How do I use a pressure washer safely?

Choosing the nozzle and distance for the surface, the things that must never be washed, and how injuries actually happen.

Difficulty: beginner
Time: 1 hr
Read: 4 min
Safety: warning

Short answer

Start with the widest nozzle at the greatest distance and work closer only if needed. Never use a zero-degree pencil jet on anything domestic. It is powerful enough to cut skin, strip render, force water behind cladding and destroy soft mortar and timber.

Pressure washers cause more damage to property than almost any other household tool, and the injuries they cause are unusually serious relative to how harmless they seem.

Safety

A pressure washer can cut skin and inject water and dirt under it, which is a surgical emergency even when the wound looks trivial — go to A&E and say it was a high-pressure injection injury. Never point it at a person or animal, never use it up a ladder, always use an RCD, and wear eye protection and closed footwear. Do not use one on asbestos cement, which is common in older garage and shed roofs.

Step by step

Understand the nozzle colours.Red or zero degrees is a pencil jet that will cut skin and strip surfaces — it has almost no domestic use. Yellow at 15 degrees is for hard cleaning, green at 25 for general work, white at 40 for gentle. Black is low pressure for detergent.
Start wide and far away.Widest nozzle, arm's length, and move closer only if it is not working. Every mistake in this job is made by starting too aggressive, and the damage is not reversible.
Test somewhere hidden.A corner, behind a bin, low on a wall. Thirty seconds tells you what the surface will tolerate before you have done it to the visible part.
Match the pressure to the surface.Concrete and stone tolerate a lot. Block paving, soft brick, render, timber and painted surfaces do not, and the damage — furring, pitting, stripping — is permanent.
Know what must never be washed.Asbestos cement roofs and panels, which release fibres. Soft lime mortar. Old render. Windows and their seals. Electrical fittings and outdoor sockets. Anything with rot behind it.
Do not force water into the building.Never aim upwards at cladding, weatherboard, soffits or under tiles. The water goes behind and stays there, and this is a common cause of damp that appears months later.
Keep it off pointing and joints.Mortar joints and block paving sand are easily washed out. A strong jet along a joint removes years of it in seconds and the paving then moves.
Never use one up a ladder.The lance kicks back and it is enough to unbalance you. Use an extension lance from the ground, or scaffolding.
Use an RCD and check the cable.Electricity and water. An RCD is essential, the cable should be uncoiled fully to prevent overheating, and no connection should sit in a puddle.
Wear eye protection and proper footwear.Debris comes back at high speed, and sandals or bare feet in the spray path is how foot injuries happen.
Use detergent at low pressure and let it dwell.Apply with the low-pressure nozzle, leave it several minutes to work, then rinse. Chemistry and patience do the work that people try to do with pressure.
Work in overlapping passes at a consistent distance.Uniform distance and speed. Varying either leaves stripes and clean patches that are more obvious than the original dirt.
Protect plants and drains.Cover nearby planting, and know where the runoff goes. Detergent should not go into a watercourse or onto a border.
Drain it before storing in winter.Water left in the pump freezes and cracks it. That is the most common way pressure washers die.
Take any injury seriously.A pressure injection injury can look like a pinprick and cause severe damage under the skin. Go to A&E immediately and say what caused it.

Tips

The widest nozzle and the greatest distance that works. Almost every ruined patio started with someone deciding to get closer.
Let detergent do the work. Five minutes of dwell time replaces a lot of pressure and is far safer for the surface.
Re-sand block paving after washing. The jet removes the joint sand and the paving loses its interlock without it.

Common mistakes

Using the zero-degree pencil jet — It concentrates the full pressure into a point that cuts skin, strips render and gouges timber and soft brick. It has almost no domestic use.
Aiming upwards at cladding or under tiles — Water is forced behind the surface and stays there, causing damp that appears months later and is hard to trace back.

If it doesn't work

Stripes and patches on a patio

Cause: Inconsistent distance and speed — Fix: Do the whole area again at a uniform distance with overlapping passes. It has to be evened out rather than touched up.

Block paving loose after washing

Cause: Joint sand washed out — Fix: Re-sand with kiln-dried sand once dry, brushing it into every joint. Use less pressure on the joints next time.

Damp appearing inside after washing outside

Cause: Water forced behind cladding, render or under tiles — Fix: Let it dry out fully with ventilation. Never aim upwards or at joints, and keep well back from openings.

Pressure dropping or pulsing

Cause: Blocked nozzle or inlet filter, or air in the system — Fix: Clean the nozzle with the pin supplied, check the inlet filter, and run water through with the trigger open before starting.

Will not start after winter

Cause: Water left in the pump has frozen and damaged it — Fix: Drain fully and store somewhere frost-free. Once the pump is cracked it is usually not economic to repair.

Questions people ask

What nozzle should I use on a pressure washer?

Start with the widest — white at 40 degrees or green at 25 — at arm's length, and only go narrower if needed. The red zero-degree pencil jet cuts skin and strips surfaces and has almost no domestic use.

What should you not pressure wash?

Asbestos cement roofs and panels, soft lime mortar, old render, painted timber, windows and seals, electrical fittings, and anything you would be aiming upwards at such as cladding or under tiles.