What is DOFF cleaning?

The basic principle

Most people assume that cleaning stone means either water pressure or chemicals. DOFF works on a different principle entirely. The system generates steam at high temperature and delivers it through a purpose-built lance at very low pressure — typically around 100 bar, which is substantially lower than a domestic pressure washer. The cleaning action comes from the heat, not the force.

At 150°C, the steam desiccates and lifts soiling from the surface rather than blasting it. Biological material — algae, lichen, moss, biofilm — is killed on contact and releases from the substrate. Atmospheric soot and carbon deposits are softened and removed. The stone surface is not touched mechanically, and because the water delivery pressure is low, there is no saturating of the masonry.

What it removes

DOFF is effective against a specific category of soiling. Understanding what it does and does not address helps set accurate expectations before a site visit.

It removes atmospheric soiling reliably: the carbon-based deposits from combustion that give stone its characteristic grey-to-black appearance in urban environments. It removes biological growth — algae, moss, lichen, and biofilm — with high effectiveness, including spores that would otherwise regrow. It removes paint from stonework, though heavily layered or alkyd-based paint may require additional treatment. It cleans terracotta and ceramic finishes gently enough to preserve glaze integrity.

It does not remove sulphate crusts from sheltered surfaces effectively. Sulphate crust — the hard, dense black layer that forms in recesses where rain cannot wash the surface — requires a different approach, typically TORC or a nebulous spray system. DOFF and TORC are often used together on complex facades where both soiling types are present.

Why low pressure matters for historic stone

The damage done to historic stone by high-pressure water cleaning is well documented in conservation literature. Portland stone, London stock brick, and most limestones are porous materials. High-pressure water does three things that compound over time: it erodes the surface layer, driving away material that cannot be replaced; it saturates the masonry, which then expands on freeze-thaw cycling; and it forces existing salts deeper into the stone body, where they crystallise and cause spalling when the stone dries.

DOFF avoids all three mechanisms. The low water volume means the stone is not saturated. The heat rather than pressure does the work. And the method is non-abrasive, so the surface profile of the stone is not disturbed.

This is why DOFF is one of the methods most commonly accepted by conservation officers for listed building work. The risk profile is lower than any water pressure system, and substantially lower than abrasive cleaning.

The trial panel

Before DOFF is applied to any building, a trial panel is required. This is not optional — it is part of correct practice, and on listed buildings it is required by the consent conditions.

The trial starts at a lower temperature than the target specification and works up incrementally in a representative area of approximately one square metre. The panel is then left to dry — typically 48–72 hours — and assessed against the control area. The assessment looks at whether the soiling has been removed to an acceptable level, whether there is any change to the surface texture of the stone, and whether any areas of the stone have responded unexpectedly.

On the basis of the trial, the temperature and dwell time are confirmed for the full works. If the trial reveals that the substrate is more vulnerable than expected — friable surfaces, spalling, previous treatment with a consolidant that changes the thermal response — the specification is adjusted or an alternative method is considered.

When DOFF is the right choice

DOFF is the correct first consideration for most stone and brick cleaning on period buildings. It works well on Portland stone, limestone, most sandstones, London stock brick, terracotta, and ceramic architectural details. It is particularly well suited to buildings where the carved detail is important and abrasive methods would pose unacceptable risk to the surface profile.

It is less suitable, or requires careful calibration, in a small number of situations: very friable stone where the structural integrity of the surface is uncertain; previously treated stone where an old waterproof coating may trap steam and cause delamination; and surfaces with active salt efflorescence, where the cleaning process should be sequenced with pointing repairs and drying-out of the substrate before works.

The decision is always made after a survey, not before one. The substrate condition, the soiling type, the presence of previous treatments, and the listing status of the building all inform the method statement. DOFF is often the right answer — but it is arrived at through assessment, not assumption.

DOFF versus pressure washing: the practical difference

This comparison comes up regularly. The headline difference is water pressure: a standard pressure washer operates at 130–200 bar. DOFF operates at 15–100 bar, and the actual cleaning work is done by temperature, not water force.

Beyond the numbers, the practical difference is in what pressure washing does to the substrate. On soft limestone or soft brick, high-pressure water removes the surface layer of the material itself — there is no sharp boundary between soiling and stone, and the pressure washer does not discriminate. The result is a clean surface that has lost its weathered detail and is more porous than before, meaning it will re-soil faster.

DOFF removes what is sitting on the surface without disturbing the surface itself. After cleaning and drying, the stone looks clean but retains its texture and patina. The worked surface of carved detail is undamaged.

DOFF and other cleaning systems

DOFF is sometimes used alongside TORC, the vortex aggregate cleaning system developed by the same manufacturer. The two methods address different types of soiling and are often combined on complex facades. TORC uses a fine calcium carbonate aggregate delivered at low pressure in a vortex pattern; it is particularly effective on sulphate crusts in sheltered recesses where DOFF cannot reach the soiling effectively. For a detailed comparison of the two systems, see our article on DOFF vs TORC.

Common questions

What temperature does DOFF cleaning use?

The DOFF system operates at up to 150°C. The temperature is adjusted during the trial panel stage to find the right setting for each substrate. Friable or damaged stone may be cleaned at lower temperatures.

Does DOFF cleaning damage stone?

When specified correctly, DOFF does not damage stone. The key factors are temperature calibration during the trial, correct dwell time, and nozzle distance. On very friable stone or unstable surfaces, a lower temperature or alternative method may be more appropriate.

Can DOFF be used on listed buildings?

Yes. DOFF is one of the methods most commonly approved by conservation officers for listed building work. Because it uses steam rather than water pressure or abrasives, the risk profile is low. A method statement and trial panel are still required before full works.

What is the difference between DOFF and pressure washing?

The difference is pressure and temperature. DOFF operates at very low water pressure — the cleaning action comes from superheated steam, not water force. Pressure washing uses high-pressure cold water, which can erode soft stone, saturate the masonry, and drive water into existing cracks.