Why SPC Flooring Gaps, Lifts and Cracks at the Joints —
and the Annealing Step That Prevents It
When SPC flooring gaps, lifts or cracks at the joints a few months after a perfect-looking installation, the usual cause is not the installer and not the click system — it is residual internal stress left in the core during manufacturing. Extrusion leaves the rigid PVC-limestone core under stress; if that stress is not relieved by proper annealing and conditioning before the planks are profiled and packed, it releases later — when the floor is warmed by sun or underfloor heating — and the plank shrinks or distorts. Ecoflors conditions every batch for 48 hours after production specifically to release that stress before the floor ever reaches site.
This is one of the most expensive failure modes in commercial resilient flooring, because it appears after handover. The floor passes inspection, the project closes, and then — one to three months later, often once heating is switched on — joints begin to open, plank ends lift, or hairline cracks appear at click edges. The replacement cost, the disruption and the reputational damage all land after the money has been spent.
For specifiers and importers, the value of understanding this mechanism is simple: it lets you ask the one manufacturing question that separates a floor that stays flat from one that fails in service — long before a container is loaded.
The Three Symptoms — and Why They Show Up Late
Under-relieved core stress presents in three recognisable ways. All three share the same signature: the floor looked perfect at handover and degraded weeks later.
The reason all three appear late is that the trigger is heat. At the stable temperature of an installation day the stress stays locked in. It is the first real thermal cycle — summer sun through glazing, or the first time underfloor heating runs — that gives the trapped stress the energy to release. The floor was always going to move; the heating simply set the timer.
The Root Cause: Residual Stress From Extrusion
SPC is made by feeding a calcium-carbonate-and-PVC compound through a twin-screw extruder at roughly 180–200°C, then calendering it to thickness and cooling it. That process is violent at the molecular level — the polymer chains are sheared, stretched and then frozen in place as the sheet cools. The result is a rigid board that carries locked-in internal stress, like a spring held under tension inside the material.
This is well-established in the industry: insufficient annealing leaves residual stress that causes warping and joint failure after installation. It is not a controversial claim — it is the reason every serious SPC line has a cooling and conditioning stage, and the reason the quality of that stage is what separates manufacturers. Core density tells you the floor was formulated correctly. Annealing tells you the floor was finished correctly. A floor can have a perfect 1.95–2.05 g/cm³ density and still fail if the stress was never relieved.
What Annealing and Conditioning Actually Do
Annealing is the controlled release of that stored stress at the factory, where it can do no harm, instead of on site, where it ruins a floor. The principle is the same one used for glass and metals: hold the material through a managed thermal cycle so the internal structure relaxes to a stable, low-stress state — and then let it rest until it reaches equilibrium with normal ambient conditions.
For SPC the sequence is a managed cool-down after the core is formed, followed by a conditioning period where the boards rest at ambient temperature before they are milled into click profiles and packed. That resting time is the part most easily cut to hit a shipping deadline — and the part that decides whether the floor stays flat. The stress has to be given time to release; you cannot rush equilibrium.
Every Ecoflors SPC batch is conditioned for 48 hours after production before profiling and packing. The boards are brought through a controlled cool-down and then rested so the core reaches dimensional equilibrium and the residual extrusion stress is released at the factory — not in your client’s building. This is why the dimensional stability we declare (≤0,10%, EN ISO 23999) holds in service, not just on the day of testing.
The Production-to-Pack Sequence
Where the 48 hours sits in the full process — and why its position matters. Conditioning happens before profiling, so the floor is milled to its final click geometry only after it has stabilised:
Why Underfloor Heating Is the Real Test
Underfloor heating is where under-annealed flooring fails most reliably, because UFH does exactly what releases trapped stress: it cycles the floor through heat, day after day. An under-conditioned plank may survive a temperate room and then fail within a season over UFH.
This is the other half of the stability story told in our 3:1 calcium-to-PVC core density guide. Density gives the core the mineral mass to resist expansion; annealing ensures there is no stored stress waiting for the heat to unlock it. You need both. A correctly formulated, properly conditioned SPC floor stays flat and joint-tight across the UFH range; a high-density floor that skipped conditioning will not.
A supplier can show you a laboratory density report of 1,95–2,05 g/cm³ and a dimensional-stability figure of ≤0,10% — both measured on a freshly tested sample — and the floor can still gap in service if the production batch wasn’t conditioned. The lab sample tells you the formulation; only the process tells you the stress was released. Ask about both.
The Parameters That Matter — and Which You Can Verify
| Parameter | Value | What it tells you |
|---|---|---|
| Post-production conditioning | 48 hours | The stress-relief step. Ask the manufacturer directly; it is a process commitment, not a lab number. |
| Core density | 1,95–2,05 g/cm³ | Formulation quality. Independently testable to ISO 1183 by any accredited lab. |
| Dimensional stability | ≤0,10% | EN ISO 23999. Across a 1.000mm plank, ≤1mm change. Verify it’s measured per batch, not once. |
| Water absorption | 0% · waterproof core | The limestone-PVC core doesn’t absorb moisture — independent of the annealing question. |
| UFH compatibility | Yes · within stated limits | Conditioned core stays flat across UFH cycling. Confirm the maximum surface-temperature limit on the TDS. |
How to Verify It Before You Order
You cannot see annealing in a sample swatch, and you cannot read it off a density report. But you can ask three questions that a manufacturer who conditions properly will answer immediately — and a manufacturer who rushes will not:
None of these requires a lab. They reveal whether stability is something the manufacturer builds in through process, or something they hope the formulation alone delivers. For the full picture on how to evaluate an SPC supplier on technical merit, see our guide to the four specs every wholesale buyer must check.
Frequently Asked Questions
Request a full-thickness SPC sample with the technical data sheet — core density (1,95–2,05 g/cm³, ISO 1183), dimensional stability (≤0,10%, EN ISO 23999) and the UFH surface-temperature limit. Every batch conditioned 48 hours before profiling. Dispatched within 5 business days.