The short answer: for a tiled roof, we recommend softwash over pressure washing. Pressure washing is quick, but tile makers and roofing bodies warn that it can damage tiles. Softwash is gentler, and it treats the growth itself. That said, no method is risk-free, so a survey matters more than any single rule.
What this guide covers
1. The two methods, explained
Pressure washing (jet washing)
A pressure washer sprays water at high pressure straight at the tile surface. Domestic machines are powerful, so the effect on a roof can be strong. For example, the Kärcher K 5 range reaches up to 145 bar, which is about 2,100 psi. The force blasts off the top layer of moss, algae and dirt, so the roof looks cleaner at once.
Softwashing (biocide treatment)
Softwashing applies a biocide at low pressure. While products vary, professional ones are usually based on sodium hypochlorite, which kills the living moss, algae and lichen. Rain then washes the dead growth away over the following weeks, so the full result builds slowly.
2. Side-by-side comparison
| Softwash | Pressure washing | |
|---|---|---|
| Method | Low-pressure biocide | High-pressure water |
| Pressure | Low | Up to 145 bar (about 2,100 psi) on a domestic Kärcher K 5 |
| Removes visible growth | Over the following weeks | Straight away |
| Treats the surface left behind | Yes, with biocide | No |
| Risk to tile finish | Lower | Higher (Wienerberger warns of damage and colour loss) |
| Water getting under the laps | Lower | Higher (Wienerberger warns of ingress) |
| Environmental care | Biocide is toxic to aquatic life, so run-off needs control | Uses a lot of water, and drainage must cope |
| Results last | Depends on shade, damp and trees | Depends on shade, damp and trees |
3. Why pressure washing can damage tiles
Wienerberger, a UK tile manufacturer, says it is crucial to avoid high-pressure jet washers on roof tiles. Its guidance says the forceful spray can damage the tile finish and strip colour from concrete tiles. It also says the water can seep into the interlocks and headlaps and cause water ingress.
The National Federation of Roofing Contractors (NFRC) raises a second risk. It warns that power washing, especially with unnecessary foot traffic, often leads to cracked or broken tiles. Since a damaged tile can let water in, a quick jet wash can create a bigger job than it solves.
The RHS adds a note on drainage. In its advice on hard surfaces it says to use pressure washers with care where drainage is poor, because the extra water could make damp problems worse. It also points out that the results are temporary and the water use is high.
Check your own tile maker
Guidance varies between tile makers and products. So before anyone cleans your roof, read the care advice for your own tiles, and ask the firm which method it plans to use. If you are unsure which tiles you have, a survey can identify them.
4. Why results differ
Moss and algae return when the conditions suit them. The RHS notes that many mosses favour damp, shady conditions, and that growth can return if those conditions persist. This is true after any cleaning method, so shade, trees and damp matter as much as the method itself.
Pressure washing removes the visible growth but does not treat the surface left behind. Softwash biocide does treat it, which is why we offer a written regrowth guarantee on our softwash. Even so, Wienerberger notes that moss treatments need ongoing maintenance, so no roof stays clean for ever. Our post on whether softwashing is safe for roof tiles looks at the tile question in more depth.
5. Comparing cost over time
A headline price hides the real cost. However, a fairer test is the cost per year of clean roof, which you get by dividing the price by the years until the roof needs treating again. We cannot give you reliable UK figures for either method, so here is an example with made-up numbers.
Suppose a cheap clean costs £300 and needs repeating after two years. That works out at £150 a year. A dearer clean at £400 that lasts five years works out at £80 a year. The numbers are only an illustration, yet they show why the cheapest quote is not always the best value.
Damage changes the sums further. A cracked tile or a leak can cost far more than the cleaning saved, so factor in the risk as well as the price. Our roof cleaning cost guide lists what to compare in each quote.
6. What tile makers and roofers say
These are the sources we relied on for this guide. We name them so you can read them yourself.
- Wienerberger advises against high-pressure jet washers, and says moss treatments need ongoing maintenance
- NFRC says moss is rarely a problem unless it blocks drainage, and warns about power washing with foot traffic
- RHS says many mosses favour damp, shady conditions, and that pressure washing results are temporary
- K Rend says very powerful pressure washers can damage render, and recommends a biocide for algae on its render
We have not quoted other tile makers here, because we could not confirm their wording first-hand. Instead, we named the sources we did read. If a firm says a particular maker approves jet washing, ask to see that maker's guidance in writing. For wall surfaces, our render softwashing service follows the same low-pressure idea.
7. How to spot a pressure washer pretending to softwash
"Softwash" is a popular label, and not every firm that uses it follows the method. Five questions help you check.
- Do they name the biocide? A real softwash uses a biocide, so ask for the product or active ingredient. If the quote mentions only "gentle pressure", ask what treats the growth.
- How long will the job take? A careful job includes survey, set-up, application and gutter clearance. A very short visit should prompt questions.
- Is the guarantee in writing? Ask for its length and what triggers a claim. A firm that cannot offer one may not trust its own method.
- What will the roof look like on the day? Softwash results build over weeks as rain washes the dead growth away. A roof that sparkles at once may have been jet washed.
- Will they walk on the tiles? The NFRC warns that unnecessary foot traffic often cracks tiles, so ask how they will reach the roof.
If you want to try it yourself, read our post on DIY roof moss removal first. It covers the risks of working at height as well as the cleaning methods, since both matter.
Want a specialist softwash?
Low-pressure biocide with 5-year regrowth protection. We survey first and tell you honestly what your roof needs.
Get my free quote8. Frequently asked questions
What is the difference between softwash and pressure washing?
Softwash applies a biocide at low pressure to kill moss, algae and lichen, and rain then washes the dead growth away. Pressure washing uses a strong jet of water to blast the growth off. Softwash treats the growth itself, whereas pressure washing mainly removes what you can see.
Does pressure washing damage roof tiles?
It can. The tile maker Wienerberger advises against high-pressure jet washers because the spray can damage the tile finish, strip colour from concrete tiles and push water into the interlocks and headlaps. The NFRC adds that power washing, together with unnecessary foot traffic, often leads to cracked or broken tiles.
How long does softwash last compared with pressure washing?
There is no fixed answer for either method, because shade, damp and nearby trees drive regrowth. The RHS notes that many mosses favour damp, shady conditions, and growth can return if those persist. Our softwash comes with 5-year regrowth protection, and we explain the likely timing at survey.
Is softwash more expensive?
It can be, because you pay for biocide, care and a written guarantee as well as labour. The right comparison is cost per year of clean roof, not the headline price. Ask each firm for a fixed price and the guarantee terms, then compare like with like.
Can pressure washing ever be safe on a roof?
Some roof systems tolerate it, but we avoid it on pitched tile and slate roofs. Wienerberger says that if anyone does use a pressure washer, it should run on low pressure and point downwards from the apex, never upwards. Check your own tile maker's guidance before you let anyone jet wash.
What do tile makers and roofing bodies say?
Wienerberger advises against high-pressure jet washers on roof tiles, and it says moss treatments need ongoing maintenance. The NFRC says moss is rarely a problem unless it blocks drainage, and it warns about power washing and unnecessary foot traffic. Always read the guidance from your own tile maker too.