Are Your Tiles Actually Flat? Why Most Aren't, and How to Check

There’s a belief that buying porcelain guarantees a flat tile. It doesn’t. A large share of the tiles we handle, porcelain included, come out of the box with a slight bow in them. It’s one of the most common reasons a finished floor or wall looks “off” even when the tiler did everything right.

Why tiles come out bent

A tile is clay (or porcelain body) pressed in a mould and fired at over 1,000 °C. Three things conspire against flatness:

• The mould and the press. Pressure isn’t perfectly even across the whole tile, so the body comes out with tiny thickness differences before it’s even fired.
• The firing. Ceramic shrinks as it cools. If the temperature isn’t identical across the whole kiln, one part of the tile shrinks slightly more than another and it pulls into a curve.
• Storage. Tiles sit stacked on pallets for weeks or months. Weight, heat and humidity all keep working on a material that’s still settling.

The result is usually a gentle dome: the centre sits a fraction higher than the edges. It’s small, often under a millimetre. But across a 60×120 cm or 120×120 cm large-format tile, a “small” bow becomes very visible once it’s on the wall catching the light.

The two-tile test (10 seconds, no tools)

You don’t need a straightedge. Take two tiles from the batch and put them face to face, pressing the centres together.

• If the edges meet cleanly all the way round, the tiles are flat. Good.
• If you can see a gap along the edges, or you can rock them, the centres are touching because they’re both domed. The middle is higher than the edges. That’s a bowed tile.

Do it with a few tiles from different boxes, not just one. Flatness varies batch to batch and even box to box.

The size problem nobody mentions

Flatness isn’t the only thing that varies. The size does too. Two tiles sold as “60×60” can differ by anywhere from a fraction of a millimetre up to 3 mm or more, depending on quality.

That destroys your grout lines. If a client wants a tight 1 mm joint, the maths only works if every tile is the same size to within a fraction of a millimetre. With cheap tiles varying by 2–3 mm, the joint has to open up to absorb the difference. Otherwise the rows drift out of line and you get lippage (one tile edge standing proud of the next). A 1 mm joint becomes a promise you physically can’t keep.

This is why rectified tiles exist: they’re mechanically ground on all four edges after firing so every piece is the same dimension. They’re what make tight joints possible. Non-rectified tiles need a wider joint by design. If the tile box doesn’t say rectified, plan for a bigger gap, not a 1 mm hairline.

A related detail: good tiles are sorted into calibre (size batch) and tone (shade batch) codes printed on the box. Mixing calibres or tones on the same floor is how you end up with visible size steps and patchy colour. It’s worth checking before a single tile is cut.

Why buying tiles online is a gamble

Everything above is the reason we’re wary of clients buying tiles online to “save money.” When the pallet arrives, it’s too late:

• You can’t inspect flatness before you commit: no two-tile test on a product photo.
• You can’t check the calibre and tone codes match across the boxes you were sent.
• If the batch is bowed or the sizing is loose, returning a pallet of heavy, fragile tiles across a delivery you’ve already paid for rarely ends well.

A cheap price on a screen tells you nothing about whether the tile is straight. Sometimes the online deal is fine. Often it isn’t, and the cost of a slow, fiddly, lippage-prone installation eats the saving many times over.

What we’d suggest instead

We’re happy to go and choose tiles with you. In a real showroom we can pull tiles from the actual batch, run the face-to-face test on the spot, read the calibre and tone codes, and tell you honestly whether a 1 mm joint is realistic for that product or whether you should plan for 2 mm.

We’ll also do the calculations (joint width, square metres, the cuts, wastage) before anything is ordered, so the numbers actually work on your floor and not just on paper.

A trusted supplier and the right tile costs a little more up front than the cheapest deal online. Over the life of the floor, it’s the cheaper option every time. If you’re choosing tiles for a project on the Costa del Sol, get in touch and we’ll come and look at them with you.