Where patio weeds come from
Most patio weeds do not grow up from the ground beneath. They start from seeds that blow in and land in the joints, where a little soil and moisture has gathered. Moss behaves the same way in shady, damp spots. A well filled, sound joint gives them nothing to hold on to; a gap of any depth gives them a home.
Failed or washed-out jointing
Jointing does not last forever. Mortar cracks as the patio moves with the seasons, and once cracked, water gets in and breaks it down further. Sand and some brush in compounds can wash out over time, particularly where water runs across the patio. Our guide comparing mortar and brush in jointing explains how each one behaves and fails.
Gaps that were too wide or too shallow
Some patios have weeds from the start because the joints were never filled properly. A joint pointed only at the surface, with a thin skin of mortar over an empty gap, cracks quickly and leaves a void underneath. Very wide joints that do not suit the jointing material can break up early too. In both cases, the weed problem is really a pointing problem.
Patio cleaning and repointing
- Slabs cleaned to suit the stone, not blasted
- Old, cracked or missing joints raked out
- Loose or rocking slabs lifted and re-bedded
Debris and seeds in the joints
Leaves, grass cuttings and soil from nearby beds break down in the joints and turn into compost. Patios next to lawns, under trees or beside borders get more of this than open ones. Keeping the surface swept, especially in autumn, reduces the material weeds can root into, though it will not fix a joint that has already opened up.
Short-term clearing
Pulling weeds by hand, using a joint scraper or a weed knife, and brushing the joints out all clear the surface. They are fine for keeping a patio tidy between bigger jobs. Their limit is that they leave the roots and the gap behind, so the same weeds return. Pressure washing can remove growth quickly but also tends to blast more jointing out, opening the joints further and making the problem worse over time.
Raking out and repointing
The lasting fix is to remove the old, failed jointing along with the roots and debris in it, down to a sound depth, then refill the joints fully and finish them neatly. Leaving old material at the bottom gives the new joint nothing to bond with and it will crack again. Done properly, a repointed patio has joints with no gaps for seeds to settle in. Cleaning the slabs at the same stage makes it easier to see which joints need attention.
When the base is the problem
If slabs rock, have sunk or the patio holds water, the joints will keep cracking however well they are filled, because the slabs underneath are moving. Weeds then return through the new cracks. In that case the base needs putting right, which means lifting and relaying. Our sub-base guide explains what a patio should sit on to stay still.
Keeping joints weed-free
Once the joints are sound, a little care keeps them that way. Sweep the patio regularly, clear fallen leaves before they rot down, and avoid aiming a close, high pressure jet along the joints. Check them each spring and fill any small crack before it widens. If you are not sure whether your patio needs repointing or something more, Ashley can look on a visit and write up what it needs.