Updated
Patio Pointing Crumbling Out
The mortar between the slabs has gone soft and sandy and can be raked out with a finger, leaving open joints.
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What's happening?
Pointing is not decoration. On a rigid patio the joints stop water reaching the bed, keep the slabs from shifting against one another, and give the surface somewhere to shed rain from. Crumbling pointing almost always means the mix or the method was wrong rather than that the patio is old. Dry-brushed sand and cement is the usual culprit: it looks tidy on the day, but without enough water and compaction it never fully hydrates, so it cures weak and washes out from the first winter. Too much sand in the mix does the same. Once joints are open, water sits under the slabs, frost lifts the bed, and what began as a cosmetic job turns into rocking slabs and a stained surface.
What usually causes it?
- Dry-brushed sand and cement that never hydrated properly, so it cured weak and porous
- A mix too rich in sand, giving mortar with little strength once it has dried out
- Joints too narrow or too shallow for mortar to hold, particularly with tightly butted flags
- A cement mortar used on porous natural stone or in joints that stay permanently damp
- Frost attacking the joints while the mortar was still curing, typically autumn or winter pointing
- Movement in the slabs below cracking the joints and letting water in behind them
- Pressure washing the joints out, which strips weak mortar in a single pass
Is it dangerous?
Failed pointing is not a hazard on its own, and no one is hurt by an open joint. The consequences matter more than the appearance. Water going through the joints saturates the bed, so slabs begin to rock and, in a hard winter, lift. Constant damp in the joints feeds the algae that makes a patio slippery. Where a patio adjoins the house, open joints let rain collect against the wall instead of running off, which turns a cosmetic job into a damp one.
Can I fix it myself?
Rake a joint out with a screwdriver and see how deep the failure goes. If the mortar is sound below the top few millimetres, the patio is fine and the surface needs re-pointing. If it comes out to the bed, water has been getting through and the slabs need checking for movement at the same time. Clear out all the loose material and let the joints dry. Small areas can be re-pointed with a proper wet mortar or a resin-based jointing compound, following the manufacturer's instructions on joint width and slab type — the two things people get wrong. Never point in frost or over damp, dirty joints.
When should you call a driveway and patio installer?
Call a patio specialist when the pointing has failed across the whole patio, when raking it out reveals the bed rather than more mortar, or when the joints are too narrow to hold anything. Widespread failure usually means the original method was wrong, and re-pointing over a wet bed simply seals water in. Get it looked at with the slabs rather than in isolation if any of them now rock.
What will the driveway and patio installer actually do?
- Rake out and test the joints to establish how deep the failure goes and whether the bed below is wet
- Check the slabs for movement, because loose slabs will crack new pointing within a season
- Choose a jointing system matched to the joint width, the slab material and the falls, rather than a single house mix
- Clean the joints back to sound material and let them dry before filling
- Fill and compact the joints fully, finishing flush so water sheds off rather than sitting in a hollow
- Protect the work from rain and frost while it cures, and clean any residue off the slab faces
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