Experts in Concrete Patio installation

Why Are My Patio Joints Crumbling? Mortar vs. Sand Fixes

If the joints between your patio slabs are breaking up, disappearing, or growing weeds, you're not alone. In UK homes, crumbling patio joints are one of the most common maintenance issues, often caused by poor installation or weather exposure.

This guide looks at what causes joint failure, compares mortar and sand jointing methods, and outlines your best options for restoring strength and appearance to your patio.

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What Causes Patio Joints to Crumble?

Joint failure can look like cracked mortar, loose grit, growing weeds, or even gaps forming between paving slabs. Here are the main reasons this happens, and what to watch for.

Poor Mortar Mix or Application

If mortar is too dry, too wet, or not properly compacted into the joints, it can weaken quickly. Over time, it becomes brittle and begins to flake away, especially during freeze–thaw weather or under regular foot traffic. Fast-setting jointing compounds may also fail if applied in cold, damp conditions.

Movement in the Slabs

If the slabs aren’t bedded firmly or the sub-base underneath is unstable, movement occurs when walked on, and that movement cracks or dislodges the joints. This is particularly common on older patios or where patios were laid on sand rather than cement mortar.

Weather Exposure

Jointing material takes a battering from the elements. Heavy rain, frost, UV light, and wind erosion can all degrade joints, particularly in exposed or high-traffic areas. Mortar tends to last longer, but once cracked, water can seep in and accelerate the failure.

Weed Growth and Moss

Weeds and moss exploit weak spots and tiny cracks, pushing jointing material apart as they grow. If joints weren’t sealed or were filled with basic kiln-dried sand, this process happens quickly, and looks worse over time as roots expand in the gaps.

Wrong Type of Jointing Product

Many patios are filled with sand when they should have used a strong mortar or polymeric jointing compound. Using the wrong material, or one unsuited to your climate or footfall, can lead to failure within a year or two.

Repair Options: Mortar vs. Jointing Sand

Repointing with Mortar

Use a strong 4:1 sand and cement mix to refill joints after clearing out old material.
Mortar offers a solid, long-term finish when mixed and compacted properly into clean, dry joints.
Allow 24–48 hours to dry fully before use, and consider sealing to protect against frost and weeds.

Brush-In Polymeric Jointing Compound

These modern alternatives cure with moisture and set hard, offering flexibility and strength in one.
Ideal for DIYers, they’re easy to apply and available in different colours to match your patio.
Make sure to apply in mild weather with no rain forecast for at least 12 hours.

Simple Sand Refills (Temporary)

Dry kiln-dried sand can be swept into gaps for a quick visual improvement, but it won’t last.
This method offers no strength or weed resistance and may need topping up every few months.
Use this only as a stopgap solution or in low-traffic, decorative areas.

Frequently Asked Questions

For most UK patios, a well-mixed mortar or polymeric jointing compound offers the best balance of strength and weather resistance. Sand alone usually fails over time.

Yes, you can remove and refill individual joints as needed. Just make sure the surface is clean and dry before applying new material to avoid poor adhesion.

Well-installed mortar joints can last 5–10 years or more. Sand or DIY compounds may last 1–3 years depending on weather, usage, and application quality.

Not usually. Crumbling joints are a cosmetic and maintenance issue. As long as your slabs are stable, you can often repair just the joints without replacing the patio.

Yes. Applying a patio sealant every few years protects jointing material from water, frost, and weed growth, all major causes of joint failure.