material.

Can you pour self-leveler over wood or old tile?

Over concrete is the simple case, covered in the prep and primer sections: clean it, profile it if it is slick or sealed, prime it, pour it. Over a wood subfloor and over old tile, the prep changes, and getting it wrong is where these pours fail.

A wood subfloor moves and outgases in ways concrete does not, so most underlayments poured over plywood or OSB need reinforcement and a tight perimeter. The common detail is galvanized expanded metal lath or wire mesh fastened down through the subfloor, which gives the underlayment something to grip and holds it together over a substrate that flexes, plus a primer and a minimum depth over the high spots so the layer is thick enough to span the movement. Some fiber-reinforced underlayments are rated to skip the lath on interior wood-framed floors, but you confirm the product is rated for wood before you pour, because not all are, and you seal the seams and the entire perimeter so the liquid does not drain through the floor. Without the primer, wood outgassing bubbles rise straight through the pour and pock the surface.

Over old tile, the tile has to be sound and bonded first. You tap it and pull anything loose or hollow, then clean it hard, because glazed tile carries wax, sealer, grease, and mop film that no topping bonds to. A glazed, non-porous surface needs a bonding or epoxy primer, sometimes sand-broadcast, to give the underlayment a grip the smooth glaze will not. A clean, well-bonded tile floor can take a pour with the right primer. A drummy, greasy, or partly loose tile floor is cheaper to remove than to chase a debond later.

Gypsum or cementitious underlayment?

Both pour and both flatten a floor, but they are different binders with different limits, and picking the wrong one for the service is a quiet, expensive mistake. Cementitious underlayment is portland-cement based, builds higher strength, and resists moisture, so it is the choice for wet areas, high-strength and high-traffic work, and anywhere the slab still moves moisture. Gypsum based underlayment, a calcium sulfate binder, is the choice for dry interiors, over wood subfloors, and over radiant heat, because it has low shrinkage, good dimensional stability, and pours large areas flat and fast.

The line that matters most is water. A gypsum, calcium sulfate, matrix softens and loses strength under sustained moisture, so gypsum underlayment stays out of bathrooms, kitchens, wet-service areas, and slabs with ongoing moisture vapor emission. Cementitious is the answer where water is in the picture. This is the rule the trade gets wrong by reaching for whatever is on the truck. Match the binder to the moisture exposure of that specific floor, not to habit.

Two more practical differences. Gypsum often pours thicker and flatter over big areas with less cracking, which is why it is common in multifamily and over radiant, while cement gives you the strength and the moisture tolerance for tough and wet service. And the flooring and adhesive over a gypsum underlayment have to be compatible with a gypsum surface, which the flooring manufacturer states, so confirm that the finished floor system accepts the underlayment you poured before you pour it.

Pouring over radiant floor heat

Radiant floor heating, hydronic tubing or electric mats, gets encapsulated in an underlayment so the heat spreads evenly into a flat floor instead of striping over the tubes. The underlayment surrounds the tubing or the cable, fills the voids, and gives the finished flooring a flat, even surface with the heating element locked in place below it. Gypsum underlayment is common over radiant because of its low shrinkage and its dimensional stability through the heating and cooling cycles, though cementitious products are also poured over radiant where the manufacturer rates them for it.

The depth over the element is the number to hold. The pour has to cover the tubing or the cable by the minimum the manufacturer states, commonly something on the order of 3/4 inch over the top of the tube, so there is enough mass over the element to spread the heat and protect the tubing, without going so deep that you slow the response and waste material. Confirm the coverage and the total depth against both the underlayment and the radiant system manufacturer.

Two cautions specific to heat. The system stays off and cool during the pour and the cure, then comes up to temperature gradually on the schedule the radiant maker gives, because firing a green floor or ramping it fast drives cracking. And the tubing has to be pressurized or otherwise verified before and during the pour, so a leak or a crushed line is caught while the floor is still open, not after it is buried under the finished flooring.

How long before you can install flooring over self-leveler?

It depends on the product and the flooring, and the spread is wide, so the data sheet governs, not a rule of thumb. Many underlayments are walkable in a few hours and will take tile and other non-moisture-sensitive flooring quickly, sometimes in as little as a few hours. Moisture-sensitive flooring, sheet vinyl, LVT, wood, and the like, waits longer, commonly in the range of roughly 16 to 72 hours, with premium fast products at the short end and standard products at the long end.

Two clocks are running and you wait for the slower one. The first is the underlayment's own set and early strength, the time before it can be walked, sanded, or covered without damage. The second is moisture, because the fresh underlayment carries its own water and the finished flooring will not bond over a surface that is still wet. The underlayment has to dry down to the flooring maker's moisture limit, the same kind of limit you checked on the slab, and on a thick pour or in a humid space that drying takes longer than the set does.

