The question comes up constantly: should we use DOFF or TORC? It's asked by clients, by architects, by conservation officers writing method statements, and occasionally by contractors who have been using one or the other for years without fully understanding why. The answer is always the same: it depends on the building, and specifically on three things. The stone or brick type. The soiling type. And the condition of the surface.
The mistake is treating the choice as a brand preference. DOFF and TORC are tools, not philosophies. Using the wrong one produces worse results and occasionally causes damage. Understanding how each works makes the decision straightforward.
What DOFF actually does
DOFF is a system of superheated low-pressure steam. The water is heated to around 150°C and delivered to the surface at pressures typically between 15 and 30 bar. The key word is low-pressure: this is much lower than a domestic pressure washer, and the mechanism of action is thermal rather than mechanical. The heat does the work.
At 150°C, biological matter — algae, lichen, moss, the black fungi that colonise stone in damp areas — is killed at the root rather than just removed at the surface. Atmospheric carbon soiling, the sulphate-rich black crust that forms on sheltered Portland stone in polluted urban environments, is loosened and lifted. The steam carries very little water volume, which matters enormously on historic masonry: lime mortar joints, porous stone, and soft brick can all be damaged by prolonged water saturation. DOFF avoids this.
The other critical property is that DOFF is non-abrasive. The steam jet carries no aggregate, no grit, no mechanical cleaning agent. This makes it appropriate for carved stonework — capitals, friezes, finials, figurative sculpture — where any abrasive action would round the arrises and erase tooling marks that are part of the historic character of the building.
What TORC actually does
TORC introduces a fine aggregate into the cleaning stream in a swirling vortex pattern. The aggregate — typically a very fine, soft material like crushed glass or chalk — adds a mechanical element to the cleaning action. Where DOFF relies on heat alone, TORC relies on heat plus controlled abrasion.
This makes TORC significantly more effective on surfaces where the soiling has calcified or carbonated — where the pollution deposit has essentially become part of the surface layer rather than sitting on top of it. This type of soiling is common on Portland stone in areas of historically high atmospheric pollution: the carbon particles bind with calcium carbonate from the stone's own surface to form a crust that is chemically bonded rather than just physically deposited. Steam alone won't shift it easily. TORC's aggregate does.
TORC is also effective on thick biological growth, paint residue (in some cases), and on robust masonry where a faster clean is needed and the surface texture can tolerate the aggregate.
The decision process
When we survey a building for cleaning specification, we're looking at five things in order:
1. Stone type. Soft stone — sandstone, Bath stone, soft limestone, friable or salt-damaged Portland — rules out TORC immediately regardless of soiling. The aggregate will cause surface damage. DOFF only.
2. Surface complexity. Carved ornamental detail — capitals, friezes, figurative work, fine mouldings — rules out TORC for those elements. DOFF only on carved work. TORC may be appropriate on adjacent flat surfaces on the same building.
3. Soiling type. Atmospheric pollution on exposed surfaces: test DOFF first. If it shifts the soiling adequately, DOFF is the specification. If a calcified crust remains after DOFF treatment on a test panel, TORC becomes the appropriate tool for those areas.
4. Listed building status. Conservation officers have a strong preference for DOFF on listed buildings, particularly those with Grade I designation. This isn't a rule — TORC is used on listed buildings when justified — but the method statement needs to evidence that DOFF was tested first and found insufficient before TORC is specified.
5. The test clean. On any building of significance, we carry out a test clean before specifying the full works. A representative area is treated, evaluated after 24 hours once dry, and the results inform the final specification. The test clean often answers the DOFF vs TORC question definitively — you can see what works on the actual stone.
Using both on the same building
The most accurate description of how we work on a complex historic facade is: DOFF on carved elements, TORC on flat work where needed. This is not unusual. A Georgian or Victorian commercial building might have Portland stone panel work with significant carbonation sitting alongside carved capitals, friezes, and ornamental keystones. Specifying TORC across the whole facade would produce faster results on the flat panels and damaged results on the carved work. Specifying DOFF across the whole facade might leave the flat panels inadequately cleaned.
A properly written specification lists each element type and the method proposed for it. This is standard practice for conservation work. It's also what a good conservation officer will expect to see.
What neither method is appropriate for
Both DOFF and TORC are appropriate for atmospheric soiling, biological growth, and general surface contamination. Neither is the correct primary approach for paint removal, for deeply penetrated mineral staining, or for salt efflorescence. These require poultice cleaning, chemical treatment, or specialist consolidation. A thorough survey identifies which soiling types are present and specifies the right approach for each.
High-pressure water cleaning and acid cleaning are not appropriate for period stone in any context. We don't use them. The damage they cause — surface erosion, mortar loss, water saturation, chemical attack on lime-based materials — is significant and often doesn't become fully visible for years after the work. By which point the responsible contractor is long gone.