Views: 0 Author: Site Editor Publish Time: 2026-09-11 Origin: Site
SPC flooring is often described as an easy flooring solution. It is waterproof, rigid, dimensionally stable, and its click-lock installation can be much faster than traditional flooring systems.
But there is one part of the installation that must be paid additional attention to:
The Subfloor.
A good SPC floor still needs a good surface beneath.
If the subfloor is uneven, damp, contaminated, unstable or structurally unsound, the problems may eventually appear on the finished floor. Depending on the situation, this can mean movement, plank separation, edge damage, telegraphing, buckling or other installation failures.
This is why professional SPC installation does not kick off a project with opening the first carton of flooring. Everything begins with checking what is underneath.
SPC flooring has a rigid composite core. This rigidity gives it excellent resistance to indentation and helps it maintain its shape during normal use.
However, rigidity also means that the flooring can NOT tolerate every imperfection underneath it.
Think of the subfloor as the foundation of the installation.
A small raised spot, loose section or moisture problem may not look serious when the subfloor is exposed. Once SPC planks are installed over it, however, the finished floor can make those problems much more noticeable.
The installation references therefore repeatedly emphasize the same basic requirements:
The subfloor needs to be flat, clean, dry and structurally sound before SPC flooring goes down.
And there is another popular misunderstanding:
SPC flooring is waterproof itself, but the flooring should not be treated as a moisture barrier for the building structure.
Excessive moisture below the flooring can still contribute to mold, hydrostatic pressure and other substrate-related problems. The installation references therefore require moisture and pH issues in concrete to be identified and corrected before installation.
So when an installer says "the floor is waterproof", this should never mean "we don't need to check the subfloor for moisture."
The basic principle is not unique to SPC.
Laminate, engineered wood, LVT and other flooring systems also require a suitable substrate. But the preparation procedure can be different because each flooring system has different construction, installation methods and tolerance for substrate movement.
For SPC, one important characteristic is that many products are designed for floating installation and use a rigid click-lock system. Some SPC products can also be installed by glue-down methods, depending on the manufacturer's instructions.
This creates several practical differences.
Some flooring systems use relatively thick or soft underlayments to compensate for minor irregularities or provide cushioning.
That should not automatically be transferred to SPC.
The RFCI reference specifically warns that an excessively thick or soft pad can allow excessive vertical movement, which may contribute to edge damage or plank separation. Where an additional sound-reducing pad is permitted, it needs to meet the product's specified requirements.
For SPC products with an attached pad, adding another layer underneath is generally not necessary unless the manufacturer's installation instructions specifically allow it.
A subfloor may look acceptable to the eye while still having excessive variation.
Professional preparation therefore involves actually checking the surface with appropriate straightedges, levels or other measuring equipment.
One RFCI SPC installation guide specifies a maximum variation of approximately 3.2 mm within 1.8 m, or 5 mm within 3 m, together with limits on slope and subfloor gaps.
The exact tolerance should always come from the installation instructions for the particular SPC product.
SPC can sometimes be installed over an existing hard floor.
For example, the reference allows certain well-adhered resilient flooring, non-cushioned vinyl, terrazzo, ceramic tile and stone tile, provided the existing surface is clean, flat, dry, secure and structurally sound. Grout lines and other depressions may need to be filled.
But carpet, cushion-backed vinyl, loose flooring, laminate and other floating floors are examples of substrates that may need to be removed rather than covered.
“Can SPC be installed over an existing floor?” is not a simple yes-or-no question.
The condition and construction of the existing floor matter.
A professional installer should look at the installation from several angles rather than checking only whether the floor “looks flat.”
A useful way to think about the preparation process is:
Site → Subfloor → Moisture → Flatness → Cleanliness → Repair → Final inspection → Installation
Each stage answers a different question.
Before the flooring arrives, construction work that introduces moisture—such as painting, drywall, concrete, masonry and plumbing—should be completed and dry.
The installation area should also be enclosed and temperature controlled. One RFCI reference specifies an environment of approximately 16–27°C with relative humidity of 30–60% for at least five days before delivery, during installation and afterward.
The exact environmental requirements should follow the flooring manufacturer's current guide.
The surface should not flex excessively, sag or move under normal loading.
For wood subfloors, loose boards and squeaks should be repaired before installation. The reference also specifies moisture limits and structural requirements for wood substrates.
If the substrate itself is unstable, simply putting SPC over it does not solve the underlying problem.
Use an appropriate straightedge or other measuring method to identify:
high spots
low spots
depressions
cracks
grooves
excessive slope
gaps or joints that could affect the installation
High spots may need to be removed, while low areas and other imperfections may require a suitable patching or leveling compound.
The goal is not necessarily to make every substrate perfectly level.
The more important question is whether it is sufficiently flat and stable for the particular SPC flooring system.
This is particularly important for concrete.
Moisture and alkalinity testing should be performed on concrete substrates, and the reference specifies ASTM F2170 or ASTM F1869 moisture testing together with ASTM F710 pH testing.
The acceptable limits depend on the specific flooring system and installation method.