Do not rush the floor on for the schedule. Lay moisture-sensitive flooring over an underlayment that has not dried down and you trap the moisture, and the floor bubbles, debonds, or telegraphs a haze later, blamed on the underlayment when it was the calendar. Protect the cured underlayment from traffic, water, and dropped material until the flooring goes on, and confirm both the set time and the moisture condition against the data sheet before you cover it.

Why did my self-leveler crack?

An underlayment cracks because something broke one of a short list of rules, and the causes rank by how often they actually bite. Find the rule that got broken and you find the crack most of the time before you ever blame the bag.

Over-watering is first and by a wide margin. A wet batch loses strength, shrinks more as the extra water leaves, and segregates, and it cracks across the field and goes hollow where the bond failed, with that white swirl at the pour as the warning. Pouring too thin is second: below the minimum depth, especially over the high spots, the layer cannot hold itself together and it crumbles and cracks. A skipped or thin primer is third, because the bare slab pulls the water out of the mix and the underlayment cures starved, weak, and crazed. Fourth is a moving joint poured over solid, which the slab cracks straight through the underlayment on the next cycle, right at the joint line.

The rest are setup failures. A pour over a debonded or contaminated slab cracks because it never bonded to sound concrete. A pour rushed into a fast set in heat, or over a substrate moving and flexing without reinforcement, cracks from the stress it cannot take. Almost every one of these is fixed before the pour, not after: mix to the exact water, hold the minimum depth, prime the slab, honor the moving joints, and bond to sound concrete. Cracking is rarely the material. It is the install.

The flatness result, the spec, and commercial floors

The whole point of the pour is flatness, so the floor has to land inside the tolerance the finished flooring actually needs, and that tolerance is tighter than people expect. Flooring specs commonly call for the floor to fall within roughly 1/8 to 3/16 inch of gap under a 10 ft straightedge, with thin and large-format finishes, large tile, and rigid plank, demanding the flatter end because they bridge a low spot and rock or crack. The flooring manufacturer's flatness requirement is the target you pour to, and it is usually stricter than the slab the concrete crew left you, which is exactly why the underlayment is in the scope.

On commercial and institutional work the flatness gets specified as a number. Floor flatness and levelness are measured as FF and FL values under ASTM E1155, and a project can call out an overall and a local FF the floor has to meet. A typical slab might be specified in the FF 25 to FF 35 range, and superflat floors for certain warehouse and equipment uses run much higher. Confirm the FF or the straightedge tolerance against the project specification and the flooring maker, because the two can differ and the stricter one wins.

Data center and raised-access-floor work pushes flatness hardest. A raised floor system stands on pedestals that need a flat, level, dimensionally stable base to sit plumb and carry the rack loads without rocking, and the slab and any underlayment under it get held to a tight flatness and levelness. The resinous floor coating guide covers the data center floor surface itself; the underlayment's job there is the same as anywhere, just to a tighter number: leave a flat, sound, dry plane for what goes on top.

Cracks and moving joints through the pour

Joints are where a flat new floor cracks first, and the rule is the same one that governs any bonded topping: static cracks get filled, moving joints get honored. Get this backward and the slab telegraphs its movement straight through your pour at the worst possible line.

A control joint or an expansion joint moves with the slab as it shrinks, expands with temperature, and shifts. Pour the underlayment solid across it and the slab cracks the underlayment at the joint on the next cycle, leaving a hard, ragged line under or through the finished flooring. So you carry the moving joint up through the floor system. Either hold the underlayment back and treat the joint as a joint, or pour across and then saw the joint back open through the cured underlayment and the finish, filling it with a flexible joint sealant or a movement-joint profile that lets it keep moving. The finished flooring carries its own movement-joint requirement over the same line, which the flooring manufacturer states.

Static, non-moving cracks are the opposite. They get cleaned, chased open if needed, and filled with a rigid repair so they do not reflect, and then the underlayment flows over them as part of the flat field. The mistake is treating the two the same. Fill a moving joint rigid and you have built the crack in. Leave a static crack open and it reflects through a thin finish. Sort which is which during the slab read, and detail each one before the pour.

Maintenance, repair, and the floor that came back low

Once the finished flooring is on, the underlayment is buried and the owner inherits a floor, not a topping to maintain, so the maintenance is the finished floor's. The underlayment only comes back into the conversation when something fails: a hollow spot underfoot, a crack that telegraphed up, or a finished floor that came up low or rocking over a soft area.

A localized failure gets cut out and repaired in kind. You open the area, find whether the problem was a debond, a void, an over-watered soft spot, or a crack, fix the cause, and re-prime and re-pour or patch the section to plane so the new flooring sits flat over it. A debonded patch that gets skimmed over without finding the cause comes back, so chase the why before you fill the where.

The honest prevention is in the install, not the repair. A floor poured over a sound, primed slab, mixed to the right water, held to the right depth, and dried before the flooring went on does not come back as a callback. The ones that come back are the ones that skipped a step, and they come back after the finished floor is down, which makes the repair a flooring tear-out, not a quick patch. That is the cost of the shortcut, paid later, by whoever owns it then.

