The short answer
Start with a wide fan nozzle at a distance and move closer only if you have to. Never use one on mortar, soft brick, render, roof tiles or old decking.
- Start with the widest fan nozzle, well back, and close in gradually.
- Keep it moving and keep the angle low rather than pointing straight down.
- Test in a hidden corner before doing a whole patio.
- Never use it on: pointing and mortar joints, soft or old brick, render, roof tiles, painted surfaces, or decking that has gone soft.
- Rinse everything afterwards, including any plants that got splashed.
What will ruin it
Blasting mortar out of paving joints or brickwork is a genuinely expensive mistake. It lets water in, the water freezes over winter, and the damage appears in spring as spalled brick and lifted slabs.
Why it works
A pressure washer cuts by removing material, and it cannot tell dirt from soft mortar or from the surface of a brick. What looks like a dramatically clean result is often a fractionally thinner, more porous surface.
Household chemicals can be dangerous in combination. Never mix them, keep the room ventilated, and test anything on a hidden patch first. Full disclaimer.