Weeds in the joints are the number one thing people ask us about once a patio has been down a few years. The grass between the slabs, the moss in the shady corner, the dandelion that appears overnight in a gap. It's annoying, it makes a tidy patio look neglected, and most of the usual fixes only work for a fortnight.
Here's why it keeps happening, and what actually stops it, based on the patios we lay and the tired ones we get called out to rescue.
Why weeds grow between slabs in the first place
Weeds don't grow up through a solid patio. They grow in the joints, in the gaps between the slabs, where windblown soil and seeds collect and sit damp. The slab itself is never the problem. The joint is.
That's the bit most people get wrong. They attack the green on top and leave the open, soil-filled gap underneath completely untouched, so it's all back by the next warm spell. If you want to stop weeds, you have to deal with the joint, not the leaf.
The pressure washer trap
A jet wash is the first thing everyone reaches for, and it's the thing that quietly makes it worse.
Blasting the joints does clear the green, and for a week the patio looks great. But the pressure also strips out whatever jointing sand was in there, leaving the gaps wider, deeper and open to the next round of seeds. So you clean it in April, and by July it looks worse than before you started. If you do pressure wash, treat it as step one, not the whole job, and re-joint afterwards.
How to clear them properly
If you want it to last, do it in this order, on a dry spell:
- Clear the joints out. A weeding knife or a thin scraper to rake out the weeds, moss and old soil down into the gap. Tedious, but this is the step that matters.
- Treat what's left. A patio-safe weedkiller on any roots you can't dig out, following the instructions. Skip this on edibles nearby.
- Let it dry fully. Jointing compound and sealer both need a properly dry patio and a dry forecast, so pick your day.
- Re-joint the gaps. Brush a resin-based jointing compound into the clean, dry joints. It sets firm and seals the gap so soil and seeds can't get back in.
That last step is the one that breaks the cycle. You're not just removing the weeds, you're removing the place they were living.
What keeps them out long term
Once the joints are clean and filled, a bit of upkeep keeps them that way:
- A proper jointing compound, not loose kiln-dried sand, which washes out and lets weeds straight back in.
- A yearly sweep and gentle rinse to clear the soil before it has chance to settle in the gaps.
- A sealant on porous stone like sandstone, which makes the surface easier to keep clean and less hospitable to moss. Our guide to cleaning and sealing a sandstone patio walks through that properly.
- Sorting out damp and shade where you can, because a patio that never dries out will always go green faster.
When it's not the joints, it's the build
Sometimes the weeds are a symptom of something bigger. If slabs are rocking, the patio holds water in puddles, or weeds are coming up through cracks rather than joints, the base underneath has usually failed. No amount of re-jointing fixes a patio that's moving.
That's the difference a proper installation makes. A patio laid on a full mortar bed over a compacted sub-base, with tight resin joints from day one, barely weeds at all. The ones that turn into a yearly battle are nearly always the ones that were spot-bedded or laid on sand to save money. If yours is past saving, it may be more sensible to start again than to keep fighting it, and our patio installation page explains how we build them to stay clean.
Need a hand with it?
If your patio has gone past the point where a weekend with a scraper will fix it, we're happy to take a look. We cover Derby and the surrounding Derbyshire towns and can advise on whether it needs re-jointing and sealing or a proper rebuild.
Get in touch for a free quote or call Jamie on 07891 632305.