Grinding Subfloor before Levelling, Removing Adhesive and Bitumen

Old adhesive and bitumen are often soft, greasy or only loosely stuck to the concrete. Levelling compound will flow over them, but it bonds only to that layer, not to the solid base. When the old adhesive moves under load or with changes in temperature, the compound peels, cracks or leaves a hollow underneath. After a few months this shows up as creaking vinyl, cracked tiles or a hollow sound when you tap the floor.

Grinding therefore has two jobs: remove everything that is not firmly attached, and roughen the surface so the primer can soak in and the compound can grip.

Before anyone switches on a grinder, it pays to know what kind of adhesive you are dealing with. In older houses and flats this is a question of health as well as technique.

• Shiny black adhesive under old parquet may be tar-based. These adhesives can contain harmful polycyclic aromatic hydrocarbons (PAHs), which are released into the dust during grinding
• Old vinyl tiles and some older adhesives may contain asbestos. If there is any suspicion, have the material tested before work starts. Asbestos removal must be carried out by a licensed contractor where local rules require it. Your local authority or national health and safety body can tell you what applies
• Ordinary water-based adhesives under carpet or vinyl tend to be lighter in colour and softer, and during grinding they smear rather than crumble

The base itself also needs assessing. A sound concrete screed is ground differently from a crumbly old screed or a timber subfloor. On timber the work is gentler so the boards are not ground too thin, and the levelling compound must be one the manufacturer specifically approves for timber.

1. Clear the room, remove skirting boards and seal off doors, thresholds and openings where dust could escape.
2. Scrape off thick lumps of adhesive that would otherwise clog the grinding discs.
3. Grind the area with a floor grinder fitted with coarse diamond tooling and connected to an industrial vacuum.
4. Finish the edges, corners and areas along the walls with a smaller grinder where the large machine cannot reach.
5. If needed, go over the floor with finer tooling to even out the surface.
6. Vacuum the whole floor thoroughly, including the corners. Sweeping alone is not enough.
7. Check the base for strength, cleanliness and flatness and mark any areas that need repair.

After grinding comes the primer, chosen to suit the base and the levelling compound, and if needed sealing of corners and pipe penetrations so the compound does not run into gaps. That is a separate job on the price list.

Grinding is not always the only option. The floor layer chooses the method according to what they find.

• Thin adhesive on sound concrete: standard grinding with diamond tooling is usually enough
• Very thick layers of bitumen or adhesive: it is often quicker to scarify or strip them mechanically first and then grind
• Crumbly, cracked or weak screed: grinding will not solve this. The weak layer has to be removed and repaired with a repair mortar, sometimes with a new screed
• Large differences in height: grinding does not level a floor. These are dealt with by a levelling compound of suitable thickness or a separate levelling layer
• Residues that cannot be removed completely: some manufacturers offer primer systems designed for old adhesive residues. Use them only where the technical data sheet explicitly allows it

Two flats with the same floor area can take very different amounts of time and effort. The job is harder with:

• Thick or soft, sticky adhesive that clogs the tooling
• Black bitumen, where dust protection becomes an extra concern
• Awkward rooms with lots of corners, alcoves and pipes coming through the floor
• A timber subfloor, which needs a finer and slower approach
• A block of flats, where quiet hours have to be respected and access and waste removal agreed with the building management or owners' association

It is easier in an empty, regular room with sound concrete and a thin layer of adhesive. It also helps if the old floor covering is already out and the thick residues have been scraped off.

• Grinding that only looks finished: small islands of adhesive remain on the surface, and that is exactly where the levelling compound later comes away
• Grinding too aggressively: the concrete ends up with grooves and pits, and timber is weakened
• The wrong tooling: soft adhesive needs different tooling from hard concrete. A poor choice means slow progress and a patchy result
• Grinding without dust extraction: dust gets everywhere in the home, and a fine film left on the floor reduces how well the primer bonds
• Ignoring moisture: a damp screed must be identified before levelling. The permitted moisture content is set by the manufacturers of the levelling compound and the floor covering, and it is usually stricter for anhydrite (calcium sulphate) screeds than for cement screeds
• Grinding unknown black adhesive without testing it: an unnecessary health risk

Before the primer and levelling compound go down, inspect the base yourself or together with the floor layer. In practice, problems are often only spotted after the compound has been poured, when it is too late.

• No continuous patches of adhesive or bitumen are visible on the surface
• The base is firm and does not crumble or dust when scratched with a key
• No hollow-sounding areas when you tap the floor
• The surface is evenly roughened, without deep grooves or pits
• Edges, corners and areas along the walls are as clean as the middle of the room
• The floor has been vacuumed and no dust is left on your palm when you run it over the surface
• Larger dips and cracks are marked for repair
• The moisture content of the base is known, or a measurement before levelling has been agreed
• Grinding waste has been removed and, where hazardous materials were involved, disposed of in line with local regulations