Grinding and Levelling Subfloors Before Installation (Concrete)

A grinder deals with the surface, not with large differences in height. It handles small bumps, concrete that has run up against the walls, and old adhesive, paint or skim coat. It also roughens concrete that is too smooth, so the primer, levelling compound or adhesive can grip properly.

Grinding is worth doing mainly when:

• There are leftovers of an old floor covering, adhesive or coating
• You can feel ridges and raised edges on the surface
• The top of the concrete is soft, dusty or crumbling
• The concrete absorbs water unevenly, so a drop soaks in at once in one place and sits on the surface in another

If the floor has sunk, has hollows or one side of the room slopes, grinding alone will not fix it. Check the floor with a long straightedge first. For floating floors a tolerance of roughly 2 to 3 mm over 2 metres is often quoted, and vinyl or glued-down coverings usually need a flatter surface. The exact figure comes from the flooring manufacturer. If the gaps are bigger than that, priming and a self-levelling compound follow the grinding. So grinding is often the first step, but not always the only one.

First the subfloor is checked. A straightedge or laser shows the high spots, and tapping with a small hammer reveals hollow or loose areas. Cracks and damaged patches are marked so they can be repaired later. If there is old adhesive on the floor, the installer needs to find out what it is. The section on mistakes explains why.

Then comes the grinding itself. The installer uses a single-disc or planetary grinder with diamond segments, connected to an industrial vacuum. Coarse segments remove adhesive and high spots, finer ones even out the surface. A large machine cannot reach right up to the wall, so edges, corners and areas around pipes are finished with a smaller hand grinder fitted with a diamond cup wheel.

Finally the subfloor is vacuumed thoroughly, ideally twice. Dust left in the pores would weaken the bond of the next layers. Primer usually goes on straight afterwards. It reduces how much the concrete soaks up and prepares it for levelling compound or adhesive.

The simplest case is solid, only slightly dirty concrete in a large, empty room. The machine runs smoothly and there is little hand work at the edges. These things make the job slower and therefore more expensive:

• Thick layers of old adhesive, or rubber and bitumen residue that clogs the segments
• Small rooms with lots of corners, where most of the work is done by hand along the walls
• Very hard concrete, which needs different segments and more passes
• A damaged subfloor with cracks and loose areas that must be repaired first
• Furniture, door frames or thresholds that have to be protected or worked around
• Work in an occupied home, where doors must be sealed carefully and dust and noise kept down

In a block of flats, let the building management or owners' association and your neighbours know beforehand. A grinder is loud, and house rules often set the hours when noisy work is allowed.

In practice it often happens that only the middle of the room gets ground and the edges are neglected. When the floor is laid, the boards near the wall do not sit flat or the click joints do not close. Another common mistake is grinding without dust extraction. Concrete dust contains fine silica dust, which is harmful to the lungs, and it settles everywhere in the home.

Other mistakes that come up again and again:

• Grinding a hollow into one spot by letting the machine stand still for too long
• Trying to remove large height differences by grinding instead of levelling
• Not vacuuming the dust thoroughly before priming
• Skipping the primer, after which the levelling layer can peel or crack
• Grinding an unknown old adhesive without checking what it is

The last point matters in older houses and flats. Black bitumen adhesives and some old floor tiles and coverings can contain asbestos. If you have any doubt, have a sample tested. Only a licensed asbestos contractor may remove asbestos-containing materials, not an ordinary floor fitter or a DIYer.

You can manage a small room with solid concrete yourself if you hire a grinder with a matching vacuum and take your time. If the old adhesive is unknown, the damage is extensive or the area is large, call a professional.

1. Clear the room, then cover and tape up doors, air vents and consumer units with plastic sheeting.
2. Check the subfloor with a straightedge and mark high spots with a marker pen.
3. Tap the surface with a small hammer and chip out any loose parts.
4. Put on an FFP3 respirator, safety glasses, ear defenders and work boots.
5. Connect the grinder to an industrial vacuum rated for fine dust.
6. Grind in even strips with a slight overlap and never let the machine stand in one place.
7. Go over the marked high spots several times and keep checking with the straightedge.
8. Finish edges and corners with a hand grinder fitted with a diamond cup wheel and a dust shroud.
9. Vacuum the whole area thoroughly twice.
10. Apply primer following the manufacturer's instructions and let it dry.

Before laying or levelling, go over the subfloor yourself, even if a company did the grinding. All you need is a long straightedge, a torch held low over the floor and your own hands.

• The surface is firm, does not crumble and a key scratched across it leaves no deep grooves
• Under a straightedge laid in different directions there are no gaps larger than the flooring manufacturer allows
• There is no adhesive, paint or grease left on the surface
• Edges, corners and areas around pipes are ground as well as the middle
• There are no hollows from over-grinding anywhere
• The subfloor is thoroughly vacuumed and your palm stays clean when you run it over the surface
• Cracks and damaged patches are repaired or clearly marked for repair
• It is agreed whether priming, levelling or direct installation comes next