Homemade Patio Cleaner with Bicarbonate of Soda: I Tested It on My Sandstone Patio

Why I tried a homemade patio cleaner

I'll be honest from the start: I wanted this to work. Every spring my little courtyard comes out of winter looking tired, with green film between the slabs and a scattering of black spots that seem to multiply every year. So when I kept seeing "homemade patio cleaner" recipes online that promised a bright patio from a box of bicarbonate of soda and a bottle of vinegar, I thought I'd give it a proper go.

My patio is Indian sandstone, the buff and grey riven kind that half the gardens in Britain seem to have. It went down about eight years ago. It's not huge, roughly 18 square metres, which felt like the perfect size for a fair test. Cheap ingredients, a free Saturday, a stiff brush. How hard could it be?

The bicarbonate of soda patio cleaner recipes I used

Most of the recipes I found boiled down to two versions, so I tried both on different sections of the patio.

Recipe 1: bicarb paste

• 500g bicarbonate of soda

• Roughly 250ml warm water, added slowly until it's like a thick yoghurt

• A squirt of washing-up liquid to help it cling

Recipe 2: bicarb and vinegar "fizz" cleaner

• 250g bicarbonate of soda sprinkled straight onto damp slabs

• 1 litre of white vinegar mixed with 1 litre of warm water, poured over the top

Between the two, I got through five 500g tubs of bicarb and two big bottles of vinegar. That's not expensive, but it's not quite the "costs pennies" I'd been promised either.

How I applied it

I picked a dry, overcast morning so nothing would bake onto the stone, then followed the recipes as closely as I could.

1. Swept the patio and scraped out the worst of the weeds from the joints.

2. Wet the slabs with the hose so the paste wouldn't just soak into dry stone.

3. Spread the paste over the left-hand half with an old paintbrush, working it into the riven texture.

4. On the right-hand half, sprinkled bicarb and poured the vinegar mix over. It fizzed nicely for about a minute, which felt very satisfying.

5. Left both for 30 minutes, as most guides suggested.

6. Scrubbed hard with a stiff deck brush, then rinsed with the hose.

The scrubbing took the best part of two hours. Riven sandstone has lots of little ridges, and the paste sits in them like icing.

Did the homemade patio cleaner work? Not really

Once everything dried, I stood back with a cup of tea and felt a bit flat. The patio was cleaner in the way that any patio is cleaner after a good scrub and a hosing. Loose dirt had gone, and the lighter green film had faded in the sunnier spots. But that's what a brush and water would have done on their own.

The black spots, the thing that actually bothered me, hadn't budged. Not one. The dark staining along the shady north wall still looked exactly the same. If anything, the lighter patches around them made the spots stand out more.

There were a couple of other niggles too. The paste half left a faint whitish bloom in the grooves of the riven stone that took two more rinses to shift. And the vinegar half didn't look any better than the paste half, fizz or no fizz.

The real disappointment came about five weeks later. The green was creeping back along the joints and the shaded slabs, right where it always starts. All that effort had bought me barely a month.

Why doesn't bicarbonate of soda clean patios properly?

Afterwards I did a bit of reading, and it made sense of what I'd seen.

Bicarbonate of soda is a very mild alkali, around pH 8 in water. That's gentle enough to use in baking, which is great for your fridge but not much of a threat to algae. It works mainly as a soft scouring powder. So it helps you scrub off loose surface grime, but it doesn't kill anything living in the stone.

Those black spots are usually lichen, not dirt. Lichen roots itself into the tiny pores of sandstone, and scrubbing the surface leaves the living part behind. That's why they looked untouched.

The vinegar fizz is mostly theatre. When bicarb meets vinegar, they neutralise each other, making carbon dioxide, water and sodium acetate. By the time the bubbling stops you've got something close to salty water. Vinegar on its own is also acidic, which isn't ideal around the cement mortar in your joints.

And because nothing killed the spores left in the stone, the green simply grew back once the weather turned damp.

A bit about my courtyard garden

The patio matters to me because it more or less is the garden. We've got a small walled courtyard at the back of a terraced house, and there's no lawn at all. It's sandstone, a narrow border along one wall, and a lot of pots.

There's a potted olive tree that I'm probably far too attached to, a climbing rose trained up the sunny wall, and a row of lavender that the bees love in July. A little bistro table sits in the corner that catches the evening sun, and that's where most summer evenings end up.

The trouble with a walled courtyard is the shade and the damp. The north wall barely sees the sun from October to March, so the slabs there stay wet for weeks. That's exactly where the green and black always starts, and it's why a proper clean matters so much in a space this small. When the whole garden is the patio, a grubby patio means a grubby garden.

What I'd do instead next time

If you're weighing up a homemade patio cleaner for sandstone, here's what I'd suggest based on my wasted Saturday.

• Use a cleaner that kills algae and lichen, not just shifts dirt. Look for a patio cleaner labelled as safe for natural stone, which works over days or weeks to clear green and black growth at the root.

• Go easy with the pressure washer. Sandstone is fairly soft, and a narrow jet can etch the surface and open up the pores, which makes regrowth quicker.

• Keep acids off the stone and joints. Vinegar and brick acid can dull sandstone and attack the mortar.

• Treat the shady spots first and more often. In my courtyard that's the north wall, every spring and again in autumn.

• Keep bicarb for the kitchen. It's still brilliant for greasy BBQ grill racks and a stained garden table top. Just not the patio.

Homemade patio cleaner FAQs

Does bicarbonate of soda kill moss and algae on patios? Not in my experience. It's too mild to kill algae, moss or lichen. It helps you scrub loose grime, but the growth comes back quickly.

Is baking soda and vinegar safe on Indian sandstone? Bicarb on its own won't harm sandstone, but it can leave a white residue in riven grooves if you don't rinse well. Vinegar is acidic, so I'd keep it off natural stone and mortar joints.

Will a homemade patio cleaner remove black spots? It didn't on my patio. Black spots on sandstone are usually lichen, which is rooted into the stone and needs a proper treatment to shift.

What's the best way to clean a sandstone patio without a pressure washer? Apply a stone-safe patio cleaner that kills algae and lichen, give it time to work, and scrub with a stiff brush. Then rinse with a hose rather than a high-pressure jet.

My verdict on bicarbonate of soda as a patio cleaner

Disappointing, if I'm honest. My homemade patio cleaner gave me a slightly cleaner patio for a few weeks, a sore back, and a white film to rinse off. The black spots never shifted, and the green was back by early summer.

If your patio just needs a quick freshen-up, a brush and hose will do the same job for free. But if you've got proper algae and lichen on sandstone, like my shady courtyard, save the bicarb for baking. Next spring I'll be using a dedicated stone-safe patio cleaner and spending that Saturday at the bistro table instead.