Ask ten flooring installers whether an SPC floor needs underlayment and you will get eleven answers. Some will tell you a pad is mandatory. Others will insist SPC already has one glued to the back. Both can be right, because both are answering different questions.

The confusion is understandable. Unlike laminate, which has almost no built-in cushioning, most SPC planks today ship with an acoustic underlayment already attached - typically a 1 to 1.5 millimetre IXPE or EVA foam layer bonded to the underside. That changes the entire conversation. What you are really asking is whether the built-in layer is enough for your particular floor.

This article separates the cases. It explains when the factory backer is all you need, when an extra layer genuinely helps soundproofing and comfort, and - critically - when adding a thick pad can actually destroy the locking mechanism you are about to pay good money for. Browse the SPC flooring range to see which formats carry built-in backers.

The rule for engineered wood and laminate is fairly settled: you almost always use an underlayment. The rule for SPC is not, because SPC sits in a different category. It is a rigid floating floor, largely waterproof, and most importantly it already has an acoustic layer in most residential formats.

When you add another underlayment on top of a built-in one, you stack two separate layers of cushioning under a rigid core. That stacking is where problems begin. It raises the floor height, changes the transition profile, and - worst of all - gives the locking tongue and groove room to move vertically.

The real question is not whether underlayment is good. It is whether your subfloor and your living situation demand more than the foam already glued to the plank. Read on to the four conditions before you buy a single roll of extra padding.

Check the back of a plank before you decide. Most SPC sold today carries a pre-attached backer. If you see a thin foam layer bonded to the underside of a 4 to 6 millimetre plank, that is your underlayment. It is already there, and the manufacturer designed the lock around it.

When that backer is present, an additional layer is usually unnecessary in these situations: a reasonably flat concrete slab, a dry and level subfloor, a room with no acoustic complaint, and a transition that has room for the installed height. On a sound, level slab in a typical single-family home, the factory backer does its job and the floor performs as intended - with no extra cost, no extra thickness, and no added risk to the lock.

There are four genuine reasons to put down extra underlayment under SPC. None of them are cosmetic. Each one solves a real performance problem.

1. Impact sound control in multi-storey buildings. This is the most common and most legitimate reason. In an apartment, or a room above a living area, footfall noise travels through the rigid core and into the ceiling below. A thicker, denser underlayment - cork or rubber - noticeably reduces the impact sound. If the floor is over a bedroom or a downstairs neighbour, this is where the extra layer earns its keep.

2. Correcting minor subfloor unevenness. SPC is rigid and will not conform to a lumpy slab the way a thin vinyl or carpet would. A thin underlayment can absorb small surface irregularities up to a point. Be clear, though: this only handles minor deviations. If the subfloor exceeds about 3 millimetres over a 2-metre span, no underlayment will fix it - the slab must be levelled.

3. Adding thermal comfort on cold slabs. Concrete is hard and cold underfoot. Many buyers choose SPC partly for its rigid feel, but in a basement or a room over exposed ground, a cushioning layer adds warmth and a more forgiving step.

4. Moisture protection on an unconditioned or unknown slab. SPC core is waterproof, but a vapour barrier underlayment can still help where a slab shows signs of dampness, especially in a basement or a slab below grade. If moisture is a real concern, a film-backed underlayment addresses it at the source.

Here is the part most guides leave out. SPC uses a rigid, mechanical locking system - the angle-and-tap profile at the edges - and that lock needs the plank to sit firmly on the floor. The rigid core relies on the subfloor to hold it and itself in place.

When you place a thick, compressible underlayment underneath, you add vertical movement. The more the pad compresses under walking pressure, the more the plank flexes at the seam. Over time, that flexing fatigues the locking tongue, and seams open up into visible gaps or the floor develops a squeak. This is the underlayment failure you read about in angry forum posts: a beautiful floor that starts to separate at the joins after a few months.

The rule of thumb: if your SPC already has a 1 millimetre backer, and you add another layer, keep the total combined underlayment beneath about 2 to 2.5 millimetres. Anything thicker, or anything too soft, starts to compromise the lock. Choose a low-compression underlayment with a high recovery rate, and always check the manufacturer's maximum allowable underlayment thickness - it is usually printed on the plank or in the technical datasheet.

If you have decided you do need extra padding, the material matters as much as the thickness. Five types dominate the market.

IXPE foam is the workhorse. It is cross-linked, low-compression, and dimensionally stable, with a reasonable acoustic rating at a thin profile. It is the default choice where you want a little extra warmth and sound control without adding height.

EVA foam is cheaper but less resilient. It recovers more slowly and can compress more over time, which makes it a riskier choice under a rigid lock. It works, but it is a budget option.

Cork is the acoustic premium. It is naturally dense, quiet, and eco-friendly, and it pairs well with the rigid core because it compresses very little. It costs more, and you must be careful about moisture, but it is the best at deadening footfall.

Rubber and felt are niche choices. Rubber gives excellent sound isolation but adds weight and height. Felt is thin and light but offers limited acoustic benefit. For most homeowners, the practical decision is between a thin IXPE and cork, depending on whether impact sound is your main concern.

Before you buy any underlayment, run this simple check on site. It takes under five minutes and answers the question with your floor, not with a habit.

First, look at the plank's underside. Is there a factory backer, and how thick is it? If the plank already carries a 1 to 1.5 millimetre foam layer, that is your baseline. Second, lay a 2-metre straightedge across the subfloor and measure the gap at its worst point. If it exceeds 3 millimetres, the floor needs levelling, not padding.

Third, ask whether anyone lives below this room. If this floor sits above a bedroom, a living space, or a shared apartment boundary, add an acoustic layer. If it is on grade or above a garage, the sound case is weaker. Fourth, check whether the subfloor feels damp and whether it is concrete over grade - a film-backed underlayment helps there, and the manufacturer's moisture threshold is the guide.

If the answer to sound is yes, choose a dense, low-thickness cork or IXPE underlayment and keep the total combined pad under 2.5 millimetres. If the answers point to no, save the money and install on the factory backer alone. That is the entire decision, and it is simpler than the market makes it sound. Follow the full installation sequence in the SPC installation guide for the practical steps.