9 Step Porcelain Patio Laying with Bradstone Slabs Over a 20-Square-Metre Base

Why porcelain needs a primer slurry and a sand slab never does

Vitrified porcelain has a water absorption below 0.5 percent, which is the property that makes it frost-resistant and stain-resistant. That same density is the reason a Bradstone Mode or Aspero slab will not bond to a standard sand and cement bed on its own. The mortar has nothing to key into on the smooth underside, and within a season or two the slab lifts under foot pressure or a hard frost. The fix is a slurry primer painted onto the back of each slab immediately before bedding.

The primer is a polymer-modified bonding slurry, sold by Bradstone and by Larsen and Marshalls under names like Slurry Primer or Pro Prime. You mix it to a thick paint consistency and apply a 1mm to 2mm coat across the full underside with a brush, covering the entire face including the edges. The slab goes onto the wet mortar bed within ten minutes, before the slurry skins over. Skip this step on porcelain and the failure mode is predictable: hollow-sounding slabs that rock by the following winter.

Step 1 to 3: marking out, excavation depth, and the dig

Mark the 20m² footprint with timber pegs and a string line set to your finished patio height, which should sit a minimum of 150mm below any damp-proof course on an adjacent wall. Two brick courses of clearance is the working figure most groundworkers use. Set a fall of 1 in 80 away from the house for surface drainage, meaning the patio drops 12.5mm for every metre of run. Over a 5-metre depth that is a 62.5mm drop from the wall edge to the garden edge.

Excavation depth for porcelain on a full mortar bed runs to roughly 200mm total below finished level: 100mm to 150mm of compacted MOT Type 1 sub-base, a 30mm to 40mm mortar bed, and the 20mm slab. Dig the full 20m² to a consistent depth, checking against your string lines with a spirit level on a long straightedge. Remove topsoil and any soft spots entirely. A wacker plate, hired for around the cost of a half-day from any tool depot, compacts the formation in passes before any aggregate goes down.

The spoil from a 20m² dig at 200mm is four cubic metres of soil. That fills most of a standard skip on its own, so order skip capacity or a grab lorry before the spade goes in.

Step 4: the MOT Type 1 sub-base

MOT Type 1 is a graded limestone or granite aggregate from 40mm down to dust, which is what lets it lock together under compaction. Spread it in two layers of 50mm to 75mm rather than one deep lift, because a single thick layer will not compact through its full depth. Run the wacker plate over each layer until the surface stops moving and rings slightly underfoot. A light spray of water on the aggregate before the final passes helps the fines bind.

For 20m² at 100mm compacted depth you need around 2 cubic metres of loose Type 1, which equates to roughly 3.6 tonnes by weight since compacted aggregate runs near 1.8 tonnes per cubic metre. Order 4 tonnes to cover the over-dig and the inevitable spillage. Check the fall again across the compacted sub-base with the straightedge and level before mixing any mortar, because correcting a fall is trivial in loose aggregate and expensive once slabs are down.

Step 5 to 7: the mortar bed, slurry, and bedding the first slab

Mix a wet, almost plastic mortar at 4 parts sharp sand to 1 part cement, wet enough to hold a ridge when troweled but not running. This is a full bed, not the five-blob method that hollow-bedded patios are built on. Five dabs of mortar under a porcelain slab leaves voids that crack the slab when a planter or a chair leg loads the unsupported centre. Lay a continuous bed 30mm to 40mm deep across an area slightly larger than one slab.

Paint the slurry primer onto the slab back, lower it onto the bed, and tap it down with a rubber mallet to your string line. A Bradstone Mode slab in mid-grey is 600mm by 600mm, so one slab covers 0.36m² and the 20m² job runs to about 56 slabs plus cutting wastage. Order 60 to allow for breakages and the cuts at the perimeter.

Work off boards laid over completed slabs so you never kneel directly on a freshly bedded one. Check every slab in two directions with a 600mm level as you go, and check the gap to its neighbour with spacers. A 3mm to 5mm joint is standard for porcelain. Lift and rebed any slab that rocks or sits proud immediately; mortar that has started to go off will not let you adjust later.

Cutting porcelain needs a diamond blade, not the abrasive disc that cuts concrete flags. A wet-cutting bench saw with a continuous-rim porcelain blade gives clean edges; an angle grinder with a porcelain blade works for notches and the occasional cut but chips more readily. Score the glazed face first and finish from the back to keep the visible edge clean.

Step 8: pointing the joints

Porcelain joints are filled with a slurry-applied or gun-applied jointing compound, not the dry brush-in kiln sand used between block pavers. Products like GftK vdw 850 or Marshalls Weatherpoint 365 are two-part or single-part resin mortars that bond to the slab edge and stay flexible enough to handle thermal movement. The slurry types are squeegeed across the whole patio and washed off the face before they cure, which is fast but unforgiving if you let the surface dry. Work in shade and in sections you can clean within the working time.

Step 9: curing, sealing, and the first load

Keep foot traffic off the bedded slabs for 24 hours and heavy loads off for a full week while the mortar gains strength. Porcelain itself does not need sealing because its absorption is already below 0.5 percent, but the jointing compound and any exposed sub-base edges benefit from being kept dry for the first 48 hours. Resin joints reach handling strength in a few hours and full cure over several days depending on temperature; below 5 degrees Celsius most resin jointing compounds will not cure at all, which rules out laying in a hard frost.

The perimeter is where most porcelain patios fail first. An edge slab with no haunching has nothing holding it laterally, and frost or a mower wheel will shift it. Haunch the outer edges with a wedge of the bedding mortar against the slab edge, sloped down and away so it sheds water and sits below the visible joint line. This is the same mortar mix as the bed, applied after the slabs are down and before the joints are filled.

One detail catches people out on the 20m² scale: the difference between the slab count on paper and the count after cutting. A 4-metre by 5-metre rectangle divides cleanly into 600mm modules with a part-slab strip down one side, but few gardens are true rectangles. Where the patio meets a curved bed or an out-of-square wall, the cut slabs eat into stock fast. The 60-slab order against a 56-slab calculation absorbs that, but only if the cuts are planned from a setting-out drawing before the first slab is bedded rather than improvised at the edges.

That raises the question this sequence does not settle: whether to set out the full slab from the house wall and push the cuts to the garden edge, or centre the slab pattern and cut both sides. The first looks tidy against the building; the second balances the visible cut widths. The drawing has to commit to one before any mortar is mixed.