Outdoor porcelain · 6 min read

Laying 20 mm porcelain outside.

Porcelain is unforgiving of shortcuts. Slurry primer, a solid bed, tight joints, and the right jointing compound.

Joint width is decided by the edge of the slab, not by taste.

Why it’s different

Porcelain absorbs almost no water, which is the point of it — and also why plain mortar won’t grip the back. Every slab needs a coat of slurry primer on the underside, applied wet and laid straight into the bed. Skip it and the slabs will lift within a winter.

Base and bed

Same base as sandstone: 100 mm compacted Type 1 with falls of 1 in 80. Bed is a full 30–40 mm of 4:1 sharp sand and cement, or a proprietary porcelain bedding mortar. Because the slabs are calibrated, the bed can be laid level and the slabs just tapped in — no working to the top face.

Cutting

A wet-cut diamond blade on a bridge saw or a rail saw. Porcelain chips with a grinder and won’t score-and-snap. Cut with the face up, slowly, and keep the blade wet. Factor the 10% cutting allowance into your order — the calculator does.

Joints

Rectified porcelain takes 3–5 mm joints. Use a two-part resin or a proper brush-in porcelain compound; standard sand-and-cement pointing will crack against a slab that doesn’t move. Clean the face as you go — resin haze is much harder to shift once cured.

Alternatives to a mortar bed

On a roof terrace or over a membrane, 20 mm porcelain goes down on adjustable pedestals or a grid system, dry, with open joints. Ask us before ordering if that’s the job — the slab is the same, the accessories aren’t.