Q & A
1. Should a roof be cleaned and why?

- Simply yes, organic growth can deteriorate roof tiles, shortening its life and often leading to smaller or bigger damages. Action should be taken as early as possible, this is usually when first signs of growth appear - yellow lichen, dark patches or first signs of moss growth. Acting early not only you'll maximise how long the roof will last but also it's the most cost effective time to take action!

2. Should I clean my roof or replace it?

- It depends:
If it's in relatively good condition, clean it.

If its not, how bad is it? are you aware of any damage? cracked tiles? ridge pointing? leaks? how old is the roof, can you access and inspect the roof felt? Are the battens rotten?

If your answer is no worse than a few cracked tiles & some repointing needed it is still a good idea to have it cleaned and repaired.

But! The main question here should be:

What is your budget and what are your plans.

Cost of a new roof on a small house usually starts in the region of £8,000, average 2-3 bed with a single level floor plan size of approx 40 square meters (small!) would be £11,000+ and the bigger the roof area the more expensive it becomes.

Cost for roof cleaning would typically range under the 10% of the new roof value, the better the condition and less growth the less expensive it'll be.

If your roof is in bad condition but your budget will not permit roof replacement in the near future then definitely, clean and repair it to give yourself maximum time to act. If you can however afford a new roof or will be able to within a year or two - do bare minimum - repairs if needed and avoid unnecessary spend on cleaning, save and replace it.

If your roof is in average condition - balance the pros and cons and again consider your budget. Is it the right time for you to replace it? or, can you get another 10-20 years or more out of the existing one? Spend for example £15,000 on a new roof or £1,500 on cleaning, the decision has to be yours.

If your roof is in good condition, take care of it! you spent a significant amount of money on it, make sure it lasts as long as it can!

Roof cleaning cannot be compared to roof replacement, cleaning is a maintenance process and you can get your roof tiles to look great again and last as long as possible but with a new roof you get a new felt, timber and new tiles with a fresh coating.

3. What about tile coating?

Tile coating is another option for the old and worn (concrete only) tiles but it's not cheap.

Re-coating roof tiles will definitely improve the look and extend the life of your roof but you have to consider the costs and benefits. Full roof clean with coating can amount to approximately 25-40% of a new roof value but you're still left with old felt and timber. 

You'll need to consider the condition of your roof including not just tiles but the structure and the felt to decide whether it's the best option for you, budget will also play a key role here.

4. Why is pressure washing bad for the roof?

If your roof is in a good condition with coating still in tact then with the right approach and adequately low pressure risk is minimal to none.
However, if your roof is in a bit more questionable condition:
Deteriorating roof tiles become much softer, whether it's concrete, slate or clay, they'll be more fragile and easy to damage, blasting it with a pressure can cause unnecessary damages.
If your roof is fully covered in moss (even if it's just spot places) you don't know what's underneath it, if the tiles below the moss are badly cracked blasting them with high pressure will not only remove the moss but also the tiles and has a chance to get right through the felt by the time operator releases the trigger.

5. How about steam cleaning?

It's still pressure washing, yes it's steam but it still comes out at high pressure, otherwise moss wouldn't come off. It's a gimmick in a way, steam or just hot water enhances cleaning process but it doesn't differ from pressure cleaning too much.

6. I've heard that the moss is natural insulation and I shouldn't touch it.

It's just not true. Google how much water can moss absorb. Not only moss keeps your roof damp for much longer creating great habitat for fungus, algae, mould and so on, when it freezes in the winter it takes time to defrost!
Compare a clean roof tile to a thin sheet of ice, when the sun rises and hits it, it defrosts in minutes.
Now, fully saturated moss is comparable to an ice cube, will it defrost quicker? It's a layer of ice around your roof.
Same applies in cold but not below 0 degrees conditions, damp moss will take longer to dry (warm up the water it holds in).
How about the summer? do you need additional insulation in the summer? - No

7. I was told not to clean the roof as it'll only cause leaks.

Someone who said that must have had a bad experience, if there are no leaks pre existing then with correct technique the risk is pretty much none. Where it comes to scrape and softwash only carelessness or accident can cause a damage.

8. Can you clean asbestos roof?

Yes and no, asbestos turns white if the treatment is applied. Moss can still be removed without the treatment but it's likely to grow back rather soon if no treatment is applied. When treatment is applied the asbestos turns white in colour what isn't really a desirable look.
There are other options available but the cost can be daunting.

9. How much does roof cleaning cost on approximate size 3 bed house?

Number of bedrooms doesn't tell much about the roof.. We really need to see a house to give you an accurate price.
When working out the cost we consider:
Access, trees and bushes, soft or hard ground, space around the house (narrow paths can be insufficient to safely setup a scaffold or position a ladder), layout of the roof and profile of the tiles, condition itself.

For the closest possible estimate please see the video below explaining how to measure the size of your roof on Google Earth and at what rate per square meter to multiply the cost based on the profile of the tile and amount of growth. This is not always accurate but that's the best way of indirect estimation we could think of, the rates provided are based on working out the average cost of the work we completed. divided by the size of the property in sqm.