You can ruin resilient flooring or tile by pouring over wet, flexible, dirty, or poorly profiled subfloors; verify RH/CM limits and stabilize movement first. Don’t skip repairs, manufacturer-approved primer, joint sealing, or edge dams. Measure water exactly, mix mechanically, and never re-temper a batch. Maintain a continuous wet edge, spike-roll fresh material, and avoid cold joints. Finally, respect thickness and cure schedules before installing finishes. These checks help prevent costly failures early.

Check Subfloor Suitability, Moisture, and Thickness

Before mixing a self-leveling compound, confirm the subfloor can support it. Perform moisture testing with CM or RH methods and compare results with the product datasheet; many systems require RH below 75%. Excess moisture disrupts curing and adhesion, causing resilient flooring or tile failures. Follow manufacturer’s instructions for thickness limits: most products perform at 5–15 mm, while excessive depths can crack, crumble, or debond.

Assess substrate behavior, not just flatness. Non-porous substrates, including glazed tile, epoxy, and power-floated concrete, require compatible bonding technology or mechanical profiling. Don’t pour over flexible boards: rigidify timber with plywood or cement board first, or movement will fracture the leveling layer. For expansive rooms, select slow-setting or pump-grade material and sequence pours to preserve a continuous wet edge.

Clean, Repair, Prime, and Seal the Subfloor

Proper subfloor preparation determines whether a self-leveling pour bonds as designed or fails at the interface. Poor preparation turns advanced materials into expensive callbacks. Your surface preparation must remove dust, paint, adhesive residue, and loose debris before placement.

• Repair cracks, spalls, and movement with compatible epoxy or cementitious systems; leveler won’t stabilize a moving base.

• Prime porous or nonporous substrates using the manufacturer’s specified product, dilution, and open time to control suction and pinholes.

• Seal joints, gaps, and control-joint edges with flexible sealant or foam backer rod; add edge dams to prevent bleed-out and weak perimeters.

• Verify dryness against published moisture limits with a calibrated meter or ASTM/BS testing; vapor disrupts curing, adhesion, and long-term performance.

Don’t skip inspection: clean, repair, prime, and seal before pouring.

Mix Self-Leveling Compound With Exact Water Ratios

Once the primed, sealed subfloor is ready, measure mixing water exactly as the manufacturer specifies—commonly 4.0–4.5 L per 20 kg bag (about 0.20–0.23 L/kg)—then maintain that ratio for every batch. Your water-to-powder ratio governs Self-Levelling Compound rheology, strength, and finish quality. Use a graduated container, and add powder or water in the sequence the technical data sheet requires. Overwatering may improve flow initially, but it reduces compressive strength and can leave a dusty, fragile surface. Underwatering produces a stiff mix that won’t reach low areas; verify the specified creamy consistency with any supplied slump test. Use a low-speed paddle mixer for the complete mixing time, typically one to three minutes, to hydrate powder uniformly. Don’t hand-stir. Track pot-life—usually 10–20 minutes—and never re-temper material with water.

Pour Continuously to Prevent Ridges and Lumps

Pour self-leveling compound continuously so each batch meets the next while both remain fluid, maintaining a wet edge and preventing cold joints. Most mixes begin setting within 15–30 minutes, so a pause can create ridges that telegraph through vinyl or tile.

• Stage bags, water, and a paddle mixer before starting.

• Assign helpers to mix staggered batches for a continuous pour.

• Use a pump or bucket relay to keep material moving across large rooms.

• Immediately spike roll each fresh section to blend joins and release trapped air.

For areas exceeding a few hundred square feet, choose a slow-setting compound to extend your working window. Plan runs around access points so you don't trap yourself. If the surface skins during an interruption, don’t pour over it; grind or scarify the edge, re-prime it, then feather in fresh material.

Cure Fully and Fix Minor Surface Defects

Curing the self-leveling compound to the manufacturer’s specified schedule protects the floor system from adhesive failure, cracking, and movement under the finished covering. Follow documented drying and curing times: standard mixes often need 24–48 hours, while rapid-setting products may permit installation within hours. Don’t rely on touch alone; test slab moisture using ASTM F1869 or F2170 requirements.

