Subfloor vs Underlayment: Key Differences Every Homeowner Should Know 🏡

When planning a flooring project, it’s easy to get confused between subfloor vs underlayment. While they work together, they serve very different purposes. Knowing the difference helps you choose the right materials and ensures your flooring lasts for years without squeaks, cracks, or uneven spots.

The subfloor is the structural base attached to the floor joists, while the underlayment is a thin layer installed above the subfloor to provide a smooth surface for finished flooring.

In this article, we’ll break down subfloor vs underlayment, explain the role of underlayment for tile on plywood, and answer whether does underlayment help with uneven floors.

Subfloor vs Underlayment: The Basics

Subfloor

• Structural base that sits on floor joists.

• Common materials: plywood, OSB (oriented strand board), or concrete.

• Provides strength and stability for the entire floor system.

Underlayment

• Thin layer (⅛–½ inch) installed over the subfloor.

• Materials: cement board, foam, cork, or plywood.

• Prepares a smooth, level surface for finished flooring like tile, laminate, or vinyl.

Pro Tip: Think of subfloor as the foundation of your house, and underlayment as the finishing touch that makes flooring look and perform its best.

Underlayment for Tile on Plywood

If you’re installing tile, proper underlayment is critical. Directly tiling onto plywood can cause cracking because wood expands and contracts with humidity.

Best options for underlayment for tile on plywood:

• Cement board (like HardieBacker) – Provides a stable, moisture-resistant surface.

• Uncoupling membranes (like Ditra) – Help absorb movement between plywood and tile.

• Plywood underlayment – Only used as a secondary layer when properly treated and paired with cement board.

Without underlayment, tiles may crack or grout lines may fail prematurely.

Does Underlayment Help With Uneven Floors?

Many homeowners ask: does underlayment help with uneven floors? The answer: yes, but only to a degree.

• Foam or cork underlayments can smooth out minor imperfections.

• For laminate or vinyl flooring, thicker underlayment adds comfort and helps hide small dips.

• For tile, cement board underlayment doesn’t level floors—it only provides stability. Significant unevenness must be corrected with a leveling compound before installation.

Underlayment improves performance but won’t fix severe structural issues. Always repair the subfloor first if it’s sagging or damaged.

FAQ: Subfloor and Underlayment

What’s the main difference between subfloor vs underlayment?

The subfloor is the structural base; the underlayment is the smooth surface layer for finished flooring.

Do I always need underlayment?

Not always—some vinyl or laminate floors come with built-in underlayment, but tile almost always requires it.

What’s the best underlayment for tile on plywood?

Cement board or uncoupling membranes for maximum strength and moisture resistance.

Does underlayment help with uneven floors?

Yes, but only for minor imperfections. Major unevenness needs floor leveling.

Can I install flooring directly on the subfloor?

Yes for some materials, but underlayment often improves durability, soundproofing, and moisture protection.

Conclusion

Understanding subfloor vs underlayment is essential for any flooring project. The subfloor provides strength and stability, while underlayment ensures a smooth, long-lasting finish. For tile installations, using proper underlayment for tile on plywood is crucial to prevent cracking. And while many ask does underlayment help with uneven floors, it only corrects small imperfections—serious dips need leveling before installation.

With the right combination, you’ll end up with floors that look great and last for decades. ✅