Do not assume that an old concrete slab is automatically dry enough simply because it looks dry.
Dust, grease, wax, paint, oils, old adhesive residue and other contaminants can interfere with the installation.
A prepared substrate should therefore be thoroughly cleaned before flooring installation.
For existing resilient flooring and old adhesives, removal methods also need special care. The RFCI guide warns against mechanically disturbing certain existing resilient flooring or adhesive materials because they may contain hazardous substances.
The exact tools will vary according to the condition and type of substrate, but a professional installer may need:
Tape measure
Straightedge
Level or appropriate flatness-checking tool
Pencil and chalk line
Moisture meter for wood
Concrete moisture-testing equipment
pH testing equipment where required
Broom or vacuum cleaner
Scraper
Suitable patching or leveling tools
Appropriate leveling compound
Utility knife
Personal protective equipment
For the flooring installation itself, commonly listed tools also include spacers, rubber mallet, tapping block, pull bar and cutting tools.
The important point is that subfloor preparation requires measuring and testing tools, not just installation tools.
A rubber mallet can help install an SPC plank.
It cannot tell you whether the concrete underneath is too wet.
Walk through the entire room before doing any preparation work.
Check:
the type of existing subfloor
visible damage
uneven areas
cracks and joints
moisture-related problems
loose boards
existing flooring
areas around doors, cabinets and other fixed elements
Do not begin installation simply because one section of the room looks acceptable.
The entire installation area needs to be considered.
Make sure the building is enclosed and the required environmental conditions can be maintained.
All relevant wet work should be finished and dry before the flooring is delivered or installed.
For temperature-controlled SPC installations, maintain the environment according to the manufacturer's requirements rather than relying on temporary heating or cooling.
Also consider whether the room has unusual exposure to direct sunlight, radiant heating or other conditions requiring special installation instructions.
The preparation method depends heavily on what is underneath.
Check:
curing condition
flatness
cracks and depressions
moisture
alkalinity/pH
surface cleanliness
strength and structural condition
Concrete imperfections should be repaired with appropriate patching or leveling materials before installation.
Check:
structural stability
moisture content
loose boards
squeaks
joints
surface flatness
Loose or noisy sections should be repaired before SPC is installed.
Determine whether the existing floor is actually suitable to remain.
It must generally be secure, clean, dry, flat and structurally sound. Some existing hard coverings can remain, while carpet, soft materials and floating flooring may need to be removed.
Now check the substrate rather than judging it by eye.
Use a straightedge or other appropriate measuring equipment to locate high and low areas.
Where the measured variation exceeds the product's permitted tolerance, correct the substrate before installation.
Do not try to solve a major subfloor problem by relying on a thicker underlayment unless that specific underlayment is approved for the SPC product.
For concrete, perform the required moisture and pH tests.
For wood, use an appropriate wood moisture meter.
For example, the RFCI reference sets a maximum wood subfloor moisture content of 12% and gives specific moisture and pH limits for concrete.
These figures should not be copied blindly to every SPC product. Different manufacturers and installation systems can specify different limits.
If the substrate fails the required test, solve the moisture problem before installing the floor.
Correct the problems identified during inspection.
Typical preparation may include:
securing loose wood sections
removing high spots
filling low areas
filling cracks or grooves where required
filling tile grout lines where applicable
repairing damaged sections
removing unsuitable existing flooring
applying an appropriate leveling or patching compound
The objective is to create a sound, flat and clean installation surface, not simply to make the floor look better.
After repair and leveling work, remove dust, debris and other contaminants.
Vacuum or sweep the entire installation area.
Pay particular attention to small particles that can remain underneath rigid flooring and create localized pressure points.
The surface should be clean and ready before the SPC planks are brought into the final installation process.
Before opening cartons and beginning installation, make one final inspection.
Ask:
Is the jobsite ready?
Is the subfloor structurally sound?
Is it flat enough?
Is it dry enough?
Is it clean?
Have all necessary repairs been completed?
Is the installation method suitable for this substrate?
If the answer to any of these questions is no, stop and solve the problem first.
This is also the stage where the installer should document relevant installation information when required. The RFCI guide, for example, recommends recording the installation date, product SKU and lot number, subfloor moisture content, relative humidity and site temperature for warranty purposes.
One of the advantages of SPC flooring is that installation can be relatively straightforward.
But that does not mean the preparation can be skipped.
A click system can lock two planks together very quickly. It cannot repair an uneven concrete slab.
A waterproof core can resist water exposure. It cannot correct excessive moisture coming from an improperly prepared substrate.
And a rigid plank can provide a durable finished surface. It cannot make an unstable subfloor structurally sound.
That is why experienced installers spend time on the part of the project that the customer may never see.
Good SPC installation starts underneath the floor.
Once the subfloor is properly inspected, tested, repaired and cleaned, the actual flooring installation becomes much more predictable—and the finished floor has a much better chance of performing as intended.
A simple rule to remember:
Before installing SPC, don't ask only "Can I install the floor here?"
——Ask "Is the subfloor ready for this floor?"