Use a spiked roller immediately after placement and again within its working window to release air and reduce peaks. After cure, verify surface flatness with a 2-meter straightedge; LVT and tile commonly require deviations no greater than 3 mm over 2 m. To repair minor surface defects, grind high spots, vacuum dust, and clean the surface thoroughly. Apply a compatible primer, then a feathering patch at the required 3–5 mm thickness for a durable, smooth surface.

Frequently Asked Questions

What Are Common Self-Leveling Mistakes?

Common self-leveling mistakes include inadequate surface prep, skipping primer, and ignoring substrate moisture or movement. You can weaken the pour by mismeasuring water, shortening mixing time, or neglecting temperature control. Poor application technique, such as interrupted pours, creates ridges and cold joints. Don’t apply outside specified thickness ranges, omit expansion joints, or rush curing duration. Confirm manufacturer limits and plan continuous placement to protect adhesion and long-term performance.

When Not to Use Self-Leveler?

Don’t throw good money after bad: avoid self-leveler when unnecessary, such as minor surface variation within flooring tolerances. Don’t apply it over unsuitable substrates with moisture, contamination, or instability; Near active cracks; Under high slope areas; or On very thin spots below manufacturer limits. You shouldn’t use it outdoors, on vertical planes, or as a structural load-bearing layer. Specify compatible primers, vapor mitigation, and repair systems first.

How Can I Fix a Bad Self-Leveling Job?

Fix a bad self-leveling job by removing failed material to sound substrate, then vacuum thoroughly. Perform bond testing, mitigate moisture, and repair cracks before priming. Use grit blasting or grinding for edge profiling, and apply compatible surface repair mortar with controlled layer feathering at approved minimum thickness. Re-pour only with exact water ratios, paddle mixing, wet-edge placement, and spiked rolling. If movement persists, correct the substrate system first.

What Happens if You Add Too Much Water to a Self-Leveler?

Adding too much water weakens your self-leveler, increases drying time, and promotes bleeding, cracking, curling, and poor adhesive bond. You’ll disrupt mixing ratios, reduce compressive strength, and create uncontrolled flow despite improved surface tension. Verify consistency with a slump test, avoid excess air entrainment during paddle mixing, and expect possible pigment fading in colored products. Measure water precisely per the manufacturer’s data sheet for predictable thickness and performance.

Conclusion

Avoid expensive callbacks by treating self-leveling work as a system, not a shortcut. Verify subfloor strength, moisture, cleanliness, primer coverage, and pour depth before mixing. Measure water precisely, blend thoroughly, and pour continuously within the product’s working time. Let the underlayment cure before installing resilient flooring or tile, then correct pinholes or ridges. Don’t let a medieval “good enough” mindset sabotage modern performance—your finished floor depends on disciplined preparation and timing.

Stop over-pouring SLU by laser-verifying each bay’s area and average depth, then calculating bags from the product’s stated yield. Stage only that bay’s bags, plus a controlled 5–10% buffer for absorption, spills, and mixing loss. Prime and seal gaps first, set laser-level gauge strips, and mark containment bays. Measure water exactly, calibrate the pump, and recover clean fluid spills within the working window. The steps below show how to tighten each control point.

Calculate Self-Leveling Underlayment Coverage

Start by calculating the floor area and your planned average pour depth, then base the order on the exact yield listed for the SLU you’re using. Measure area in square feet, convert thickness to inches, and calculate volume: bags = (area × thickness ÷ 12) ÷ bag yield in cubic feet. A typical 50-pound bag delivers about 0.45–0.6 ft³, or 20–30 ft² at 1/4 inch, but product data overrides averages. Build your bag estimate from laser-verified elevations, not nominal slab conditions. For fills beyond standard limits, evaluate the manufacturer’s aggregate protocol; some systems reach 5 inches with approved aggregate, while unextended SLU may cap near 1.5 inches. Track priming absorption and mixing losses during trial batches, and include a documented procurement contingency. This data-first workflow prevents underordering and eliminates inflated coverage assumptions.

Add a 5–10% SLU Waste Allowance

Build a 5–10% waste allowance into your SLU order after calculating the base bag count. For a 1,000-square-foot pour at 1/4 inch requiring 40 bags, add two to four bags. Use 5% for simple, open rooms; specify 8–10% for complex layouts, long runs, or elevated placements. This reorder buffer prevents costly rush deliveries and schedule interruptions.

