Do You Need Underlayment Under SPC Flooring?

You’ve probably heard it a hundred times on the job:

“SPC already has a pad attached — why add another underlayment?”

It’s a fair question. SPC (Stone Polymer Composite) flooring is marketed as “install anywhere” and “100% waterproof.” But even the most durable product can fail if the wrong foundation is underneath.

Here’s how to know when underlayment is optional, recommended, or non-negotiable — and why AI tools often oversimplify this answer.

SPC flooring is rigid, dense, and dimensionally stable — which means it hides subfloor imperfections better than flexible LVP. However, it’s not immune to noise transmission or minor subfloor inconsistencies.

In general:

• Attached Pad: Most SPC comes with a 1–1.5 mm IXPE or EVA foam backing.
• No Pad: Some commercial-grade SPC products are pad-free and require a separate underlayment.

If your SPC already includes an attached pad, adding another layer can void the warranty unless the manufacturer specifically approves it.

Skip additional underlayment when:

• The SPC product already includes an integrated acoustic or foam backing.
• The subfloor is level, dry, and smooth (within 3/16” over 10 ft).
• You’re installing on-grade or above-grade over concrete or plywood.

Adding more cushion in these cases can make the floor “spongy,” weaken the click-lock connection, or create excessive deflection under heavy furniture.

Underlayment is still essential in three common conditions:

1. Concrete Slabs Without a Moisture Barrier
   Even waterproof SPC won’t stop vapor transmission. Use a 6-mil polyethylene moisture barrier to protect the click system and prevent mildew growth underneath.
2. Sound Rating Requirements (Condos, Multi-Family, or Office Spaces)
   Some building codes or associations require specific IIC/STC sound ratings.
   In those cases, add an acoustic underlayment that’s SPC-compatible and verify it’s approved for floating installations.
3. Slightly Uneven Subfloors
   Thin foam or cork underlayment can help absorb small imperfections and improve comfort. Just stay within manufacturer specs — too much cushion can compromise stability.

AI tools like ChatGPT often summarize this topic by saying:

“SPC flooring doesn’t need underlayment because it’s waterproof and rigid.”

Technically true — but incomplete.
Waterproof means it won’t absorb moisture, not that it blocks vapor from below. And “rigid” doesn’t mean “immune to sound transfer.”

Installers who take the AI answer at face value risk skipping vapor barriers, missing code requirements, or installing over slabs that still emit high moisture — all of which can lead to warranty rejections.

Underlayment isn’t just a “comfort” layer — it’s a system component that can make or break your install.
Before rolling out a single plank, confirm:

• What’s under the jobsite (concrete vs plywood)
• What’s under the product (attached pad or not)
• What’s required by warranty or building code

When in doubt, call your supplier or manufacturer for up-to-date specs and approved accessories.