Moss and algae on a shaded patio, and why it keeps coming back

A patio tucked under trees or facing north holds onto moisture far longer than one that sees direct sun most of the day, and that extra moisture is exactly the environment moss and algae need to take hold and spread. Scrubbing the surface clears it for a few weeks at most, then the same shaded, damp conditions bring it right back in the same corner, which is why so many homeowners feel like they are fighting a losing battle with the same patch of stone every single season. This guide covers why that corner keeps losing the fight, what actually breaks the cycle, and what part of the fix is a wash and what part comes down to the joints themselves.

A north facing patio, or one shaded for most of the day by a fence, a deck or mature trees, gets far less direct sun than a section of the yard that faces south or west. Less sun means the surface stays damp for longer after rain, morning dew or an evening watering, and that extra moisture is exactly the environment moss and algae need to establish themselves and spread across stone or concrete. It is why the same corner of a patio tends to green up first every year, while a sunnier section a few feet away barely shows any growth at all.

Scrubbing clears whatever is currently growing on the surface, but it does nothing about the shade or the moisture sitting underneath it. The conditions that grew the moss in the first place are still exactly as they were once you put the brush and the bucket away, so it is usually only a matter of weeks before the same patch of green starts creeping back into the same spot. It becomes a cycle that repeats every season unless something about the surface itself actually changes.

A proper pressure wash lifts moss, algae and general oxidation off the surface of stone or concrete, using a pressure matched to what that surface can handle, and brings the original colour back out from underneath the growth. It is a far deeper clean than scrubbing by hand will ever get you, reaching into texture and low spots a brush tends to skip over. That said, the shade and the moisture are still there once we leave, which is why a wash on its own is not a permanent, once and done fix for a patio in a shaded spot.

If the patio in question is interlock rather than a poured slab, moss and weeds can both take root inside an open or washed out joint the exact same way a seed does. Packing those joints with fresh polymeric sand closes that gap so growth has nowhere left to establish itself, which is the same mechanism we cover in more detail in our guide on weeds between interlock pavers. Washing the surface and packing the joints work together, not as separate fixes for separate problems.

A pressure wash about once a year is the general rhythm we recommend across every property, shaded or not, as part of keeping joints from getting overgrown and stone looking the way it should. That rhythm tends to matter more on a shaded patio specifically, since the moisture that brings moss back is working against you for most of the season rather than only showing up after one particularly wet stretch of weather.

We usually do not recommend a sealer, and it would not solve this particular problem even if we did. A sealer does not change the amount of shade a patio gets or the moisture sitting on its surface, and depending on the product it can actually trap moisture underneath the stone instead of letting it dry out properly between rainfalls. Our guide on whether you should seal your interlock goes through the full reasoning behind that recommendation.

We keep cleaning solution off your plants and grass throughout the job, and rinse anything that gets overspray with plain water right away. The edges of the patio get rinsed too, so nothing runs off onto a neighbour's lawn or garden bed, and we walk the whole area with you before we leave to confirm everything looks the way it should.

If you are also noticing weeds, ant hills, or stones that shift or wobble underfoot alongside the moss and algae, the joints on that patio have likely already failed and a wash on its own will not fix that part of the problem. We will tell you plainly on the visit whether a full resand is worth doing at the same time, rather than sending a crew out twice for something that could have been handled in one visit.

Every quote is free, and we measure the area in square metres and price it off that rather than guessing at a flat rate. Send a photo of the patio, mention which corners stay shaded the longest, and we will get you a real number for the wash and, if the joints need it, the resand as well. The price is fixed for the scope on that quote before we start, so there is no surprise if a resand turns out to be part of the visit.