How to Repoint Patio Slabs – DIY Guide
To repoint patio slabs, rake out all loose mortar to a depth of at least 20mm, brush the joints clean, mix a dry mortar or brush-in compound, work it into the joints, and finish flush with the slab surface. Allow to cure fully before use.
Clean and Prepare the Joints
Once the old mortar is out, go over every joint with a stiff wire brush to remove dust, grit, and organic debris. Blow or brush the joints clear — any loose material left behind will stop the new mortar bonding properly. Dampen the joints lightly with a water spray or brush; dry joints will pull moisture out of the new mix too quickly and cause it to crack. Do not soak them — you want damp, not wet.
Mix Your Pointing Mortar
For a traditional wet mortar, mix four parts sharp sand to one part cement and add water gradually until you reach a firm, stiff consistency — it should hold its shape when squeezed in your hand without crumbling or slumping. If you prefer a simpler method, use a brush-in dry jointing compound: no mixing required, and it suits most standard patio joints. Avoid ready-mixed wall mortar — it is too weak for external, trafficked surfaces and will deteriorate quickly.
Work the Mortar Into the Joints
Using a pointing trowel, press the mortar firmly into each joint in small sections, packing it tightly to eliminate air gaps. Work the mortar slightly proud of the slab surface, then use the edge of the trowel to strike it flush or finish it with a slight inward slope to help water run off. For brush-in compound, tip the dry material across the slabs, sweep it into the joints with a stiff brush, and repeat until all gaps are packed full. This is a good opportunity to check whether any slabs are sunken or uneven and need resetting before pointing.
Remove Excess Mortar and Protect the Surface
Before the mortar sets, use a damp sponge or cloth to wipe excess from the face of the slabs — dried mortar smears are difficult to remove and can stain porous stone. Cover the newly pointed area with plastic sheeting if rain is forecast within 24 hours, and keep foot traffic off for at least 24 hours (48 hours in cold weather below 5°C). For brush-in compound, activate it with a light watering from a fine rose on a watering can once the joints are packed, then leave undisturbed. After the patio has fully cured, consider our guide on how to clean and pressure wash a patio to finish the job properly.
Inspect and Touch Up After Curing
After 48–72 hours, walk the patio and look for any joints that have shrunk, cracked, or pulled away from the slab edges — this is common in a first pass and is easy to fix while the surrounding mortar is still relatively fresh. Mix a small fresh batch and press it into any voids, striking it flush as before. Once the pointing is fully hard, the patio is ready for use and should resist weed growth and water ingress for several years before needing attention again.
Frequently Asked Questions
What is the best mix for repointing patio slabs?
A four-to-one mix of sharp sand and cement works well for most patio slabs. For a simpler approach, a brush-in dry jointing compound is increasingly popular and performs well on standard concrete or natural stone — just avoid it on very wide joints over 15mm.
Can I repoint a patio in cold weather?
Avoid repointing when temperatures are at or below 5°C. Frost prevents the cement from curing and will cause the mortar to crumble out. Early spring or late summer are the best times to repoint in the UK.
How long does repointing a patio last?
Should I use wet mortar or a brush-in compound for patio pointing?
Both work. Wet mortar is harder-wearing and better for high-traffic areas or wide joints. Brush-in dry compound is quicker, cleaner to apply, and more forgiving for beginners — it suits most domestic patios with standard 8–12mm joints.
How do I stop weeds coming back after repointing?
Fully packed, dense joints leave no gap for weed seeds to germinate. Treat any existing weeds before you start, rake all organic material out of the joints, and ensure the finished pointing sits flush with the slab. For further patio maintenance advice, see our Garden Guide for a full overview of external upkeep tasks.
When striking wet mortar flush with the slab, use the flat back of the pointing trowel rather than the edge — it compresses the surface of the joint slightly, which closes micro-cracks before they form and produces a denser, more water-resistant finish. Doing this in one smooth pass per joint also prevents the drag marks that cause crumbling in the first winter.
Sources
- HSE — Working with cement: skin and health risks — hse.gov.uk
- Which? — How to repair and repoint patio paving — which.co.uk
- RHS — Paving: maintenance and repair — rhs.org.uk
This guide is for general information only. Always work safely and follow manufacturer instructions. DIYnut accepts no liability for injury or damage arising from DIY work.
