Cleaning Concrete and Clay Tile Roofs

Roof Cleaning to remove moss, algaes and lichens can be a tricky business. On one hand, you want to remove the growths to stop the damage to your roof. On the other hand, you don't want to damage the roof itself or create leaks in the process.

Choosing the right Outdoor Cleaner for DIY and using the correct application techniques will give you a clean roof that a professional operator would be proud of and your neighbours will envy.

Roof Cleaning with Bio-Shield® has been proven effective in over 38 years of commercial use. With regular applications every 2 to 4 years, Bio-Shield will keep the roof clean and free of moss, algae and lichens.

This maintenance program protects the original coating on your roof, extends the service life and eliminates the major expense of having a damaged roof recoated.

We have put together a few tips for roof cleaning that will help you get the job done safely and without causing any further damage to the roof.

If you don't want to DIY we recommend using a professional roof cleaning company for the application. Its surprisingly inexpensive and ensures you will stay safe on the ground. Here's an expert and professional roof cleaning company we recommend, they give free quotes and a 2 year Guarantee on the work.

Should I Water Blast a Concrete or Clay Tile Roof to Remove Moss?

Outdoor Cleaners like Bio-Shield® and water blast cleaning are the 2 most common methods for roof cleaning. Outdoor Cleaners are the best option to maintain a roof in a clean condition over a long period of time and protect the original protective or decorative coating from damage. Outdoor Cleaners are cost effective and easy to apply.

There is no doubt that water blast cleaning is harsh, it uses high pressures up to 3000psi, and 4000L to 6000L of water per roof and the process creates a large amount of debris that must be cleaned up afterwards. Concrete and clay tiles are just not designed to be water blast cleaned so It is not possible to water blast a tile roof without it leaking - be prepared for water inside your ceiling cavity if you choose this cleaning method.

The only time a concrete or clay tile roof should be water blast cleaned is in preparation for repainting where it is essential to get the surface clean and sterile before the paint is applied. For repainting, best practice is to apply Bio-Shield® then waterblast the roof down after 3 to 4 weeks. This recommended process will minimise surface damage, leaks, reduce water usage and give you a clean, sound and ultra sterile surface to apply paint onto

Safety to Consider When Cleaning a Concrete or Clay Tile Roof

In New Zealand, the majority of falls off ladders and roofs occur when homeowners are working on their roofs during the weekend. If you have any doubts or hesitations about your ability to carry out this work safely then do not attempt it, better to contact a professional.

If you do choose to do the work yourself, tie the ladder off at the top, always have 3 points of contact on the ladder and ensure you have a safety rope or harness attached to you to prevent any slips or falls.

Walking on Tile Roofs

Modern concrete tiles are extremely strong and durable so actually very easy to to walk on without damaging them. Modern clay tiles are also strong but can be brittle. The older Winstone or Marseille clay tiles are best left to the professionals to clean.

The strongest place to walk is the middle of the tile, at the bottom where it overlaps on the row of tiles below. The design and inherent strength of this area is supported by both the underlapping tile and tile batten below, making it the strongest portion of the tile.

Avoid the overlapping side edge of the tile, which is where it's most vulnerable to breaking. Avoid walking on dirty and wet tiles, they are slippery and can lead to a fall and injury.

Preparing your Roof For Outdoor Cleaning Solution

Preparation is key for a successful do-it-yourself project. Following these simple instructions will guarantee a professional looking finish and long-term protection from moss, algae and lichen growth that are damaging your roof. Place your ladder on solid ground at the point where the distance between roof and ground is minimal. The base of the ladder should be 1 metre out from the spouting for every four metres in height. For utmost safety, ensure that the top of the ladder extends at least 1 metre above the edge of the roof so when you enter and exit the roof the top of the ladder provides support and stability.

Placing the ladder against the spouting brackets will reduce the possibility of breaking PVC spouting or deforming metal spoutings.

Always have 3 points of contact on the ladder, e.g. two feet and 1 hand. The first trip up the ladder should be used to attach it to the spouting, and this is preferable with a bungee cord or luggage tie to prevent it from sliding. To avoid scratching of the paint, Coloursteel or painted spoutings should have some padding between them and the ladder.

Remove any loose debris or tree leaves from the roof surface. Cutting back overhanging trees will assist in keeping the roof clean. A concrete roof surface can be either dry or damp but clay tiles and Colorsteel must be dry to avoid slipping.

Check your local council rules on discharging wash water into the storm sewer system.

If you're on tank water, disconnect the tank from the roof. It will need to be disconnected for two to four weeks, and at least 40mm of rainfall to clear residue of the exterior cleaning solution. 

Do not reconnect the tank until the rain water runoff is free of frothing or odour.

Application of Exterior Cleaning Solution

Make sure you take the time to read the exact specification, especially the dilution ratio of water to the solution, before beginning the application of your chosen exterior cleaning solution.

While Bio-Shield® has very low toxicity for skin contact we do recommend taking the usual chemical spraying precautions -put on a respirator if you have one, long pants, soft soled shoes, and gloves. If you do not have a respirator, ensure you apply the solution with yourself downwind, as the solution can evaporate into vapour in warm conditions, which should be avoided being breathed in whenever possible.

Saturation of the tiles is the key to getting a good result. Apply the cleaning solution in slow even strokes to ensure saturation of the surface. Pay particular attention to saturating the thicker lichen growths on the ridge line and under the TV aerial, and any thicker moss that grows in the tile laps and on the bottom edge of the tile.

On concrete tiles, one saturation coat of Bio-Shield® outdoor cleaner is enough to effectively kill and provide a faster clean up.

On clay tiles we strongly recommend a 2nd lighter coat, 10 to 15 minutes after the 1st coat. This 2nd coat will ensure a good cleanup of the hard to kill black algae.

You will notice distinct colour changes in the growths over the next few days and weeks as the dead growths start to break down.

Once the dead growths are broken down enough, rain and wind will begin to clean up your roof. Initial clean ups may take anything from 3 to 12 months, depending on the type of roof, how much growth was present and how much rainfall son the roof.


Most outdoor cleaning solutions recommend reapplying the solution at the first sight of regrowth to maintain the roof in a clean condition. This is usually once every 2-3 years. Maintenance applications will be easier, faster and use less diluted solution; any spores or new growths will die off and the roof will clean up quickly, sometimes in as little as a few weeks.