Facade cleaning without pressure washing: which methods are there?
Team OPPY4 min read
Facade cleaning without pressure washing can be done in several ways: letting a product work and rinsing gently, low-pressure washing, long misting, steam or hot water, a fine abrasive at low pressure, or a poultice. The right method depends on the surface and the soiling, and a trial panel settles it.
A pressure washer removes dirt quickly, but the jet that loosens the dirt can also wear away the surface of the stone and the mortar joints. Below you will find why, which gentler methods exist and what each one suits.
Why can high pressure damage a facade?
Because a high-pressure water jet acts as an abrasive. Hard materials cope with it, but soft stone, some brick and weak joints lose material.
The Cultural Heritage Agency of the Netherlands (RCE) writes that high pressure is mainly meant for hard, dense materials such as granite and brickwork of hard bricks with hard joints, and that it should be used with care because the surface is easily damaged (RCE). The US National Park Service is equally clear: water at too high a pressure is very abrasive and can etch marble, other soft stones and some types of brick. If grains of stone or sand show up in the run-off on the ground, the pressure is probably too high (NPS).
A second risk applies to any cleaning with water: if the joints are unsound or the facade is not watertight, water can seep through the wall and damage interior plaster and metal anchors (NPS).
Which methods allow facade cleaning without pressure washing?
There are six common methods, each suited to a different kind of soiling. Most use water, and a few are meant only for local stains.
Letting a product work and rinsing gently
A cleaning product is sprayed on at very low pressure or brushed on, left to dwell, then rinsed off with water at low or moderate pressure. Dilution and dwell time are best set by testing (NPS). How aggressive these products are varies widely: soap-like cleaners are mild, inorganic acids are aggressive and risky (RCE). This principle is also called soft washing; read more in what is soft washing. Chlorine-based products are covered in bleach on facades. A product that kills algae or moss is a biocide and falls under the EU Biocidal Products Regulation 528/2012 (Ctgb).
Washing with water at low pressure
According to the NPS, this is probably the most common way to clean historic masonry. You start at very low pressure, even with a garden hose, and raise it only as needed. Soft brushes help on stubborn dirt, while metal brushes scratch the surface. A non-ionic detergent helps with oily soiling (NPS). The US General Services Administration names water and soft bristle brushes as the gentlest method for masonry (GSA).
Misting
Misting wets the facade for a long time without pressure, so the soiling slowly dissolves and runs off. It is mainly used against black gypsum crusts (RCE). The NPS calls it particularly effective on limestone and marble, and very gentle but slow: it can take several days to a week (NPS). The drawback is the amount of water. Wetted for too long, the facade becomes saturated, with a risk of frost damage in winter, more biological growth, and damp or salt damage inside (RCE).
Steam or hot water
Steam cleaning is really low-pressure hot water washing, because the steam condenses almost as soon as it leaves the nozzle. The NPS describes it as gentle and effective, especially for acid-sensitive stone, built-up soiling and dried plant remains such as ivy tendrils (NPS). The RCE describes saturated steam at 100 to 150 °C as relatively safe, because the surface and any patina are usually left intact, and as best suited to dense natural stone and low-porosity materials such as unpainted concrete (RCE).
Gentle cleaning with a fine abrasive
For historic facades, where blasting is needed at all, the RCE recommends low pressure of a few bar at most and a fine-grained abrasive such as olivine sand or limestone powder (RCE). The NPS points out that even these fine methods clean by removing a small part of the surface. They therefore belong on limited, hard-to-clean areas under the supervision of a conservator, and never on a whole building (NPS).
Pastes and poultices
A poultice is a paste of an absorbent material, such as kaolin or another clay, mixed with a liquid. As it dries, it draws the stain out of the stone. It suits stains and graffiti that have soaked into the stone, and also salts (NPS). The RCE mentions chelating pastes for stubborn local deposits, followed by steam or warm water (RCE).
How do you choose the right method for your facade?
