Pressure Cleaning Melbourne

Terracotta roof cleaning

Terracotta Roof Cleaning - Pressure Cleaning or Soft Wash

You may have looked up at your roof on the way home from work one day and thought to yourself “that roof could do with a bit of tlc, its looking a bit untidy up there”
Sometimes when you’ve no detailed knowledge of a structure like the roof of your home its difficult to know where to start. A couple of leak marks may have started to show on the ceiling but you’re not sure what they’re caused by, a real leak or just the paint finish getting a bit tired. “I’ll get to it one day”
One of the easiest jobs to start with might be to get your terracotta roof cleaned. A good local roofing contractor can give you some guidance on this but the point of this blog and this article is to help you to get some clarity yourself about what to do next. A bit of knowledge goes a long way and that is equally important when deciding what to do with your roof.
I’m not here to say, get a full roof restoration or get the roof pointing done, that’s not the purpose of this article. All I want to achieve, is to give you the lay person when it comes to roofing a bit of knowledge so you don’t become a victim of an unscrupulous roofing contractor. I like my sleep. I like to keep customers happy, its that simple. I do the work, you are happy, you pay, you tell others, I get more work…….that’s it. No more complex than that however some people like to make life really complicated.
Lets get on with talking about roofs, more specifically I only want to talk about roof cleaning in this article. Seems simple enough doesn’t it? But like a lot of things that seem simple enough at the outset there are plenty of issues that can trip you up.
Get some knowledge, get some quotes, have a think about it, write down anything you are not sure about and ask some more questions.

Roof cleaning and the different types of roof dirt and grime

We see dirty roofs all the time, every street has roofs that appear dirty. But do you know what that dirty appearance is made up of? We need to know or we won’t understand how to get rid of it.
There are two main issues, dirt and organic matter. Roof dirt is pretty simple, pollution, airborne dust, bird droppings (yes roofs can be pretty disgusting places, especially near the coast with seagulls), rodent and possum droppings and leaves. Organic matter gets a bit more complex and we venture into an area we call the biofilm (or scientists do).

Roof Biofilms

Biofilms can be difficult to analyse as they are made up from a number of components, and all are inter related. So the grey discolouration on your roof is a mixture of dirt and the biofilm. It's never just dirt. The biofilm is a complex layered eco system of organisms the simplest of which has landed on your roof blown in by the wind and has attached itself to the tiles. This would be an algae similar to the algae you can see on any stagnant pond, albeit a different species. They love damp spots and will colonise the south facing side of your roof very easily in winter, giving it a nice green colour, especially where water flows around chimneys and down flashings. So we see this on the surface of the roof and if your roof is tiles then the algae will colonise every single tile on all sides of your home not necessarily just on the surface of the tile but in between the overlaps, that’s thousands of places for them to grow.
Lets just assume for the time being that they are only on the surface of your roof. The next layer of biofilm are lichens. These are voracious feeders and what do they like to feed on? yep the algae. So we start to build up this complex layer. Now given that there are thousands of species of algae and lichens, and they all have different preferences as to where they live and what they feed on you can see that things become complex.
On top of this we have moss with again many different species, with varying abilities to adapt to heat and cold by colonising different parts of your roof.

Plant conservation

One of the great places to see these biofilms at work and close up are in cemetarys with old gravestones where different coloured algae, lichens and moss can be seen flourishing.
Conservation matters and where these life forms exist in nature it is good to leave them alone as it is a fact that some rare species of lichens are making a comeback because we have cleaner air as a result of reduced emissions from vehicles. So removing them from archaeology would not be acceptable.
Roof lichens tend to be of the common varieties and are classed as leafy (foliose) or crustose. They are very adaptable and thrive in moist conditions and go dormant when extreme heat on your roof forces them to shut down for their own survival. When dry and dormant they are almost impossible to remove by scraping without damaging the building substrate ie your roof tiles.

Roof materials

The biofilm exists in a wide variety of conditions on your roof and is dependent from what your roof is made. Tiles can be terracotta or concrete, and metal roofing could be galvanised steel, zincalume or modern Colorbond.
At some time or other all these surfaces will be populated with a biofilm. Its fair to say that with metals, the colonisation is less overall, and the negative effect of the plant material is also less but problems do exist.

Terracotta Tiles

Algae loves the pores in unglazed terracotta tiles (the orange colour tile) and very few untreated roofs of this style are untouched by the biofilm. Most are heavily colonised by lichen and moss also.
Semi glazed and fully glazed terracotta tiles are colonised more slowly over time but ultimately end up with severe infestations.

