How to use a pressure washer — what the nozzle colours mean, PSI versus GPM, and the surfaces you should never blast.
A pressure washer transforms a grey, green, tired-looking patio in an afternoon. Driveways, decking, fencing, garden furniture, wheelie bins, brickwork — the results are genuinely dramatic, and it is one of the most satisfying jobs in home maintenance.
It is also a tool that damages things very quickly, and the damage is usually permanent.
Pressure washers strip soft wood out of decking and leave it furry. They blast mortar out of old pointing. They force water behind cladding, where it sits and rots. They etch patterns into soft stone. And a narrow, high-pressure jet at close range can cause a serious injury to skin.
Nearly all of it comes from the same root cause: too much pressure, too close, with too narrow a nozzle.
The fix is a habit that takes no extra time. Pick a wide nozzle, start further away than you think you need to, and move in gradually until it is cleaning. This guide covers that, plus the nozzle system and the washing sequence that actually works. 💡
Alt text: “Pressure washer surface cleaner attachment cleaning a patio with an even edge and no stripes”
- PSI and GPM explained
- Nozzle colours — the key knowledge
- Surfaces you should not pressure wash
- Safety, including the injury risk
- Step by step
- Detergent: bottom up, rinse top down
- Stripes, zebra marks and how to avoid them
- Technique by surface
- Decking — the one to be careful with
- Aftercare and storage
- Common mistakes (and how to fix them)
- Pro tips worth knowing
- What I recommend
- Frequently asked questions
- Your final checklist
PSI and GPM Explained
Two numbers on every machine, and most people only look at one.
PSI (pounds per square inch) is pressure — how hard the water hits. This is the cutting and lifting power that breaks dirt off a surface.
GPM (gallons per minute) is flow — how much water is delivered. This is what carries the loosened dirt away and determines how quickly you cover ground.
| PSI (pressure) | GPM (flow) | |
|---|---|---|
| What it does | Breaks dirt loose | ✅ Washes it away |
| Matters most for | Stubborn stains, moss | ✅ Large areas, speed |
| Too much causes | ⚠️ Surface damage | Water use |
| Usually advertised? | ✅ Prominently | ⚠️ Often buried |
| Underrated? | No | ✅ Yes — check it |
Both matter, and they multiply. A common way to compare machines is to multiply PSI by GPM — sometimes called cleaning units — which gives a better picture than PSI alone. A very high-pressure machine with poor flow is slow on a big patio, because it cuts dirt loose but does not carry it away.
For most home use, more pressure is not what you need. Higher PSI mainly increases the risk of damaging the surface. If you are cleaning a large driveway, flow and a surface cleaner attachment will save you far more time than extra pressure.
Nozzle Colours — The Key Knowledge
Most pressure washers come with colour-coded nozzles, and the colours are broadly standardised. Learning them is the single most useful thing you can do.
| Colour | Angle | Use | Risk |
|---|---|---|---|
| Red | 0° | Pinpoint blasting | ⚠️ Damages almost everything |
| Yellow | 15° | Heavy stripping, concrete | ⚠️ Aggressive — test first |
| Green | 25° | ✅ General cleaning | Moderate |
| White | 40° | ✅ Gentle — wood, cars, siding | ✅ Low |
| Black | 65° | ✅ Applying detergent | ✅ Very low pressure |
| Turbo / rotary | Spinning 0° | Hard surfaces, fast | ⚠️ Very aggressive |
The practical approach: start with the widest nozzle that could work, at a distance. If it is not cleaning, first move closer. If it still is not cleaning, then consider a narrower nozzle. Work up gradually — you can always increase aggression, but you cannot undo an etched surface.
The black soap nozzle is not a cleaning nozzle. Its very wide angle drops the pressure enough for the machine’s detergent siphon to work. Many people never realise their detergent will not draw unless this nozzle is fitted.
Surfaces You Should Not Pressure Wash
| Surface | Verdict | Why |
|---|---|---|
| Old or soft lime mortar | ❌ No | Blasts the pointing out |
| Roof tiles and slates | ❌ No | Forces water under; lifts tiles; fall risk |
| Asbestos cement sheeting | ❌ Never | ⚠️ Releases fibres — get advice |
| Painted surfaces you want to keep | ⚠️ Careful | Strips paint readily |
| Windows and seals | ⚠️ Careful | Forces water past seals |
| Soft or crumbling stone | ⚠️ Test first | Etches permanently |
| Electrical fittings, meters, vents | ❌ No | Water ingress |
| Living plants | ❌ No | Strips leaves and bark |
| Anything with lead paint | ❌ No | ⚠️ Spreads contamination widely |
Safety, Including the Injury Risk
Pressure washers feel harmless because it is only water. It is not harmless.
