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Concrete overlays are a popular choice for transforming tired or uneven floors into smooth, durable, and visually striking surfaces. Whether you’re using Rapid Set TRU PC, TRU SP, or TRU Self-Levelling, success depends on one crucial factor: the condition of the subfloor and finding the best Subfloor for the concrete overlay.
In this complete guide, we explain:
- What makes the best subfloor for concrete overlays
- Surfaces that overlays should never be applied to
- Common subfloor problems to watch out for
- How to properly prepare your concrete base
Whether you’re a contractor, DIYer, or working with a professional flooring supplier, this guide will help you avoid costly mistakes and achieve a flawless finish.
What Is the Ideal Subfloor for a Concrete Overlay?
The perfect base for a self-levelling concrete overlay or polishable topping must be:
Clean and Contaminant-Free
Use mechanical surface preparation—like diamond grinding or shot blasting—to remove old adhesives, oils, paint, and surface laitance. A clean surface ensures proper bonding and long-term durability.
Dry and Moisture-Tested
Moisture is the silent killer of many overlay jobs. Use relative humidity (RH) probes or calcium chloride tests to confirm the slab is dry enough for overlay application. If moisture is high, install a vapor barrier or use a compatible overlay system.
Structurally Sound and Crack-Free
The substrate should be strong and stable, with a compressive strength of at least 25 MPa. Repair any cracks or weak areas before applying the overlay using epoxy resin or cementitious repair mortars like Cement All.
Flat or Pre-Levelled
Self-levelling overlays like TRU Self-Levelling require a relatively level base. Large dips or high spots should be corrected beforehand using screeds or grinding equipment.
Common Subfloor Problems That Can Ruin Your Overlay
Even on concrete, there are plenty of red flags to watch for:
Concrete That’s Too Thin or Weak
Concrete slabs less than 50mm thick may not handle the added load from the overlay and can flex or crack. Always test for compressive strength and reinforce weak areas if needed.
Previous Coatings and Sealers
Existing coatings like epoxy or acrylic block adhesion. You must mechanically remove these using floor grinders, scarifiers, or shot blasters.
Contamination from Oils, Adhesives, or Grease
Oils and chemicals penetrate concrete and interfere with bonding. Use C2 Degreaser, then mechanically clean the surface.
Excessive Moisture
High moisture levels lead to delamination, efflorescence, or bubbling. Always test, and if necessary, use a damp-proof membrane or moisture-tolerant Epoxy primer.
Movement Cracks or Structural Issues
Dynamic cracks will transfer through the overlay unless properly addressed with stitching, epoxy injection, or (flexible joint) Polyurea systems like Rapid Refloor.
Subfloors You Should NEVER Overlay
Avoid overlaying these types of substrates unless completely removed or isolated:
- Wood, chipboard, or plywood – too much movement
- Bitumen or asphalt floors – too soft and unstable
- Old paint or sealed surfaces – no mechanical bond
- Anhydrite screed – This is never recommended, due to the poor bond properties
- Contaminated industrial concrete – risk of long-term adhesion failure
How to Prepare a Subfloor for a Concrete Overlay
Here’s a simple checklist before pouring a concrete overlay:
- Mechanical Preparation
Use a floor grinder, shot blaster, or scarifier depending on the condition of the existing floor. - Moisture Testing
Check moisture content with RH or calcium chloride tests. Apply moisture barriers if needed. - Priming the Surface
Use the correct primer for your chosen overlay (e.g., Epoxy primer for TRU overlays). - Repairing the Substrate
Fill cracks, holes, and damaged sections with compatible repair compounds. Like Cement All or Metzger Mcguire Rapid Refloor. - Edge and Joint Protection
Protect joints and edges from cracking with expansion joints, tape, or formwork.
Final Thoughts
Concrete overlays offer a fast and effective way to resurface floors—but only if the subfloor is properly prepared. A perfect subfloor is clean, dry, sound, and strong enough to support the overlay. Thin, weak, or contaminated concrete can cause problems down the line. Avoid shortcuts and incompatible surfaces, and your overlay will look great and last for years.
If you’re unsure about your subfloor or need help choosing the right prep equipment, Multi-Hire Power Tools is here to help. We supply hire equipment and consumables, plus full training on TRU overlays—so you can prep and pour with confidence.
Get in Touch Today
Call us, visit our showroom, or message us online for tool hire, consumables, and expert overlay advice. Let’s make your next overlay project a flawless success.
For more information on The Perfect Subfloor for a Concrete Overlay: What to Know, What to Avoid talk to Smartfloor UK