So, you’ve pulled up that crusty old carpet or vinyl tile and instead of a pristine canvas, you’ve found a subfloor that looks like a topographical map of the Rolling Hills.
Don't panic. Whether you're dealing with a "vintage" dip or a plywood mountain, leveling your subfloor is the difference between a floor that feels like a luxury hotel and one that squeaks, bounces, and eventually cracks your expensive new floor.
Here is how to get your floor on the level without losing your mind.
1. The "Crime Scene" Investigation
Before you buy a single bag of mix, you need to find the trouble spots.
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The Straightedge Test: Take a 6-foot or 8-foot level (or a very straight 2x4) and slide it across the floor.
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The Pivot: When the level rocks like a seesaw, you’ve found a high spot.
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The Gap: When you can see light under the level, you’ve found a low spot (a "birdbath").
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Mark it up: Use a carpenter's pencil to circle the dips and "X" the peaks.
2. Prep: The Most Boring (and Important) Part
If you skip prep, your leveling compound will peel up like a bad sunburn in six months.
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Screw it Down: Walk the floor. Hear a squeak? Drive a subfloor screw into the joist. This is your only chance to silence the floor forever.
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Clean Freak Mode: Vacuum every spec of dust. Leveler won't stick to dirt.
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Prime Time: If you’re using self-leveling underlayment (SLU), you must use the manufacturer's primer. It seals the wood so the water in the leveler doesn't soak in too fast, causing it to crack.
3. Choose Your Weapon
Different problems require different solutions:
| The Problem | The Solution | Why? |
| High Ridges/Humps | Belt Sander or Planer | You can't fill a mountain; you have to grind it down. |
| Deep Dips (> 1/2") | Self-Leveling Underlayment | It’s liquid magic that finds its own level via gravity. |
| Minor Imperfections | Floor Patch (Skim Coat) | Thicker consistency; great for feathering out small transitions. |
4. The Pour (The Point of No Return)
If you’re using Self-Leveling Underlayment, speed is your best friend. This stuff sets up fast.
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Seal the Perimeter: Use foam sill sealer or even painters tape to block vents and gaps. You don’t want your expensive leveler leaking into the basement or the HVAC ducts.
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Mix Like a Pro: Use a high-torque drill and a mixing paddle. Follow the water-to-powder ratio on the bag to the gram. Too watery? It'll be weak. Too thick? It won't flow.
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The Pour: Start at the furthest corner and work toward the door.
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Help it Along: Use a long-handled squeegee or a spiked roller to spread it and pop any air bubbles. Then? Walk away. Do not touch it while it’s drying.
Pro Tips for a Flawless Finish
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The Two-Person Rule: If you’re leveling a large room, have one person dedicated to mixing and one person dedicated to pouring/spreading. If you stop to mix a second bag, the first bag will already be "skinning over," creating a nasty seam.
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Check for "Flat" vs. "Level": In an old house, the whole floor might lean 2 degrees. You don't necessarily need it perfectly level (earth-parallel), you just need it flat (no bumps or dips) so your flooring sits right.
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Mind the Mesh: If your subfloor has a lot of movement, stapling down some plastic lath (mesh) before pouring can help prevent the leveler from cracking later.