Other essentials:
- Eye protection. Debris comes back at you at speed — grit, moss, paint flakes.
- Closed shoes, not sandals. People do spray their own feet.
- Expect kickback when you pull the trigger, particularly with narrow nozzles. Brace yourself.
- Beware of ladders. The recoil while working overhead is a real fall risk. Use an extension lance from the ground instead.
- Electrical safety. Use an RCD, keep connections off wet ground, and never spray toward sockets, outdoor lights or the machine itself.
- Watch the surface underfoot. A freshly washed patio is extremely slippery, especially if you have used detergent.
- Never leave the machine running unattended with the trigger locked off — many pumps overheat if they run without flow for long.
The two rules that prevent nearly everything: never point it at a living thing, and always start further away than you think necessary. Neither costs a second, and between them they cover both the injuries and the ruined surfaces.
Step By Step
Step 1 — Prepare the area
Move furniture, pots and anything that will be sprayed. Cover plants you cannot move. Close windows and doors. Check for anything electrical nearby.
Step 2 — Sweep first
Remove loose debris, leaves and moss with a brush. Blasting loose material just relocates it and slows the job.
Step 3 — Connect water and check the supply
Turn the water on before starting the machine. Running a pump dry, even briefly, damages it.
Step 4 — Purge the air
Squeeze the trigger with the machine off until water flows steadily, then start the motor.
Step 5 — Fit the widest sensible nozzle
White 40° or green 25° to begin.
Step 6 — Test in an inconspicuous spot
Always. A corner behind a plant pot, a section that will be covered. Find out how the surface responds before you do the visible area.
Step 7 — Start far back and move in
Begin around a metre away and close the distance gradually until it is cleaning effectively. That distance is now your working distance — keep it consistent.
Step 8 — Apply detergent if needed
Black nozzle, bottom upward. See below.
Step 9 — Let it dwell, then rinse
Top downward, overlapping passes.
Step 10 — Work in a consistent pattern
Overlap each pass by about half, keep the lance at a steady angle and distance, and keep moving.
Step 11 — Shut down properly
Switch off the machine, turn off the water, then squeeze the trigger to release residual pressure before disconnecting anything.
Detergent: Bottom Up, Rinse Top Down
This sequence sounds arbitrary and it is genuinely important.
Apply detergent from the bottom upward. If you start at the top, the cleaner runs down over dry, dirty surface below. Where it runs, it starts working — and it leaves streaks that are visible after rinsing and can be very difficult to remove. Starting at the bottom means everything below the area you are working on is already wet with detergent, so runs have nowhere to leave a mark.
Rinse from the top downward. Gravity carries the dirty water down over surface you have not yet rinsed, which is fine — you are about to rinse it. Rinsing bottom-up means dirty water runs over areas you have already cleaned.
Use detergent designed for pressure washers, and check it suits the surface. Some cleaners are harmful to plants, some will discolour certain stone, and some should not go into a drain that leads to a watercourse. Read the label, and rinse surrounding planting with clean water afterwards.
Stripes, Zebra Marks and How to Avoid Them
The classic pressure washing failure: a patio covered in wand-width stripes, lighter where you dwelt and darker where you moved fast.
Why it happens: a hand-held lance cleans in a narrow arc. Any variation in distance, angle, speed or overlap changes how much dirt comes off, and on a large flat area those variations are extremely visible once it dries.
| Cause | Fix |
|---|---|
| Inconsistent distance | ✅ Lock your arm position and keep it |
| Inconsistent speed | ✅ Steady sweeps, no pausing |
| Not overlapping enough | ✅ Overlap each pass by about half |
| Changing angle mid-pass | ✅ Keep the lance at a constant angle |
| Stopping mid-area | ✅ Finish a whole section before breaking |
| Hand lance on a large flat area | ✅ Use a surface cleaner attachment |
The proper solution for patios and driveways is a surface cleaner attachment. It houses two spinning jets inside a covered disc that you push across the ground. Because the jets are at a fixed height and rotating, the coverage is completely even — no stripes are possible. It is also far faster and it contains the spray, so you are not covered in dirty water.
