Tile grading sounds like a technical detail and it is really a set of plain answers to practical questions. Will this survive the traffic. Will it stay put when wet. Is it suitable outdoors. Once you know which numbers answer which question, specifying tiles becomes straightforward.
Here is everything worth knowing, in order of how much it matters.
Water absorption, the first division
This is the number that separates porcelain from other ceramic tiles, and it drives most of the other properties.
Porcelain absorbs 0.5 percent of its weight in water or less. It is fired at a higher temperature from a finer body, which makes it dense, strong and suitable for wet areas, outdoor use and anywhere that freezes.
Other ceramic tiles absorb more, commonly between 3 and 10 percent. They are lighter, easier to cut and perfectly good on interior walls. They are not the right choice for a floor with heavy traffic or for anywhere exposed to frost.
If a specification says porcelain, the absorption figure is what proves it.
PEI, the surface wear rating
PEI runs from 1 to 5 and describes how well a glazed surface resists visible wear from foot traffic.
PEI 1 and 2 suit walls and very light use such as a bathroom floor. PEI 3 suits normal residential floors. PEI 4 suits light commercial use, so shops, offices and restaurants. PEI 5 suits heavy commercial traffic, so entrances, corridors, airports and shopping centres.
One point often missed. PEI applies to glazed tiles. Full body and unglazed technical porcelain do not carry a meaningful PEI rating because there is no glaze layer to wear through, and they are generally at least as durable.
Slip resistance
Two systems are in common use and you may see either.
The R rating runs from R9 to R13 and is measured on a ramp. R9 suits dry interior areas. R10 suits most interior commercial floors and domestic bathrooms. R11 suits wet areas such as changing rooms and covered terraces. R12 and R13 suit commercial kitchens and industrial wet areas.
The DCOF figure describes the coefficient of friction, and a value of 0.42 or above is the common threshold for level interior floors expected to be walked on when wet.
Higher grip means more texture, and more texture means slightly more cleaning effort. Choose the grip the space genuinely needs rather than the highest available.
Breaking strength and thickness
Breaking strength matters where trolleys, pallet trucks or heavy loads run across a floor. It is quoted in newtons and the required figure depends on the application and the thickness of the tile.
Thickness itself is a useful shorthand. Around 8 to 10mm is normal for interior floors. 12mm and above suits heavy commercial use. 20mm pavers are made for outdoor and raised floor applications. Thin porcelain panels of 3 to 6mm exist for wall cladding and for tiling over an existing surface.
Frost resistance
For any outdoor tile in a climate that freezes, this is essential. Water absorbed into a tile expands when it freezes and breaks the tile from the inside. Low absorption porcelain handles it. Higher absorption ceramic does not.
If the project is outdoors and cold, the specification is porcelain with a stated frost resistance and a suitable slip rating.
Rectified or not
Rectified means the edges are mechanically cut after firing so every tile is the same size to a fine tolerance. That allows a 2mm joint instead of 3 or 4mm.
Non rectified tiles are fired to size and vary very slightly, which needs a wider joint to absorb the difference. Both are perfectly good. Rectified gives a sharper result and needs a flatter substrate and a more careful installation.
Shade variation
Ranges are usually marked with a shade variation code from V1 to V4. V1 means the tiles are almost identical. V2 means slight variation. V3 means moderate variation, which is common in stone and timber effects. V4 means substantial variation, chosen deliberately for a natural look.
Check this before ordering, because a client expecting uniformity and receiving a V4 range will be surprised, and the tiles are exactly as specified.
Chemical and stain resistance
Relevant for commercial kitchens, laboratories and anywhere aggressive cleaning is used. It is quoted as a class, and a good technical porcelain performs well across the range.
How to specify in one line
A complete tile specification names the format, the finish, the absorption class, the PEI where the tile is glazed, the slip rating, the shade variation code and whether it is rectified. For example, a 600 by 1200 matt porcelain, absorption under 0.5 percent, PEI 4, R10, V2, rectified.
Ask for the technical data sheet with these figures printed on it and keep it with the project file. It answers most later questions before they are asked.
If you are specifying tiles for a project and want help matching grades to real production, our team in Foshan works with these ranges every day.