Porcelain is a brilliant choice for a concrete floor. It is dense, hard-wearing and handles underfloor heating beautifully, but it is also unforgiving. Unlike a soft ceramic, porcelain has very little flex, so any hollow, dip or ridge in the slab underneath will show up as a lippage problem, a cracked tile or a bond failure within a year or two. The good news is that preparation is straightforward once you know what you are checking for. Set aside a full day before the first tile goes down and work through the steps below in order.

Start With a Proper Inspection of the Slab

Before you clean anything, walk the floor and look at it honestly. You are checking for four things: dust, contamination, movement and moisture.

• Dust and laitance: new concrete has a weak, chalky top layer that must come off. Rub a finger across it — if it powders, it needs grinding or scarifying.
• Contamination: old adhesive, paint, oil, cured plaster and floor polish all stop primer and adhesive from gripping. Scrape and grind back to clean, sound concrete.
• Cracks and movement: hairline shrinkage cracks are usually fine to bridge, but wide or moving cracks suggest a structural issue. If in doubt, get it assessed before you spend money on tiles.
• Moisture: a new slab needs to be dry. As a rough rule, allow one week per 25mm of thickness for a standard slab, and longer in winter. A simple polythene test — taping a square down for 24 hours and checking for condensation — gives a decent indication, though a proper moisture meter reading is far more reliable for a large area.

Deal With Levels, Dips and Hollows

Flat is not the same as level, and porcelain cares about flat. A floor can slope gently towards a drain without causing any trouble, but a 3mm dip across a 2m straightedge will cause lippage on large-format tiles and can be felt underfoot through thin ones.

Run a long straightedge or spirit level across the floor in several directions, marking every high spot and hollow with a pencil. High spots get ground down with a diamond cup wheel. Hollows and low areas get filled with a suitable smoothing compound, poured and spread with a trowel and a spiked roller to release trapped air.

Choose a compound rated for the thickness you need — feather-edge products are fine for 1–3mm, but a deep hollow may need a mid-weight leveller applied in two passes, letting each one cure fully. Always dampen or prime the concrete as the manufacturer instructs first, otherwise the compound will dry too fast and craze.

If you are laying large-format porcelain, aim for a tolerance of no more than 2mm deviation under a 2m straightedge. Large tiles magnify every imperfection, so this stage is worth the extra patience.

Grind, Clean and Remove Every Trace of Dust

Once the floor is flat, remove the weak surface layer. A diamond cup grinder on an angle grinder works for small rooms; hire a dedicated floor grinder for anything larger, and use dust extraction. Respirable silica dust is genuinely harmful, so wear a suitable mask, eye protection and ear defenders, and seal doorways with plastic sheeting.

After grinding, vacuum thoroughly — not with a household vacuum you care about. Then wash the floor with clean water and let it dry completely. Repeat the vacuuming once dry. Dust is the single most common reason primer and adhesive fail on concrete, and it hides in the pores of the slab where a brush will not reach.

Prime the Concrete Correctly

Primer is not optional on concrete, especially on a porous or freshly ground slab. It does two jobs: it seals the surface so the adhesive does not lose its water too quickly, and it creates a key that improves the bond.

• Check whether your primer is neat or diluted — acrylic primers are often diluted 1:3 with water, but products vary enormously.
• Apply with a roller or soft brush in one even coat, working it into the surface rather than leaving puddles.
• Let it dry to the recommended tacky or clear stage. Too wet and the adhesive will slide; too dry and it will not transfer.
• On very porous slabs, a second coat is often wise. If the first coat disappears instantly, the concrete is thirsty.

Do not prime and then walk around in dusty trainers. Keep the area clean and, ideally, lay tiles the same day.

Choose the Right Adhesive and Get the Bed Right

Porcelain is close to impervious, so it needs a cement-based adhesive designed for low-absorption tiles, and it needs to be applied as a full bed — combed with a notched trowel and then back-buttered on the tile. Aim for at least 90% coverage on dry internal areas and full coverage in wet rooms or where underfloor heating is present.

If you are tiling over a slab with underfloor heating, run the system up to temperature gradually, then switch it off and let it cool for at least 48 hours before tiling. Never tile onto a hot floor.

Finally, leave expansion joints at the perimeter and across large areas as the standard requires. Concrete and porcelain both move, and a little allowance for that movement at the edges is what keeps the middle of the floor flat and quiet for years.