If you already have stripes, the only real fix is to clean the whole area to the same standard — go over everything again evenly, ideally with a surface cleaner.
Technique by Surface
Concrete and paving
The most robust surface. Green 25° or a surface cleaner. Watch out for kiln-dried sand in block paving joints — pressure washing strips it out, and it needs re-sanding afterwards.
Brick and stone
Wider nozzle, more distance. Check the mortar first — if it is soft, crumbly or lime-based, do not pressure wash it at all. A stiff brush and a masonry cleaner is the safer route.
Render and painted walls
White 40° from a distance, testing first. Pressure washing removes loose paint very effectively, which is either helpful or a disaster depending on your intention.
Cladding and siding
Spray downward or straight on, never upward. Angling up drives water behind boards and laps, where it sits and causes rot.
Garden furniture
Wide nozzle, well back. Plastic marks easily; softwood furrs up.
Cars
White 40° minimum, at a distance, and never on badges, trim, seals or damaged paint. A dedicated snow foam lance is far kinder than pressure.
Fences
Wide nozzle, along the grain, consistent distance. Softwood fencing furs up easily — see the decking notes below, which apply equally.
Decking — The One to Be Careful With
Decking is where pressure washers do the most damage, because timber is much softer than people assume and the damage is not obvious until it dries.
What goes wrong: wood is made of alternating soft early growth and harder late growth. A pressure jet erodes the soft bands faster, leaving a raised, ridged, furry surface. It looks clean while wet and terrible once dry — and the only fix is sanding the whole deck.
If you do pressure wash decking:
- White 40° nozzle only, or a surface cleaner rated for wood.
- Keep well back — considerably further than for concrete.
- Move constantly and never dwell.
- Work along the boards, with the grain, never across.
- Keep a consistent distance and speed or you will get stripes on every board.
- Test on a board that is hidden or at the edge.
- Let it dry fully and inspect before deciding it went well.
The honest alternative: a deck cleaner, a stiff brush and a hose does a good job with no risk of furring. And if the deck needs refinishing anyway, expect to sand afterwards — our guide on sanding a deck covers doing it properly, and sanding a fence covers the same issue on fencing.
Aftercare and Storage
- Release the pressure before disconnecting hoses — switch off, turn off the water, squeeze the trigger.
- Drain the machine and hoses. Water left inside is the main cause of failure.
- Protect from frost. Freezing water cracks pumps, and it is the single most common way pressure washers die over winter. Store indoors, or use pump antifreeze if the machine will live in an unheated space.
- Clean the inlet filter. Grit from the water supply damages the pump.
- Clear nozzles with the pin provided, never with wire that could enlarge the orifice.
- Coil hoses loosely — kinks become permanent weak points.
- Re-sand block paving joints after cleaning, and consider re-sealing patios once fully dry.
Common Mistakes (And How to Fix Them)
Mistake 1 — Reaching for the red 0° nozzle
The fix: start with white 40° or green 25°. The red nozzle damages nearly everything.
Mistake 2 — Starting too close
The fix: begin about a metre back and move in gradually. This alone prevents most damage.
Mistake 3 — No test patch
The fix: always test somewhere hidden first.
Mistake 4 — Detergent applied top-down
The fix: detergent bottom-up, rinse top-down. Runs on dry surface leave permanent streaks.
Mistake 5 — Letting detergent dry
The fix: work in sections and keep it wet, especially in sun.
Mistake 6 — Inconsistent distance and speed
The fix: lock your arm, steady sweeps, 50% overlap — or use a surface cleaner.
Mistake 7 — Spraying up under cladding
The fix: spray downward or straight on. Upward drives water behind boards.
Mistake 8 — Pressure washing soft mortar or old stone
The fix: check first. Soft pointing blasts out and repointing is expensive.
Mistake 9 — Running the pump without water
The fix: water on first, purge the air, then start the motor.
Mistake 10 — Leaving it out over winter
The fix: drain it and store frost-free. Frozen water cracks pumps.
Pro Tips Worth Knowing
- Sweep and pre-treat first. Removing loose debris and letting a moss killer work beforehand halves the pressure washing time.
- Work in shade or on a cool day. Detergent that dries before rinsing leaves marks that are hard to remove.
- Do the whole area in one session. Cleaning half a patio and finishing next weekend leaves a visible line.
- Wear waterproofs. You will get soaked, and dirty water rebounds further than you expect.
- Use an extension lance rather than a ladder. Recoil at height is a genuine fall risk.