See this picture as an example of a heavy growth of leak causing lichen


Heavy infestation of lichens can cause huge problems as they grow to inhabit all the overlapping joints. A roof tile is designed to have grooves or channels in the lower overlap intended to shed storm water. If lichen grows in there and blocks it then the water has nowhere to go except into your roof space. It does this slowly. The lichen gets saturated and water weeps between the tiles making the roof battens wet first. This may not be enough water to drip onto your ceiling but of course the damage is being done. The next piece of timber vulnerable to rot is the rafter, or more accurately where the batten is nailed to the rafter. Water gets into the nail hole and rusts the nail, the batten becomes rotten and infection then moves to the rafter. At this point of time things are getting serious and you havn’t had a drop of water on your ceiling to give you a clue as to what’s going on. The trouble is if this situation gets worse when a storm materialises its likely your roof will not be secure enough to weather the storm and some tiles may fly off taking the battens with them. Try to repair or add new battens and we find that the rafter won’t accept a nail, and needs to be strengthened.

To avoid all of these issues keep your roof clean.

In addition moss tends to cling to the lower edge of each individual roof tile and the same thing happens as per lichen. Moss hold a lot more water and if you live where there are frosts, the freeze/thaw process can do damage. As the water in the moss expands when its frozen it can crack slate and terracotta tiles. Concrete tiles tend to be tougher unless they are very old.

How not to clean your roof

The very best way to clean a terracotta or slate roof is NOT to use pressure washing. The high pressure water erodes the surface of terracotta and make the pores in the clay even larger. One pressure clean is the equivalent to more than 5 years of weather exposure on your roof. To pressure clean an operator must walk on your roof. This means cracked tiles are inevitable. A concrete tile makes a loud noise when it cracks, but not so terracotta, so you could be left with cracked tiles on your roof after a pressure wash unknown to the roof contractor. In addition the biofilm spores that nestle between the tile overlaps cannot be removed by pressure cleaning. Bits of lichen and algae spores will be left behind and because the pressure clean has opened up the pores in the clay surface re colonisation will be much faster than the initial growth period.
The very best way is to clean the roof is from the top of a ladder or from the ground level.

Roof Structure damage when pressure cleaning

Look at your tiled roof and you will see its made up from interlocking tiles and what are called ridge caps, covering the joins in the roof where the roof changes direction on an external corner. The ridge caps are fixed to your roof with a cement bedding and pointed with either a cement based product or a modern acrylic based pointing material. What do you think happens when an operator goes on to your roof with a high powered pressure cleaner with 4000 psi blasting on to your roof ridge caps. It is inevitable that some or most of the pointing material will be damaged by this process. Meaning that in addition to paying for roof cleaning you will need to find the money to get it re pointed.
Where your roof changes direction on an internal corner invariable you will find some metal work called the valley. In Victoria valleys are generally pointed and pressure cleaning will pose a risk to damaging the valley pointing, and with the massive amount of dirt and organic growth stripped away by pressure cleaning all at once, the valleys are at risk of being flooded by the pressure cleaning process.

A low pressure biocide application causes no damage whatsoever.

Roof cleaning solution

The very best product to use is a biocide that attacks the biofilm, from algae right through to moss. Special equipment is required to get this right as the biocide product must be applied at a low pressure. Temperature is also a factor, apply too hot and it evaporates, too cold and it remains inert and with both these options it will be ineffective.
The roof requires a thorough soaking and remember the overlap between the tiles I mentioned, this is where the biocide excels. By gently soaking the roof the biocide will seep into all the gaps between the tiles and the best thing is that because it is applied with just a shower spray pressure there is no tile erosion or risk of flooding your roof space.

Low pressure biocides

This group of products are used widely as an antiseptic in hospitals where they are very effective at killing bacteria and viruses. In fact the biocide used on a roof is a strong commercial version, however it is still safe to be applied by a human and because it breaks down on contact with organic matter it is safe to use around your home. Its is not like a bleach or an acid that has a nasty odour - it has no odour. The EPA warn people about using toxic chemicals around the home, industrial strength bleach is sometimes called a biocide but its not the product you want to go into drains and end up in waterways. The same with high strength acids that some roofers use.

Residual roof cleaning effect

Even after rain, the biocide is re activated and keeps working for a couple of years stopping re growth of the biofilm. This means that it might be 4 years before you notice any new lichen or moss.
Our biocide is safe for people pets and plants.

Other surfaces

Our biocide can be used for any surface from concrete driveways, paths and decking. Its great for decking because it kills the algae and fungi that causes wood to rot, so a regular application every couple of years will keep your fencing and decking with no sign of deterioration. It will retain a nice weathered look or your can re oil or re coat to get that as new finish.
Tennis courts and swimming pool surrounds can all benefit from the residual effect of a low pressure biocide application.