Refine every SLU takeoff with field data. Track actual usage by recording starting bags, leftover bags, and installed area. Isolate edge/corner waste and pump losses, which often consume the first 1–3% of material. Compare results across crews, substrates, and placement methods to establish a predictive allowance. Treat the buffer as controlled inventory, not expected loss: collect spills immediately, clean edges, and reuse uncontaminated leftover mix when feasible. That approach reduces true material waste while protecting production certainty.

Prime and Seal the Subfloor First

Before you mix the first bag, prime and seal the subfloor to control SLU flow, adhesion, and material consumption. Use the manufacturer-approved primer from Ardex, Mapei, Uzin, or your specified system. A correctly prepared surface stops self-leveling underlayment from losing water into a porous substrate, which otherwise accelerates set, disrupts flow, and drives overpour.

Seal cracks and joints, penetrations, and voids with compatible elastomeric sealant or backer rod before priming. Follow the data sheet for dilution and primer open time; many systems need 1–4 hours to dry. Applying primer too wet or too dry changes bonding behavior. On high-suction surfaces, prime and seal with a second coat or approved bonding slurry. Run a small mock pour in each area to validate compatibility, flow rate, and coverage before committing material.

Mark Self-Leveling Underlayment Pour Bays

Mark each SLU pour bay with paint or tape 12–18 inches wide and label the target thickness so your crew has fixed placement limits. Install removable screed rails or cleats at 3–6 foot intervals to control flow at edges while the material self-levels. Before pouring, stage the exact bag count using manufacturer coverage—about 20–30 sq. ft. per 50-lb bag at 1/4 inch—and post the mix ratio and pot life at each bay. This is the old-fashioned method that LEVELPEGS can help with most. We mark the depths in 3-10 ft increments and place a cut LEVELPEG to indicate the depth at every interval.  

Plan Pour Bay Boundaries

Pour-bay planning controls where self-leveling underlayment flows and lets your crew place each batch within its workable window. Define pour bay boundaries using 1–2-inch temporary plastic or metal screeds at planned widths to contain SLU flow and prevent spillover. Size marked bays around actual batch yield: at 1/4 inch, many mixes cover 20–30 square feet per 50-pound bag. This data-driven layout reduces stop/start seams and avoids surplus material.

Apply paint or tape as high‑visibility lines, add bay IDs, and map cleat locations, outlet ports, and expansion-joint treatments before pumping. Pre‑wet and prime only within each bay, then seal drains, gaps, and control joints with silicone or tape. Finally, coordinate pump output and mixing cycles with bay capacity, targeting 10–20-minute work windows so crews fill and finish before fluidity drops.

Install Clear Edge Markers

Clear, high-contrast edge markers give pump operators and finishers an immediate visual limit for each SLU pour bay, helping prevent material from flowing into adjacent areas. Build a visible containment system before pumping self-leveling underlayment:

1. Apply 2–3-inch fluorescent tape or plastic battening around pour bays for instant boundary recognition.

2. Secure edge markers flush to the substrate every 12–18 inches along long runs, eliminating gaps where material can seep and feather.

3. Install temporary stop rails in aluminum or PVC, 1/4–3/8 inch tall, matched to your planned placement depth.

4. Color-code markers: green for 1/4 inch, yellow for 1/2 inch, and red for depths above 1 inch.

Inspect rails and tape immediately before each pour. Replace lifted sections, gaps, or damaged stop rails. This low-cost visual control reduces material waste, protects adjacent zones, and keeps placement disciplined.

Verify Volume Before Pouring

Verifying volume before mixing prevents the most common SLU placement error: sending excess material into a bay that can’t contain it. Measure the bay area, confirm the target depth, and calculate the required SLU from the manufacturer’s coverage rate. A 50-pound bag may cover 20–30 square feet at 1/4 inch, but calculate each bay rather than relying on estimates.

Mark pour bays with chalk or tape, then label target depth, volume in liters, and bags required. Install pre-cut stopping controls, such as cleats or screed rails, at the planned elevation to contain flow. Stage bags by bay and mix only the calculated quantity for the next zone; with 10–20 minutes of working time, surplus creates risk. Use a check sheet listing dimensions, thickness, bags required, and bags used. Assign one worker to validate each bay before release.

Set Gauge Strips for Target Depth

Before mixing, install gauge strips at the planned finished elevation to create physical depth controls for the SLU. You’ll turn estimated coverage into a repeatable, measurable pour.

