How do I point the joints between paving slabs?

Paving joints do a job — they lock the slabs together and stop water getting under them. Almost all pointing failure is preparation failure: shallow joints, dirty joints, or a mix that was too wet.

Safety

Step by step

1. Rake out and clean the joints properly.At least 20mm deep for mortar, and check the requirement for resin compounds — many need 25mm depth and a minimum 5mm width. Remove all loose material, weeds and old mortar.
2. Check the joints go all the way down to sound bedding.A joint over a void does not hold pointing. If you find hollows, that slab needs rebedding before pointing.
3. Choose the method to the slabs and the weather.Mortar for hard-wearing traditional paving. Resin compounds for narrow joints, sandstone and anything where staining would be a disaster — but they need dry weather within their specified temperature range.
4. For mortar, mix it barely damp.3:1 or 4:1 sharp sand to cement, mixed until it holds a shape when squeezed but does not glisten. A wet mix runs onto the slabs and stains them permanently.
5. Press mortar in hard and in layers.Fill the joint, compress it with a pointing iron, add more, compress again. Air pockets are where frost gets in. Finish slightly recessed or flush, not proud.
6. For resin compounds, wet the paving first.Most require the slabs and joints to be thoroughly wet, the compound spread over the surface with a squeegee, worked into the joints, and then the surface swept completely clean before it cures.
7. Get the residue off before it sets.The single most important step with any wet-applied product. A soft brush over the whole area, then a second pass. Anything left on the slab face cures there.
8. Protect it while it cures.Keep foot traffic off for 24 hours and rain off if the product requires it. Sheet the area if there is any doubt about the weather.
9. Check the joints after the first frost.Any joint that has failed will show in the first winter. Repointing a few joints early is much easier than repointing the whole patio later.

Tips

• Do a test area in a corner first, especially with any wet-applied product on porous stone. Some sandstones stain and cannot be recovered.
• Point in mild, dry, overcast weather. Direct summer sun dries mortar before it cures and frost destroys it.
• A pointing iron sized to the joint width gives a far neater finish than a trowel edge, and costs very little.

Common mistakes

• Pointing joints that are only 5-10mm deep — There is not enough material to grip and it pops out within a year. Rake to at least 20mm, or 25mm for many resin compounds.
• Using a wet mortar mix and brushing off later — Cement slurry soaks into porous slabs and the staining is permanent. A semi-dry mix that can be brushed cleanly off is the only safe approach with mortar.

If it doesn't work

Alternatives

• Semi-dry mortar pointing: A 3:1 or 4:1 sharp sand and cement mix, just damp, pressed into the joint and finished with an iron. Cheap and durable, slow, and it stains slabs if it is too wet or brushed across the surface.
• Wet slurry grout: A pourable cement-based grout washed across the whole area and squeegeed into the joints. Fast over large areas, unforgiving on porous or textured slabs which can stain permanently.
• Resin or polymeric jointing compound: A ready-mixed compound brushed into wet joints and left to cure. Quick, flexible, weed and frost resistant, and considerably more expensive. Strict rules on joint depth, temperature and weather.
• Kiln-dried sand: For block paving only, brushed in dry and compacted. Not suitable for slab paving where the joints are wider and need to be sealed.

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