What to document

The moisture and flatness numbers you log are what carry you the day a section reads hollow or the finished floor fails and someone asks whether the underlayment was ever poured to spec. The record is the deliverable as much as the floor, and the failure will be local, so capture it by area.

For each area, record the moisture test method and result against the limit, the prep and the surface profile achieved, the primer and how many coats, the underlayment product and the depth poured, and the flatness the floor met against the spec. Add the conditions, the mix water and batch consistency, the dates, and who did the work. That table is the spine of the closeout package and the first thing a manufacturer's rep asks for on a claim, the same way it is on a coating job.

Common mistakes

• Pouring over a slab that was never moisture tested, then blaming the flooring failure on the underlayment.
• Skipping or skimping the prep, so the underlayment debonds from a sealed, dusty, or contaminated slab.
• Skipping the primer, so the slab pulls water out of the mix, outgasses pinholes, and the bond fails.
• Over-watering the mix for easier flow, which weakens it, drives shrinkage and segregation, and cracks the floor.
• Pouring below the minimum depth over the high spots so the thin layer crumbles and cracks.
• Pouring past the maximum neat depth in one lift instead of extending with aggregate or building in lifts.
• Installing moisture-sensitive flooring before the underlayment has set and dried down to the flooring limit.
• Pouring solid over a moving control or expansion joint so the slab cracks the floor at the joint line.
• Pouring over a wood subfloor with no lath or fiber reinforcement and no sealed perimeter.

Field checklist

Standards and references

Concrete moisture testing runs on the ASTM methods, the same ones that govern any flooring. ASTM F2170 covers in-situ relative humidity by probe, ASTM F1869 covers the calcium chloride moisture vapor emission rate, and ASTM F2659 covers the non-destructive electronic moisture meter for a preliminary read on concrete, gypsum cement, and other slabs. ASTM F710 frames preparing concrete to receive resilient flooring, the standard a flooring spec leans on for the substrate condition. The acceptance limits are not in those methods. They are set by the underlayment and the flooring manufacturer and the project specification.

Flatness is measured and specified separately. ASTM E1155 is the method for floor flatness and levelness, the FF and FL numbers a commercial spec calls out, while the practical flooring tolerance is often written as a maximum gap under a 10 ft straightedge. ACI documents frame concrete floor flatness and finishing on the slab side. Confirm the FF target or the straightedge tolerance against the project specification and the flooring manufacturer, since the finished floor usually drives a tighter number than the slab.

The controlling document on any underlayment pour is the manufacturer's data sheet. It governs the water ratio, the working time, the primer and the coats, the minimum and maximum depth, the deep-fill aggregate, the reinforcement over wood, the radiant coverage, and the time and moisture condition before flooring, and the warranty is written against it. Where the project specification or the flooring system is stricter, that wins. Name the standard that controls the point, follow the manufacturer for the numbers, and confirm any edition-specific section against the version the project adopted before citing it.

Units, terms, and conversions

Underlayment borrows terms from concrete, flooring, and moisture testing, so the same floor reads differently across a spec, a data sheet, and a moisture report. Depth is given in inches or millimeters, where 1/4 inch is about 6 mm and 1 inch is 25.4 mm. Flatness shows up two ways: a gap under a 10 ft straightedge in fractions of an inch, and the dimensionless FF and FL values from ASTM E1155, where a higher FF is a flatter floor.

Moisture shows up the same two ways it does on any slab. Relative humidity from an F2170 probe is a percentage of the slab interior, and the moisture vapor emission rate from an F1869 test is in pounds per 1000 square feet over 24 hours. They do not convert directly, because they measure different things, the slab interior versus the surface emission.

SLU

Self-leveling underlayment, a flowable cement or gypsum topping poured to flatten a slab for finish flooring

Featheredge

Where the pour tapers to nothing at a high spot or transition; thins below minimum depth and gets fragile

Deep fill

A pour past the neat maximum, extended with specified aggregate or built in lifts

Gauge rake

An adjustable rake that meters the underlayment to a set depth as it is spread

Spiked roller

A roller that breaks surface tension to release entrained and outgassing air from the wet pour

Working time / flow life

The window after mixing to place and roll the material, often about 15 to 20 minutes at 70 degrees F

FF / FL (ASTM E1155)

Floor flatness and levelness numbers; higher FF is a flatter floor

RH (ASTM F2170)

Relative humidity inside the slab, read by in-situ probe as a percentage

MVER (ASTM F1869)

Moisture vapor emission rate in pounds per 1000 square feet per 24 hours

Related tools

Calculators and readiness checks for this work

Compare your options

FAQ

People also ask

• How do you prep concrete for epoxy?
• How long do you cure concrete?
• Why does commercial flooring need a slab moisture test?
• How do you test a concrete slab for moisture before flooring?
• What is roofing underlayment?