1. Fasten perimeter gauge strips, 1/4–3/8 inch hardwood or plastic, to maintain the specified 1/4–1/2 inch gap.

2. Place intermediate gauge strips every 4–6 feet; tighten spacing on uneven substrates or deeper fills so your gauge rake bridges reliably.

3. Set every strip with a laser level, holding ±1/16 inch for thin applications and ±1/8 inch for thicker fills.

4. Choose screwed removable cleats or foam blocks, then patch their small voids after removal.

For pours above 1 inch or variable slabs, add depth markers on barrels and calibrate pump flow. These redundant controls prevent the pump from exceeding your gauge-defined volume and protect material margins.

Mix SLU With Exact Water Ratios

Exact water dosing controls SLU flow, spread, and yield, so measure every batch to the manufacturer’s stated water-to-bag ratio—commonly 4.5–6.0 pints (2.1–2.8 L) per 50-lb (22.7-kg) bag. Use a calibrated measuring jug or fixed-volume bucket; don’t estimate by eye. Measured mixing keeps viscosity, working time, and coverage predictable while preventing excess spread that consumes unnecessary material.

Record ambient temperature before mixing, then adjust water only within product tolerances. When scaling production, weigh powder and calculate total water by multiplying bags by the specified liters per bag. For mechanized placement, configure pump dosing to the exact rate and verify output with a test batch before the full pour. This controlled workflow gives you repeatable rheology, cleaner forecasting, and tighter material-use data across every placement.

Recover Clean, Fresh SLU Immediately

During placement, assign one or two workers to trail the pump or trolley and capture clean, fresh SLU spills before they begin to set. This recovery role turns drips into usable material instead of hardened waste.

1. Keep a collection bucket or a clean wheelbarrow within arm’s reach of the placement path.

2. Collect recovered SLU from fluid ribbons, then transfer within working time—typically 10–20 minutes.

3. Clean edges and corners, plus formwork, continuously to prevent SLU setting in thin, costly accumulations.

4. Label and reuse only uncontaminated material; avoid contamination from debris, cured chunks, or added water.

After finishing, use immediate water curing, such as mist or damp burlap, only when the manufacturer permits rework. Otherwise, segregate remaining material for approved secondary use or disposal. Track recovered volume per pour to quantify savings and refine crew positioning.

Frequently Asked Questions

Where to Stop Concrete Pour in Slab?

Stop the concrete pour at planned control joints, curb lines, slab changeover points, or approved construction joints. Set forms to verified finish grade and use laser checks to control elevation. Maintain required reinforcement overlap where pours terminate. Plan edge jointing before placement, then schedule saw cutting at the specified curing window. Don’t remove forms until concrete reaches required strength; premature form removal can damage edges, misalign joints, and compromise tolerances.

What Are Common Mistakes When Pouring a Concrete Driveway?

Common mistakes include Incorrect slope, which traps water; Improper formwork, which causes edge failure; and undertamping, which leaves voids. You’ll weaken concrete through overwatering, Weak reinforcement, or Debris inclusion. Avoid Cold joints by placing continuously and coordinating truck timing. Poor curing reduces surface strength and crack resistance. Verify subgrade compaction, use the specified mix design, maintain joint spacing, and monitor placement with laser-grade controls.

How to Mess up Concrete?

You can mess up concrete by introducing excess water, which lowers strength and raises shrinkage. Avoid overmixing, slump, segregation, and uneven consolidation, since each creates weak zones. Use clean inputs; contaminated aggregate compromises bond and durability. Don’t create cold joints by delaying placement, and don’t rush improper curing. If you use insufficient reinforcement, cracking accelerates under load. Track water-cement ratio, temperature, vibration time, and cure moisture to prevent predictable failures.

Conclusion

You’ll cut SLU waste when you calculate coverage accurately, add only a 5–10% allowance, and control every pour variable. Prime, seal, mark bays, and set gauge strips so material reaches the specified depth—not beyond it. Measure water precisely to protect flow and yield, then recover clean fresh mix immediately. These steps turn guesswork into repeatable production. Don’t let over-pouring become your project’s telegram from 1925: expensive, avoidable, and too late to retract.

