How do I cut a paving slab?

The tool matters less than the method. Concrete slabs split reasonably along a scored line; natural stone splits unpredictably; porcelain only cuts, and only with the right blade and plenty of water. Match the approach to the material and half the difficulty disappears.

Safety

What you'll need

• A bolster chisel and a lump hammer
• An angle grinder or disc cutter with a diamond blade (optional)
• Water for dust suppression
• FFP3 mask, eye and ear protection
• A straightedge and a chalk or wax marker

Step by step

1. Mark the cut on all faces.Top, both edges and, where you can, the underside. A line on one face only means the cut wanders and the underside breaks out ragged.
2. Support the slab fully.Flat on sand, or on two bearers with the waste side unsupported for a hand split. An unsupported middle cracks the slab in the wrong place.
3. For a hand split, score deeply.Work the bolster along the line, top face first, then both edges, building the score up over several passes. Do not try to break it on the first pass.
4. Split with one firm blow.Move the slab so the score line is over the edge of a bearer with the waste overhanging, then strike along the score. Concrete slabs split cleanly this way; natural stone is less predictable.
5. For a disc cut, always cut wet.A wet-cut bench saw, or a grinder with a water feed. Dry cutting concrete and stone produces silica dust at levels that a nuisance mask does not touch.
6. Score first, then deepen in passes.A 3-5mm scoring pass along the full line, then repeated passes 5-10mm deeper each time. Trying to cut through in one pass overheats the blade and wanders off the line.
7. Turn the slab and meet the cut from the other face.For thick slabs, cut halfway from each face and let the cuts meet. It gives a cleaner underside and avoids blowout.
8. Use the right blade for porcelain.Porcelain paving needs a continuous-rim diamond blade specified for porcelain and a wet cut. A general-purpose segmented blade chips the glazed face badly.
9. Tidy the cut edge.A rub with a masonry stone or the edge of another slab takes the sharpness off. Cut edges usually face into a wall or an edge where they show least.

Tips

• Lay all the full slabs first and mark cuts from the actual gap, not from a drawing. Openings are never quite the nominal size.
• Hire a wet-cut bench saw for a whole patio's worth of cuts. It is faster, safer and far cleaner than a grinder for repeat straight cuts.
• Cut curves as a series of straight relief cuts to the line, then nibble the waste out with the bolster.

Common mistakes

• Dry cutting with an angle grinder — The dust is respirable crystalline silica, and the exposure from a few minutes of dry cutting is very high. Wet cutting or on-tool extraction is not optional.
• Trying to cut through in a single pass — The blade wanders, overheats and can bind and kick back. Score, then deepen progressively.

If it doesn't work

Questions people ask