Almost every weed you pull out of a patio joint started life on the surface, not underneath the slabs. Once you understand that, the job stops being an endless weekly chore and becomes a fixable problem with a proper joint. Here is what actually works on Buckinghamshire patios, and what is a waste of a Saturday.
Weeds between paving are not pushing up through the sub-base. On a correctly laid patio there is a full mortar bed, a compacted sub-base and often a membrane, and nothing is getting through that. What happens instead is that windblown seed, leaf mould, moss fragments and general grit collect in the joint. Add rain and you have a shallow, free-draining seed bed sitting a few millimetres above your slabs.
That is why the usual suspects are shallow-rooted opportunists: annual meadow grass, hairy bittercress, groundsel, dandelion and moss. In the Vale of Aylesbury the heavy clay holds moisture and shaded north-facing patios stay damp well into May, which favours moss and liverwort. Up towards Wendover and the chalk slopes the drainage is sharper, so you tend to see more grass and bittercress than moss.
The practical conclusion is simple. If the joint is full, hard and sealed, there is nowhere for seed to germinate. If the joint is crumbly, hollow or half missing, you will be weeding it forever no matter what you spray.
Start dry and mechanical. A block paving brush or an old wood chisel run along each joint lifts the growth and the soft debris together. Take your time on a small area rather than skimming the whole patio; you want the joint empty and clean, not just the tops knocked off.
For heavier moss, a patio cleaner based on benzalkonium chloride is the sensible choice. Apply it on a dry, still day with rain unlikely for 24 hours, leave it for a fortnight and brush off the dead residue. It carries on working for months, which is why it beats bleach, and unlike bleach it will not lighten Indian sandstone or bleed onto the lawn.
Two warnings. Boiling water and vinegar both work on the leaves and do nothing to the roots, so expect regrowth within weeks. And a pressure washer, while satisfying, is the main reason patios end up weedy in the first place: a 2,000 psi lance blasts out jointing sand and etches soft sandstone. If you must use one, keep it above 2,000 psi only with a fan nozzle, hold it at least 300mm back, and accept that you will be rejointing afterwards.
Once the joints are clean and dry, refill them. What you use depends on the patio.
Whichever you choose, the joint needs to be at least 20mm deep and the slabs must be genuinely dry underneath, or the compound will not cure and will stain the surface. On a typical 25 square metre rear patio in Aylesbury, expect materials somewhere in the region of 200 to 400 pounds for a two-part resin compound, plus a day or two of work. Costs vary a lot with joint width and slab type, so treat that as a starting point rather than a quote.
Maintenance is mostly about denying seed a place to land. Sweep the patio properly in late autumn so leaves do not rot down into the joints over winter, and again in early spring before the meadow grass sets seed in April. Ten minutes with a stiff brush twice a year is worth more than any spray.
Check the drainage too. If water sits in a puddle after rain, the fall is wrong and that spot will always grow moss. Paving should shed water away from the house at roughly 1 in 80. If you see standing water, ponding along one edge or slabs that rock underfoot, the problem is the bedding, not the weeds, and no amount of jointing compound will hide it.
No. Membrane stops perennial roots coming up from the soil, which is useful under a sub-base, but the weeds in your joints germinated from seed on the surface. A sound joint is the only thing that prevents them.
Both scorch the top growth but leave the roots, so the weeds return within a few weeks. Salt in particular can leach into borders and lawn edges and cause lasting damage to the soil, so it is best avoided on a domestic patio.
A good two-part resin compound, applied to clean, dry joints of adequate depth, typically gives 5 to 10 years. Kiln-dried sand needs topping up every year or so, and mortar joints last longest but crack if the slabs move at all.
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