You prepare a durable floor by verifying flatness, moisture, and deflection before installation. Check the entire subfloor with a 6–10-foot straightedge, then measure gaps against the flooring manufacturer’s tolerance, often 3/16 inch over 10 feet. Mark rocking high spots and low birdbath dips. Grind or sand highs first, then patch lows with approved materials. Test moisture and secure loose panels before leveling. The steps below show how to correct and verify each condition.

Confirm Your Subfloor Flatness Tolerance

Before installation, confirm that your subfloor meets the flooring manufacturer’s flatness requirement, not just a general rule of thumb. Manufacturers specify limits such as 3/16 inch over 10 feet, 1/8 inch over 6 feet, or, for advanced rigid-core systems, 1/8 inch over 10 feet.  The NIVCOMP replaces a 10-foot straight edge, and LEVELPEGS replace feeler gauges or stacked coins. Those were the old ways - we are selling the new ways. Identify low areas where daylight appears beneath the tool or concave birdbaths exceed the stated tolerance. A floating floor still needs continuous support; unevenness creates stressed joints, gaps, and squeaks. Confirm tolerances before moisture testing or selecting corrective methods.

Test Subfloor Moisture and Deflection First

Before prep, test concrete moisture with in-situ RH probes or calcium chloride kits, and test wood in multiple locations with a species-correct moisture meter. Compare results to the flooring manufacturer’s moisture limits, including allowable wood-to-flooring MC differentials. Walk the floor and measure suspected spans with a straightedge or level; correct excessive deflection before installation.

Measure Moisture Accurately

Accurate moisture and deflection testing establishes whether a subfloor can support the finished flooring without premature failure. Don’t judge concrete by surface dryness or age. Quantify internal moisture with a 60–72-hour calcium chloride test or in-situ relative humidity probes equilibrated for at least 24 hours. Compare results directly with the flooring manufacturer’s limits.

For wood, use a calibrated moisture meter and take readings across the entire subfloor. Set the instrument for the correct substrate and species-specific gravity. Verify that the moisture differential meets product requirements: typically no more than 4% for solid strip flooring and 2% for wide plank. Capture and document results by location to create a defensible installation record. If readings exceed limits, pause the project, apply an approved mitigation system or replace affected material, then allow complete cure and equilibration before installation.

Check Deflection Limits

Verification comes first: test subfloor moisture, then confirm the joist-and-subfloor assembly meets the finished-floor manufacturer’s deflection limit. Moisture testing reveals hidden conditions that can undermine every correction. Don’t level a moving or damp substrate.

• For concrete, use calcium chloride tests or in-situ RH probes.

• For wood, use a pin-type or adjusted impedance meter.

• Verify joist deflection; rigid tile typically requires about L/360, or under 1/3 inch across 10 feet.

• Secure loose panels with screws before correcting subfloor deflection.

• Use a long straightedge to map high spots and birdbaths; compare gaps with product tolerances.

Address moisture and movement before applying patch or self-leveling underlayment. A digital layout map can prioritize repairs, but it can’t replace sound fastening, cure time, and manufacturer-specific limits.

Find High Spots and Birdbath Dips

Use a 6- to 10-foot straightedge to map the subfloor for ridges and “birdbath” depressions before choosing a repair method. For accurate subfloor preparation, move a long straightedge across the room in a grid pattern. Check for daylight beneath it and measure gaps with feeler gauges. Compare readings against required flatness tolerances, commonly 3/16 inch over 10 feet or 1/8 inch over 6 feet, while following your flooring manufacturer’s specification.

Mark high spots with chalk where the straightedge rocks or touches only at its ends. Mark birdbath dips where both ends contact but a gap remains in the center. This data-driven map shows whether isolated corrections or full-room underlayment leveling provides the most efficient solution. Plan materials precisely to fill low spots, then verify every marked area after correction before you begin flooring installation.

Grind or Sand High Subfloor Areas

Flatten marked high spots before filling low areas to avoid creating new dips or exceeding the floor’s flatness tolerance. This subfloor prep sequence protects finished-floor performance.

• Verify each mark with a 6–10-foot straightedge; gaps identify material to remove.

• On wood, use a floor edger or orbital sander with 36–60 grit, avoiding panel-joint cutthrough.

• On concrete subfloors, grind localized ridges using diamond cup or planetary tooling.

• Make small, controlled passes, then recheck; target 3/16 inch in 10 feet or 1/8 inch in 6 feet.

