Porcelain is the best-selling paving slab in the UK domestic market, with an infinite range of colours, textures, styles, sizes and effects. It has an extremely low water absorption, high resistance to weathering, and relatively low maintenance, are all reasons for its popularity.
It is made from a blend of clays, quartzitic sands, feldspar and minerals that are finely ground and blended together. White Kaolin is the most expensive clay, and is predominantly used to produce higher quality porcelain.
Water is thoroughly mixed with the powder to create a clay-like substance that can easily be shaped. The mixture is formed into ‘biscuits’ using presses and extrusions to suit the size and thickness required. These biscuits are then dried to remove any excess moisture and compacted to between 40 and 50 Mpa.
The pattern or colour is then applied to the surface, in a manner somewhat similar to an ink-jet. The biscuits are then fired in kilns to temperatures up to 1200 Celsius. This firing process transforms the raw materials into a solid, hardened state, ensuring that the slabs (or tiles) are durable and virtually non-water absorbent at less than 0.5%.
Porcelain is available in a range of thicknesses between 6mm and 30mm. 6mm to 16mm are classed as tiles, which should be laid on a solid concrete base. 20mm is currently the most common thickness for patios, with 30mm available for heavier use on driveways, which are laid on unbound bases. (From 2024, the industry will be moving towards 16mm as the favoured thickness for patios).
Porcelain paving may be rectified or unrectified. Rectified slabs have less dimensional tolerances at +/- 0.4mm, than non-rectified with a tolerance of +/- 0.7mm. Rectified slabs are generally of a higher quality and should always be used in stack-bond or chequerboard patterns as any minute differentials in tolerance may become evident over a longer distance/larger projects.
Slabs over 800mm in length, especially ‘planks’ (1200mm x 300mm or 200mm) should be laid to a one third off-set (1/3rd) – the reason being the longer the unit, the more likely the surface will become convex and the corners dip away, leaving a high point in the middle at half bond, which may hold water or create a ‘lip’ unless the 1/3rd principle is applied.
The highest quality porcelain comes from Italy, although Spanish products are not far behind. The higher quality porcelain is known as ‘full-body’, as the colour goes right through the slab, and as is not simply a surface imprint. This makes full-body porcelain paving ideal for areas with heavy foot traffic, and for copings or bull-nosed steps (See Bull-nosing) as you will not have to contend with the ‘baking lines’ which occur in the middle of the product. Full-body paving has a constant and consistent colour throughout, and won’t leave a two-tone effect, which may be visible on budget priced porcelain. (The more expensive products have between 24 and 32 ‘layers’ whilst cheaper versions only have 6 – 12 layers of ‘faces’ when viewed ‘side-on’
Having removed the slabs from their crates, although they are covered in cardboard to protect them, best practice is to store them on softwood timber skids or strips (not oak, as this will leach tannins) and not directly onto the ground, where they can easily become chipped or spalled.
Porcelain paving must have a slip rating (See Slip Ratings) of a minimum of R11 to meet British Standards. Around swimming pools, showers and hot tubs, slip rating of R12 or R13 are recommended. They must be laid to a minimum fall of 1:60 to ensure sufficient water run-off due to its propensity to hold water with surface tension.
Some slabs will have directional arrows printed on the base to indicate the manner in which they should be laid (See Directional Arrows) especially those with a pattern surface design.
Porcelain should be laid on either a bound or unbound base, depending on the sub-grade and end usage. The bedding mortar should be 1:4 cement/sharp sand mix, to an even 30 – 40mm depth. Some contractors prefer to work using a wet bed, others a reduced water bed. It is down to personal preference. However, a reduced water mix – if compacted correctly – will set harder than a wet mix.
Prior to laying the slabs, the backs should be brushed thoroughly to remove the kiln release dust, and ensure the best possible contact with the bonding mortar. Bonding mortar must be applied at 2 – 3mm thickness on the back of the slabs, sufficient to fill the pattern on the back. Due to porcelain being virtually non-water absorbent, the polymers in the bonding mortar need to work harder to achieve the required 2Mpa bond strength required to meet BS 7533. We also recommend using a BS7533 compliant bonding mortar as opposed to SBR. BS7533 compliant bonding mortars are independently tested and specifically designed to bind paving, whereas SBR is often inconsistently mixed and unregulated.
The paving should be jointed with an appropriate jointing material (See Jointing Materials). Joints should be a minimum of 5mm, anything less may fail due to thermal shock. 5 – 6mm being the most common joint width, joints up to 12mm may be used if the correct fall is not achievable.
Being extremely hard, porcelain paving should be cut as required using appropriate diamond blades and machines (See Diamond Blade Use). Regular blade maintenance is essential, as porcelain quickly dulls and glazes the discs. We recommend sharpening blades after every two lengthy cuts, using a bridge saw (See Bridge Saws & Cutting Benches) to ensure clean cuts with minimal chipping. Battery hand operated machines may be used for fine detailed cutting, although micro-chipping may occur. These may be polished out using a buffing disc on a hand grinder.
If a bridge saw is not available, use a flat bed of compacted sharp sand, or piece of Kingspan insulation to help steady and secure the slab during cutting operations and reduce vibration which may cause the slab to shatter under pressure. Drilling the corners of 90 degree cuts may also relieve the tension within the slab during cutting operations.
Although considered extremely hard-wearing, porcelain will require a regular cleaning regime, as it may easily become stained with substances such as yew berries, oak leaf tannins, red wine, rubber soled shoes etc, and should never be seen as maintenance-free. (See Cleaning Issues)