- Rinse plants with clean water after using any detergent nearby.
- Take a photo before you start. The results are dramatic, and you will want the comparison.
- Check the drainage. Large volumes of dirty water need somewhere to go that is not a neighbour’s garden.
- Re-sand and re-seal afterwards. Cleaning strips joint sand and any existing sealer, so budget for both.
- Consider hiring for one big job. A powerful machine with a surface cleaner for a day may beat buying a modest one.
What I Recommend
If you are cleaning a patio, driveway or path, the accessory that changes the result most is not a more powerful machine. It is the thing that makes coverage perfectly even — because striping is the failure people actually end up with.
Kärcher 15″ Pressure Washer Surface Cleaner
🏆 Best Patio UpgradeA 15-inch surface cleaner attachment suitable for machines rated roughly 2600 to 3200 PSI. Two jets spin inside a covered housing at a fixed height, so every part of the surface gets identical treatment — which is exactly why striping becomes impossible rather than merely less likely.
- ✅ Eliminates stripes — fixed height and rotating jets mean perfectly even coverage
- ✅ Much faster than a lance on any large flat area
- ✅ Contains the spray, so you and the surroundings stay far cleaner
- ✅ Gentler on the surface than a narrow jet held close
- ✅ Turns a whole-weekend patio job into an afternoon
Prices and availability change often — check Amazon for the current price. Check your machine’s PSI rating and connector type before ordering.
Frequently Asked Questions
What do the pressure washer nozzle colours mean?
They indicate the spray angle. Red is 0° — a pinpoint jet that damages almost anything and has very little home use. Yellow is 15° for heavy stripping, green 25° for general cleaning, white 40° for gentler work like wood, cars and siding, and black is around 65° and used for applying detergent at low pressure. Start with the widest nozzle that could work and only go narrower if you must.
Is PSI or GPM more important?
Both, and they work together. PSI is pressure, which breaks dirt loose; GPM is flow, which washes it away and determines how fast you cover ground. Manufacturers advertise PSI prominently and often bury GPM, but on a large patio flow matters enormously. For home use, more pressure mostly means more risk of damaging the surface — flow and a surface cleaner will save more time.
Why does my patio have stripes after pressure washing?
Because a hand-held lance cleans in a narrow arc, so any variation in distance, speed, angle or overlap shows up as a band once the surface dries. Keep your arm position locked, sweep at a steady speed without pausing, and overlap each pass by about half. The reliable fix for large flat areas is a surface cleaner attachment, whose fixed-height rotating jets make uneven coverage impossible.
Can I pressure wash my decking?
You can, but carefully — decking is where pressure washers do the most damage. The jet erodes the soft grain faster than the hard, leaving a ridged, furry surface that looks fine wet and terrible dry. Use the white 40° nozzle only, stay well back, keep moving, work along the boards with the grain, and test somewhere hidden first. A deck cleaner and a stiff brush is genuinely safer, and expect to sand afterwards either way.
Why apply detergent from the bottom up?
Because detergent applied at the top runs down over dry, dirty surface and starts working where it runs, leaving streaks that show after rinsing and can be very hard to remove. Working upward means everything below is already wet with detergent, so runs leave no mark. Rinsing is the opposite — top down, so dirty water only runs over areas you have not yet rinsed.
Are pressure washers actually dangerous?
Yes, more than people expect. A high-pressure jet can inject water and debris beneath the skin, producing a tiny-looking wound with severe damage underneath and a real infection risk — that is a medical emergency. Never point the lance at a person or animal, never clear a blocked nozzle with the machine running, engage the trigger lock when moving, wear eye protection and closed shoes, and be aware of recoil, especially on a ladder.
Your Final Checklist
- Move or cover furniture, plants and anything electrical
- Check for soft mortar, asbestos cement or lead paint — do not wash these
- Sweep loose debris first
- Water on before the motor, and purge the air
- Eye protection and closed shoes
- Widest sensible nozzle — white 40° or green 25° to start
- Test in a hidden spot every time
- Start about a metre back and move in gradually
- Detergent bottom-up, black nozzle, do not let it dry
- Rinse top-down with 50% overlap
- Consistent distance, angle and speed — or use a surface cleaner
- Spray downward on cladding, never upward
- Never point it at a person or animal
- Release pressure before disconnecting
- Drain and store frost-free; re-sand joints and let timber dry fully
Next in this series: how to sand a deck and how to use a leaf blower.