• Connect HEPA extraction, wear PPE, and contain dust to support cleaner, data-driven installation workflows.

Remove only enough material to restore plane. Aggressive grinding wastes substrate, increases labor, and can compromise engineered assemblies. Keep the tool moving, feather shifts, and inspect continuously for repeatable results.

Patch Low Spots and Birdbaths

Fill low spots and concave birdbaths only after removing adjacent highs, bringing the subfloor within the flooring manufacturer’s flatness tolerance—commonly 3/16 inch in 10 feet or 1/8 inch in 6 feet. For shallow dips, typically under 1/8 to 1/4 inch, apply a feather-fill or Portland-cement patch in thin, controlled passes. For deeper depressions, use a bonded patch system or multiple lift coats as specified; don’t exceed product depth limits.

On concrete, clean, mechanically profile, and prime before placing a cementitious patch or self-leveling underlayment engineered for the required depth and traffic. On a wood subfloor, correct substantial low spots with glued and screwed plywood, or use an approved patch over sound backer material. Don’t use compressible underlayment as filler; it can create movement and stress flooring joints. Allow specified cure time before installation.

Complete Final Subfloor Prep Checks

Before installation, verify that the subfloor meets the flooring system’s moisture, flatness, cleanliness, and structural requirements. A properly prepped subfloor protects performance because it's the foundation for every finish layer.

• Record moisture content at multiple locations using approved RH, calcium chloride, or wood-MC methods; compare results with manufacturer limits.

• Sweep a 6–10-foot straightedge across doorways, junctions, and room centers. Confirm Flat Enough tolerances, such as 3/16 inch over 10 feet.

• Mark and correct remaining high spots by sanding or grinding; patch birdbaths with approved underlayment.

• Remove nails, staples, adhesive ridges, and debris. Refasten loose panels with screws at specified edge and field spacing.

• Vacuum thoroughly, allow patches to cure, then recheck critical readings. Document results before releasing the floor for installation.

Don’t assume a clean surface is ready; validate each metric.

Frequently Asked Questions

How to Get a Little High Spot Out of a Sub Floor?

Mark the high spot with a straightedge, then choose the least invasive correction. On wood, use hand scraping, spot planing, or a sanding disc to gradually lower a seam; use a router rebate only for controlled, localized removal. On concrete, grind and vacuum thoroughly. Don’t use shim tapping to hide a hump. Recheck flatness, fill adjacent lows with a compatible self-leveler, and let it cure before installing flooring.

Why Should You Leave a 1.8 Gap Between Plywood Sheets in a Subfloor?

Leave a 1.8 mm gap to allow expansion as moisture and temperature change. This prevents panel edges from crushing together and reduces cupping, ridges, and squeaks. Maintain proper fastener spacing so sheets can move predictably without loosening. Although thermal contraction is minor in wood, the gap supports stable performance and helps your assembly meet installation tolerances over time, protecting the finished floor.

Should I Put Anything in Between the Subfloor and Underlayment?

Yes—install only materials your flooring system specifies between the subfloor and underlayment. You may need a vapor barrier or moisture membrane over concrete, an acoustic underlayment for sound control, thermal insulation in approved assemblies, or an adhesive primer before bonded products. Use anti-fracture membranes only where the flooring manufacturer permits them. Don’t add cushioning to hide defects; clean, fasten, patch, and level the subfloor first.

Is It Normal for a Subfloor to Be Uneven?

Yes, you’ll commonly find minor floor irregularities in a subfloor. Joist settlement, thermal movement, moisture warping, old adhesive, and installation variance can create waves, dips, or ridges. Verify conditions with a 6–10-foot straightedge and compare gaps to your flooring manufacturer’s flatness specification. Don’t assume a room-wide slope is a defect; target localized highs and lows. Sand high spots, remove adhesive, and patch low areas before installing underlayment or flooring.

Conclusion

You can’t skip subfloor prep and expect flooring to hide the problems. Once you confirm tolerance, moisture, and deflection, you’ll know exactly where to grind high spots and fill birdbaths. Even if the floor looks “close enough,” small deviations can cause movement, cracks, and visible seams. Recheck with a straightedge after every correction. When your subfloor meets the manufacturer’s requirements, you create a stable, flat surface that lets your finished floor perform as intended.