Known in the industry as nominal sizing, stated sizes for both porcelain and natural stone tiles are subject to standard industry tolerances. The nominal size, that stated on the box or website, can be an approximate size which is affected by manufacturing and production processes. This means that there may be acceptable size variation within the batch of tiles you receive and doesn’t mean your tiles are in any way seconds or poor quality.
The manufacture of porcelain tiles starts out with a combination of various materials in granular form that are pressed into a mold under high pressure then fired in kilns. The finished product is a tile in the shape of the mould which, like a batch of baked cakes, may not be perfectly the same. Porcelain tiles sold straight from the mold have a slightly angled edge - known as a ‘natural’ or ‘moulded’ edge. The tolerance to European standards on this finish of tiles is up to 3mm. They don’t normally vary this much but these are the tolerances that are deemed acceptable. It follows that a joint width of 3-5mm is recommended for natural edged porcelain tiles.
Porcelain tiles can also come with a ‘rectified’ edge. This is where the tiles taken from the mold are processed through machines that trim the edges of the tiles leaving a sharp, square rectified edge. The tolerance on the machines cutting these edges is very small indeed, making each tile practically the exact same size as the next one. As with ‘natural’ edge tiles, there is always a tolerance but a minimal 0.5mm. This rectifying process enables installation with a minimal 2mm grout line, giving a sharp finish to the project.
As a manufactured product you can expect porcelain tiles to always be the same thickness however, overall dimensions may differ from published sizes and from batch to batch as detailed above. Also, in the construction and engineering industries the term nominal size has been adopted to describe a manufactured products approximate size. For example, in the tiling trade you’ll read or hear sizes of 600x600 to describe a medium sized square tile, when in fact the size could be 595mm x 595mm due to variation in manufacturers machinery, processes, and materials used.
Natural stone tiles
All of our polished and honed natural marble and limestone tiles, along with our quartz tiles have ‘rectified edges’, giving a square cut edge with a sharp corner to the surface of the tile. To give an antiqued ‘olde worlde’ effect our flagstone tiles have what is known as “tumbled edges”. They undergo an additional process to rectified edged tiles which rounds off the edges giving them that worn aged look, desirable for older properties and barn-conversion projects.
Generally due to advances in cutting machinery, and the fact that we only source 1st grade product, you can expect all of our limestones and marbles with a straight edge to be accurate and square to within 1mm. The tolerance on the dimensions of the tile means that it’s unlikely all the tiles to line up exactly on the corners, therefore we recommend using a 3mm grout line and using spacers on the edges of the tiles rather than the corners. Working this way once grouted and dressed any discrepancies in tile sizes won’t be noticeable
Producing natural stone tiles isn’t subject to the same strict European standards as manufactured porcelain tiles as variances in their geological makeup and differences in production techniques make it difficult to standardise regulations. This is particularly relevant to natural stone tiles with a tumbled edges and textured surfaces which undergo processes to give them desired look, as a result the thickness of the tile can vary by +-2mm, so a tile stated as 15mm could be between 13-17mm. Any perceived installation issues can simply be overcome by using a thick bed of adhesive with a 10-12mm trowel.
If you’d like further advice on tile sizing and laying of tiles call 0161 277 6850.