Removing old adhesive from a concrete slab is one of the most important — and often most underestimated — steps before laying new flooring. A clean, flat, and sound substrate is essential for any new floor covering to bond correctly and last the distance. Depending on the type of adhesive, its age, and how well it has cured into the slab, removal can range from straightforward to quite labour-intensive.
Why Proper Adhesive Removal Matters
Residual adhesive left on a slab causes real problems: it creates an uneven surface that can telegraph through vinyl, carpet, or tiles; it can prevent new adhesive from bonding properly; and it may cause new floor coverings to lift or bubble over time. Most flooring manufacturers and adhesive suppliers — including Ardex and Mapei — specify that the substrate must be clean, dry, and free of contaminants before their products are applied. Skipping this step can void product warranties.
Identify the Adhesive Type First
Before you start, try to identify what adhesive you're dealing with. Common types found on NZ slabs include:
- Bitumen-based (black mastic) — very common under older vinyl and carpet tiles, especially in homes built before the 1990s
- Acrylic or latex-based — used under vinyl sheet and planks, usually tan or cream coloured
- Epoxy adhesives — very hard, often used under tiles or heavy-duty flooring
- Rubber-based contact adhesives — used under carpet and some vinyl
⚠️ Important: Asbestos risk. Some older adhesives — particularly black bitumen mastics used before the mid-1980s — may contain asbestos fibres. If your home was built before 1990 and you're unsure of the adhesive's history, do not sand, grind, or scrape it dry. Have a sample tested by a licensed asbestos assessor before proceeding. WorkSafe NZ has guidance on managing asbestos in homes.
Tools and Materials You'll Need
- Floor scraper (long-handled, with replaceable blades)
- Angle grinder with a diamond cup wheel or scarifier attachment
- Orbital floor grinder (available to hire from most tool hire shops)
- Heat gun or hot air blower (useful for softening stubborn adhesive)
- Chemical adhesive remover/stripper (water-based products are safer indoors)
- Stiff-bristle brush and dustpan
- PPE: safety glasses, dust mask (P2 rated minimum), gloves, knee pads
- Vacuum with HEPA filter
Step-by-Step Removal Methods
1. Mechanical Scraping
For thicker or more brittle adhesive, start with a long-handled floor scraper. Work at a low angle and use firm, consistent strokes. This is effective for acrylic and rubber-based adhesives that have dried out and become friable. Replace blades regularly — a sharp blade makes a huge difference.
2. Heat Application
A heat gun softens many adhesives, making them easier to scrape away. Keep the gun moving to avoid scorching the concrete. This works particularly well on rubber-based and some acrylic adhesives. Avoid using heat on bitumen-based products as it can make them smear and spread.
3. Chemical Strippers
Water-based adhesive removers are the safest option for indoor use and are widely available at Bunnings and Mitre 10. Apply the stripper as directed, allow it to dwell (usually 15–30 minutes), then scrape away the softened adhesive. Always follow the manufacturer's instructions and ensure good ventilation. Solvent-based strippers are more aggressive but require careful handling and thorough ventilation.
4. Grinding and Scarifying
For stubborn residue or large areas, hiring a floor grinder with a diamond cup wheel is the most efficient approach. This removes adhesive and lightly profiles the concrete surface, which actually improves adhesion for new products. Wet grinding reduces dust; if grinding dry, use a vacuum with a HEPA filter. This method is best for acrylic and epoxy adhesives.
For bitumen mastic that is confirmed asbestos-free, grinding is effective but generates significant dust — always use appropriate respiratory protection.
Dealing with Bitumen (Black Mastic) Adhesive
Bitumen mastic is notoriously difficult to remove completely. Once you've confirmed it's asbestos-free, you have a few options:
- Encapsulate it — some flooring systems and self-levelling compounds (such as those from Ardex or Mapei) are specifically formulated to go over well-bonded, thin bitumen residue. Check the product datasheet carefully.
- Grind it back — a scarifier or angle grinder can remove the bulk, though it's messy work.
- Chemical removal — some specialist products are designed for bitumen, but results vary.
If the mastic is thick and uneven, grinding or encapsulation with a suitable levelling compound is usually the most practical path.
Preparing the Surface After Removal
Once the adhesive is gone:
- Vacuum thoroughly with a HEPA vacuum to remove all dust and debris.
- Check for laitance — the weak, powdery surface layer on concrete. Lightly grind or acid-etch if present.
- Fill any holes or cracks with a suitable patching compound.
- Check flatness — most flooring manufacturers require the slab to be within 3 mm over a 1.8 m straightedge. Use a self-levelling compound if needed.
- Allow to dry — concrete must be sufficiently dry before laying moisture-sensitive flooring. A moisture test (calcium chloride or in-situ probe) is recommended, especially in NZ's variable climate.
When to Call a Professional
Call a licensed flooring installer or remediation specialist if:
- You suspect the adhesive may contain asbestos — this is not a DIY job
- The adhesive covers a large area and grinding equipment is required
- The slab has significant damage, cracks, or moisture issues that need assessment
- You're unsure about the correct preparation for your chosen new flooring product
A qualified flooring installer will have the right equipment and know-how to get the substrate right the first time. You can find experienced local flooring professionals on tradedirectory.co.nz.
