How Much Pressure Do You Need to Clean a Patio?

For a typical UK patio, somewhere between 100 and 130 bar (roughly 1,450 to 1,900 PSI) does the job on concrete slabs, and most domestic pressure washers sit comfortably in that range. Softer stone, older slabs and block paving want less, closer to 80 to 100 bar. The number on the box matters far less than the nozzle you fit and how close you hold it.

Bar, PSI and what the numbers actually promise

UK machines are usually rated in bar, US ones in PSI. One bar is about 14.5 PSI, so a 120 bar washer is around 1,740 PSI. That figure is the maximum pressure the pump can generate at the outlet under ideal conditions, not what lands on your patio. By the time water has travelled down the hose, through the lance and spread out across a fan, the working figure is meaningfully lower.

Manufacturers also quote a working pressure alongside the peak. The working figure is the honest one. A washer advertised at 150 bar might have a working pressure of 110, and that is the number to compare between models.

Pressure by surface type

Different patio materials tolerate very different amounts of force. As a rough guide:

• Poured concrete and concrete slabs: 110 to 130 bar. Tough stuff, hard to damage with a domestic machine unless you sit in one spot.
• Block paving: 80 to 100 bar. The blocks themselves are fine, but the jointing sand between them washes out fast.
• Indian sandstone and natural stone: 80 to 100 bar. Sandstone is porous and can pit or go fuzzy if you push too hard.
• Limestone, slate and older weathered slabs: 60 to 90 bar. Go gentle and test somewhere out of the way first.
• Decking and timber: 50 to 80 bar with a wide fan. Wood fibres lift easily and will not lie back down.

If your washer has no pressure adjustment, and plenty of budget models do not, you control the force by swapping nozzles and changing your distance from the surface.

Flow rate matters as much as pressure

This is the part most people miss. Pressure breaks the dirt loose. Flow rate, measured in litres per minute, carries it away. A 150 bar machine pushing 5.5 l/min will clean a patio noticeably slower than a 130 bar machine pushing 8 l/min, because the second one is shifting far more water across the slab.

For patio work, aim for at least 6 l/min. Anything under 5 turns a Saturday afternoon job into a whole weekend. When you are comparing two washers at similar prices, the higher flow rate is usually the better patio machine even if the bar figure is lower.

Nozzle choice does the real work

Most washers come with a colour coded set. For patios, the 25 degree (green) nozzle is the sensible default. It spreads the force over a usable width while still having enough bite to lift algae and general grime.

The 15 degree (yellow) nozzle is for stubborn patches, black spot lichen and stained corners. Use it in short bursts, keep it moving, and expect to see a lighter stripe where it has been. The red 0 degree pencil jet has no business anywhere near a patio. It concentrates the full output into a spot the size of a pea and will carve a visible line into concrete in under two seconds.

If you have any real square meterage to cover, a rotary surface cleaner attachment is the upgrade worth making. It houses two spinning jets under a shroud, holds a consistent distance automatically, and gives an even finish with no tiger stripes. It also cuts the job time by more than half and keeps the spray off your windows.

Distance is your pressure dial

Pressure at the surface falls off quickly as you pull the lance back. At 10cm you are getting close to the machine's full output. At 30cm you might be at half. At 50cm you are mostly just rinsing.

A sensible working distance for patio slabs is 20 to 30cm, held at a slight angle rather than straight down, so the water pushes debris ahead of it instead of blasting it in all directions. Start further out than you think you need, then close in until you see the dirt lifting. That habit alone prevents most of the damage people do to their paving.

Signs you are using too much pressure

Stop and back off if you see any of these:

1. Sand visibly lifting out of the joints between blocks or slabs.
2. A rough, sugary texture appearing on stone that was smooth before.
3. Distinct lighter stripes that do not blend when the surface dries.
4. Bits of the slab surface coming away as fine grit in the runoff.
5. Pointing or mortar crumbling out from between the slabs.

None of that undoes itself. Sand can be re-brushed into joints, but a pitted sandstone surface stays pitted.

A workable approach

Sweep first so you are not turning loose leaves into a paste. If the patio is properly green, a patio cleaner solution left to dwell for ten minutes does more than extra bar ever will, and lets you work at a lower pressure. Then work in overlapping passes across a manageable strip at a time, always moving, always keeping the same distance. Finish by rinsing in one direction so you are not left with a tide line where you stopped. Brush kiln dried sand back into block paving joints once everything has dried out.