Washing up liquid gets recommended constantly as a patio cleaner online, and it's true that it does something, but it's worth being honest about what it actually achieves versus what people expect from it. It's a mild degreaser and surfactant, not a biocide, and that distinction explains most of the disappointment when patios green up again within weeks.
This guide separates what washing up liquid genuinely helps with from the myths, and covers what actually works for algae, moss and properly ground-in grime.
What washing up liquid actually does
Washing up liquid is a mild detergent, effective at lifting general grease, food residue, and light surface dirt when scrubbed with warm water, which is genuinely useful for a patio that's picked up spilled drinks, barbecue grease, or everyday surface grime. For that kind of light, non-biological soiling, it works reasonably well and is a perfectly fine cheap first step.
It's not formulated to kill algae, moss or lichen, though, which is the growth actually responsible for most of the green, slippery staining people are trying to fix when they reach for it. Understanding that distinction from the start saves a lot of repeated, frustrating scrubbing.
Why it doesn't fix algae and moss for long
Scrubbing green algae with washing up liquid and water can lift some of the visible layer, giving a temporary improvement, but it does nothing to kill the algae at a biological level, so regrowth is typically fast, often within weeks, especially on shaded or damp patio areas. This is the core myth worth busting: washing up liquid cleans dirt, it doesn't treat biological growth.
People often conclude their patio just needs cleaning more often when really the underlying issue is using the wrong product for what's actually growing on the surface.
What genuinely works
For algae, moss and lichen, a proper biocide-based patio cleaner, applied and left to dwell per the product's instructions before rinsing, kills growth at the root and gives a meaningfully longer-lasting result. For ground-in grime that's stained rather than just sitting on the surface, a dedicated patio cleaner or, for stubborn cases, pressure washing at an appropriate pressure for the slab material handles what detergent alone can't touch.
A sensible combined approach: washing up liquid or a mild patio cleaner for routine light grime and spills, biocide treatment for green staining and moss, and pressure washing reserved for periodic deeper cleans rather than a first resort for everyday maintenance.
Costs and when to call a professional
DIY washing up liquid cleaning costs almost nothing and is fine for light, routine maintenance between deeper cleans. Professional patio cleaning with proper biocide treatment and pressure washing appropriate to the slab material typically costs around £3-£6 per square metre, worth it for patios with established algae, moss, or years of accumulated grime that DIY scrubbing alone hasn't shifted.
A useful way to think about it: washing up liquid is for maintaining a patio that's already in good condition between proper cleans, not for reclaiming one that's been neglected for a year or more. For that heavier job, the time and product spent scrubbing with the wrong approach usually costs more in effort than a single proper biocide treatment would.
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