How to pressure wash a roof

Constantly exposed to weathering and pollution, roofs are home to built-up dirt, moss and algae. Depending on where you live, your roof should be cleaned as much as twice a year – ideally, at the start of spring and the end of autumn. Keeping on top of cleaning your roof should help to prevent moss growth and stains.

Of course you can always call on the help of a professional to remove stubborn dirt from a roof. However, it can be just as efficient, quick and more cost-effective to pressure wash it yourself. Pressure washers harness the force of pressurised water to deep clean all types of materials, including roof coverings!

That said, it's essential to follow a few precautions to ensure your own safety and to prevent damage to the roof. First up, you'll need a pressure washer with a pressure rating of about about 140 to 160 bar. You'll also need a few roof-cleaning accessories and products.

Electric vs. petrol pressure washers

Electric pressure washers are usually ideal for cleaning roofs as they are compact and lightweight. What's more, an electric pressure washer is also less expensive and easier to maintain than a petrol model. There's no need to use a highly powered petrol machine for this type of task. In fact, doing so may even damage your roof covering. Ideally, you'll need a 2000W pressure washer with a pressure rating of 140 to 160 bar. The tool should also be equipped to release around 500 litres of water per hour (or even more for a large roof).

Roof-cleaning pressure washer accessories

Among some of the most useful pressure washer accessories for cleaning a roof, you'll find:

• an extension hose to reach every corner of the roof;

• a telescopic lance stretching up to 5.5 metres; depending on the model you choose, these lances should be able to spray up to about 15 metres in height;

• a rotary nozzle to cover a large area with speed;

• a gutter cleaning attachment to remove dead leaves from guttering and downpipes;

• a lance extension to increase the overall length of the lance for greater user comfort;

• an extension cable to avoid having to move the pressure washer around too much when you're working at a height;

• a foam cannon to dilute and spread your cleaning product as evenly as possible.

Roof cleaning products

You can choose from a few different cleaning products designed especially for roofs.

 Roof cleaner

Using a specially designed roof cleaner will allow you to wash your roof covering without removing any of its waterproofing properties (which is not the case for ordinary detergent). Added to your pressure washer tank or to a foam cannon, this type of cleaning product will effectively remove all dirt and stains.

Moss control 

Applied with a pressure washer, moss control can help to clean off and eliminate moss, lichens and algae from your roof. These types of products range in price to match your needs and budget. Some need to be rinsed while others do not.

Waterproofing roof treatment

If the moss control product you have chosen does not contain a waterproofing treatment, it is advisable to apply a protective product after cleaning to protect your roof from weathering and help to increase its lifespan.

Pressure washers cannot be used on every type of roof covering; terracotta or slate tiles, for example, are particularly fragile.

You also need to take care of your own safety. A wet roof is a slippery roof so be sure to wear non-slip shoes if you plan to climb on the roof itself. Similarly, always be sure to wear a safety harness with the correct type of fall protection (including a lanyard and lifeline) as well as safety goggles, gloves and a mask to protect your respiratory system. Before attempting to clean your roof, take the time to properly plan out several anchor points to reduce the risk of falling. You may also want to install a horizontal lifeline to help you move around safely.

Opt for a ladder specially designed for roofs to gain access and install a roof ladder on the roof itself if required. It goes without saying that you should always be accompanied when working at a height.

Only attempt to use your pressure washer when conditions are dry and wind-free. Ideally, you want this type of weather to last for a few days after you finish cleaning. Be sure to adapt the pressure setting of your pressure washer to match your roof covering.

1. Start by wetting the roof before spraying on your cleaning product with the pressure washer. Leave it to work for a few minutes then rinse.

2. Now it is time to apply your moss control. If the roof is covered in thick moss, it's best to brush off as much as you can before applying your product. Work from top to bottom using your pressure washer. Check your product to see if it has to be rinsed off or not.

3. Spray on your waterproofing treatment (unless your moss control has waterproofing properties).

4. Finish by using your gutter attachment to remove all debris from your gutters.

Once you're done, take the time to clean your accessories and store your pressure washer properly.