Roof Cleaning v Roof Washing?

By Andrew Fisher | 7 July 2020

Roof Cleaning - Roof Washing - Moss and Mould Spraying - No Rinse Roof Cleaner - Spray and Walk Away ? 

These are all terms used to describe roof 'cleaning' in a broad sense and they can all mean the same thing - or different things depending on who's telling the story!

The actual cleaning methodology is different though - and worth talking about because there is still some ambiguity in the market when it comes to what you ask for versus what you get.

First and foremost - roof cleaning is important, building owners be reminded that a roof is a big deal, they are not cheap to replace and a little bit of cleaning maintenance can extend the life span of a roof considerably - probably double if not more.

Before we go on - if you want to know the condition of your roof and don't have easy access, talk to us about a drone inspection.

Here is what you need to know about roof cleaning

If your roof looks something like this – it is quite dirty, just like your car would be if you didn’t wash it for 12 months. 

Dirt gathers on the flat tray profile and on the ends of the iron, especially evident on a low pitch roof like this and most other commercial roofs. There are likely moss or lichen spores in amongst this lot but not immediately visible as a green tinge just yet. 

A relatively easy clean, it will come up well with minimal effort. 

If your roof looks something like this – it is obviously very dirty, and it also has moss & lichen growth – this likely hasn’t been cleaned or treated for moss in the last 10 years! 

The roof coating in this case, is at risk of been penetrated by Lichen and thus exposed to the elements. In a salt zone like most of coastal NZ, corrosion will set in quickly and start to rot the roof. 

This roof has been left far too long and it will involve a considerable amount of careful work to clean it up. 

What does yours look like?

When it comes to cleaning a roof - you have 3 options;

Roof Wash #1 - A Moss & Lichen Spray Treatment (sometimes called a Soft Wash)

This is a high-volume soaking using a Benzylamonium Chloride Solution - designed to saturate the roof and kill any moss & lichen spores. It remains active for a long time and is reactivated when wet - so every time it rains a bit more washes off and away.

PROS Low cost, effective, longer lasting, usually uses less access as a larger area of roof can be sprayed from a distance.

CONS Results are not instant, heavy on chemical use, rain water collection systems have to be disconnected for several months after to prevent contamination, does not remove dirt build up, does not remove bird droppings or debris on the roof surfaces, does not clean gutters.

Note - if your cleaning company mentions they'll be using bleach on the roof - tell them to run a mile.

Roof Wash #2 - Low Pressure Chemical Clean

This is akin to your Saturday morning car wash, apply the soap, let it work and wash off - using low pressure water blaster. This is a good method for many roofs that have a medium/low level of dirt as pictured in Example 1 above.

PROS Instant and effective, removes all dirt, debris and bird droppings, gutters can be easily washed out at the same time.

CONS Potential contamination from chemicals (if not managed correctly), rainwater collection systems should be disconnected for the duration of the clean, slower process, complete fall arrest systems usually needed, noisy - more disruption to building occupants low pressure potentially exposes leaks.

Roof Wash #3 - Pressure Wash/Hot Waterblast

A straightforward med-high pressure roof wash using plain water - sometimes heated to 50-60 degrees for extra cleaning effect. This is a good method for all metal roofs, especially those with high level of dirt and lichen as pictured in Example 2 above. Even so higher pressure and even hot water may not remove the most stubborn lichen spores or roots, so it is good practice to follow through with a moss treatment spray after cleaning a badly soiled roof.

PROS Instant and effective, removes all dirt, debris and bird droppings, Chemical-free, gutters can be easily washed out at the same time.

CONS Can be slow - depending on the level of dirt, complete fall arrest systems usually needed, noisy - more disruption to building occupants, potentially exposes leaks with higher pressure.

What you need to know - not everyone who provides you with a quote for your roof wash will have inspected the roof up close - so will default to the 1st option as it is low risk. By the time you remember to check the results in 3-6 months time it's a bit difficult to account for.

The cost difference between option #1 and options #2 & #3 could be in the thousands of dollars - potentially double or more - so it is important to understand;

a The bottom line - what you are paying for versus your expectation of the Scope of work for your Roof Clean

b That you know what result you want - instant results (for pre-sale) - or are you happy with more drawn-out result?

c The numbers, roof cleaning is high-risk work by default, and the largest cost factor is TIME. If the quotes vary enough to raise your eyebrows, then it begs the question: 

* What safety practices are being followed?

* How long will it take?

* What cleaning method has been quoted for?

A roof cleaning equation might go something like this;

Time = Labour input

Temperature = Hot or cold water (hot water cleans up to 80% better than cold)

Chemical = Chemical speeds up the cleaning process by breaking down dirt faster

Agitation = Manual scrubbing to loosen dirt, or specialist cleaning nozzles to agitate the dirt better

Reduce or take one or two of these elements away completely and the others will rise to compensate, i.e. Moss Spray uses less time - more chemical.

A low-pressure chemical wash can take more than twice as long - the chemical is the main accelerant.

A high-pressure hot wash with a turbo nozzle will be quicker.

Labour is generally around 80% of the total cost of a cleaning project so you can see why Vendor A's price of $500 can look a look a lot different to Vendor's B & Vendor's C pricing which could be around $2000...!

The above scenarios are hypothetical and there may be other factors at play when considering a roof cleaning project, is an access platform required? are there fixed anchor points in place or will a temporary system need to be rigged up?, how dirty is the roof in the first place?

A clean roof is easy to keep clean

Sounds obvious but it is good to remember that Lichens only grow if the environment is right for them. They need moist air, a substrate that doesn't move (i.e. a roof), and nutrients (i.e. dirt, bird poop, moss and light) which the environment is happy to provide in generous proportions free of charge.

If you keep the dirt levels down, they will struggle to form.

We recommend an annual roof clean of some sort - an initial pressure clean in year 1, maintain it with a moss treatment spray the following year and repeat. 

That is the recipe for a long life roof. 

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