By identifying the surface and the soiling, then making trial panels, starting with the gentlest method. The RCE, the NPS and the GSA all recommend this test before a choice is made.
The Belgian building research centre Buildwise (formerly BBRI) links cleaning techniques to facade materials and types of soiling in its technical guidance on facade cleaning, from natural stone and brick to concrete and coatings (Buildwise). A trial panel goes in an inconspicuous spot, ideally both on a heavily soiled area and on a less soiled one (RCE).
Six ways to clean a facade without pressure washing
| How it works | Suited to | Limits | |
|---|---|---|---|
| Product left to work, gentle rinse | A product loosens the dirt, low-pressure water rinses it off | Organic dirt and growth on large areas | The product must suit the surface; aggressive products risk damage and run marks |
| Low-pressure washing | Water, possibly with a soft brush or a non-ionic detergent | Loose dirt on most masonry | Less suited to badly deteriorated masonry or gypsum |
| Misting | Long wetting without pressure, the dirt slowly dissolves | Black crusts on limestone and marble | Slow, a lot of water in the wall, risk of frost and damp damage |
| Steam or hot water | Hot water at low pressure | Built-up dirt, grease, plant remains, dense stone | Needs specific equipment; joints and facade must be sound |
| Fine abrasive | Fine abrasive at low pressure | Limited, hard-to-clean areas | Removes a small part of the surface; not for a whole facade |
| Poultice | A paste draws the stain out of the stone | Local stains, graffiti, salts | Local use only; needs a thorough rinse |
The NPS therefore advises against water cleaning in the days before the first expected frost (NPS).
How we do it at OPPY
We use both softwash and pressure washing up to 200 bar, and choose the most efficient approach for each surface and type of soiling. With softwash, a product suited to the surface loosens the dirt, and we then rinse with purified water at low pressure. We choose the product and its dilution for each surface, to put as little strain as possible on joints, render and paint finishes. We use enzymatic and chemical products; for sandwich panels and metal cladding we have specialised products. We only claim an effect against algae or moss for a product authorised for it.
We clean with technology controlled from the ground, such as drones and robotics. We assess every facade first, and if we spot failing joints or cracked render, we tell you before we start. Thick crusts or graffiti sometimes call for a different technique, and we will say so.
How a clean runs is set out on Facade cleaning.
Sources
- Gevelreiniging (Facade cleaning), Rijksdienst voor het Cultureel Erfgoed (RCE, Cultural Heritage Agency of the Netherlands), Kennisbank
- Preservation Brief 1: Assessing Cleaning and Water-Repellent Treatments for Historic Masonry Buildings, U.S. National Park Service, Technical Preservation Services
- Cleaning historic masonry, U.S. General Services Administration (GSA)
- Gevelreiniging (TV 197), Buildwise (formerly BBRI)
- De Europese Biocideverordening (The European Biocidal Products Regulation), Ctgb (Dutch Board for the Authorisation of Plant Protection Products and Biocides)
Frequently asked questions
Can you clean a facade with a pressure washer?
On hard, dense materials such as granite, or hard brick with hard mortar joints, it can be done with care. On soft stone, some types of brick and weak joints, too much pressure can wear away the surface. Heritage bodies advise starting at the lowest pressure and raising it only when needed.
Is steam better than pressure washing for a facade?
Steam cleaning is in practice hot water at low pressure. Heritage bodies describe it as gentle and relatively safe, especially for dense natural stone and low-porosity materials. Whether it is the best choice for your facade depends on the surface and the soiling, and a trial panel will show it.
What is the difference between soft washing and pressure washing?
With pressure washing, the force of the jet loosens the dirt. With soft washing, a product does that work: it is left to act, then rinsed off at low pressure. Our article on soft washing explains the method in more detail.
Can you clean a facade with water in winter?
Better not when frost is forecast. Water that freezes inside masonry can cause spalling and cracking, and a wall can take more than a week to dry after cleaning.
About the author
Team OPPY
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