Damp is the single most misdiagnosed problem in Central London property. Not because it is difficult to understand, but because there is an industry built around one answer to a question that usually has a different one.
The pattern is familiar. Black mould appears in the corner of a bedroom in a Bayswater conversion or a Maida Vale mansion flat. Someone says "rising damp". A survey is arranged, at no cost, by a firm that installs damp-proofing. The survey finds rising damp. A chemical damp-proof course is injected, the plaster is hacked off and replaced with a waterproof render, and a guarantee is issued. Eighteen months later the mould is back, in the same place, and the guarantee does not cover it — because the problem was never rising damp, and the work never addressed the cause.
Getting the diagnosis right first is not a technicality. It is the whole job.
Three different problems that look similar
Almost all damp in a flat is one of three things, and they behave differently.
Condensation
Warm air holds moisture. When that air touches a surface cold enough, the moisture comes out of the air onto the surface. That is condensation, and it is a physics problem, not a building defect. It shows up on the coldest surfaces in the flat — window reveals, the external corners of rooms, behind a wardrobe pushed against an outside wall, on a solid wall that has no insulation behind it, in a bathroom with no working extract. Mould follows because mould grows where surfaces are persistently wet.
The signature is important: condensation is surface wet. The wall behind is usually dry. It is worse in winter, worse in the coldest parts of the room, worse behind furniture, worse in rooms that are heated intermittently, and worse where ventilation has been reduced.
Penetrating damp
Water coming in from outside through the fabric — a failed roof, a blocked or split gutter, defective pointing, a cracked render or stucco, a failed window seal, a leaking pipe in a wall or void, or a stack that is passing water. This is a defect and it has a source. The signature is a defined patch that is wet through rather than wet on the surface, often worse after rain (if the source is external) or constant and unrelated to weather (if the source is a pipe).
Rising damp
Moisture drawn up from the ground through the base of a wall by capillary action. It is a real phenomenon. It is also, in a flat above ground level, essentially impossible — there is no ground in contact with your wall. A first-floor bedroom in a mansion block cannot have rising damp. A ground-floor or lower-ground flat can, in principle, though even there the more common causes are a raised external ground level, a bridged or blocked existing damp-proof course, a blocked airbrick, or a leak.
Why flats are nearly always condensation
Central London flat stock is close to a perfect condensation machine, and the reasons are structural rather than behavioural.
- Solid walls. Victorian and Edwardian brick, and stucco-fronted terraces, have no cavity and usually no insulation. The internal face of an external wall gets genuinely cold in winter, and cold surfaces are where condensation forms.
- Ventilation has been removed over a century. Chimneys blocked up. Original airbricks covered. Draughty sashes replaced or draught-proofed. Every one of those was a sensible improvement, and cumulatively they sealed a building that was designed to leak air.
- The flats are full of moisture. Showers, cooking, drying clothes indoors — in a flat with no outside space, laundry dries in the living room, and all of that water goes into the air.
- Intermittent heating. A flat heated only in the evening spends most of the day with cold surfaces. Continuous low heat produces less condensation than short bursts of high heat, which is counter-intuitive and true.
- Bathrooms with fans that do nothing. Extract fans in period flats are frequently disconnected, ducted into a void rather than outside, blocked, or too weak for the duct run they're on. A fan that makes noise is not a fan that moves air. This is common enough that we have written about bathroom ventilation and condensation separately.
Put those together and you get warm wet air, cold surfaces, and nowhere for the moisture to go. The mould in the corner is the entirely predictable result.
Why the wrong diagnosis is so expensive
An injected chemical damp-proof course into a first-floor wall does nothing whatsoever, because there was never any ground moisture rising into it. But the associated work does real harm. Hacking off original lime plaster and replacing it with a dense waterproof sand-and-cement render makes the wall surface colder and less able to absorb and release moisture — which means condensation gets worse, not better, and now appears above the treated band. In a period building it also removes breathable original fabric, and if the building is listed, unauthorised work of that kind carries its own consequences.
Meanwhile the actual cause — no ventilation, a cold uninsulated wall, a broken fan, a leaking gutter — is untouched. So the mould returns, the guarantee is unenforceable because the failure is "condensation", and the money is gone.
The test for a damp diagnosis is simple: does it explain everything you can see? Why that wall and not the identical one next to it. Why worse in winter. Why worse behind the wardrobe. Why the wall behind the plaster is dry. If the diagnosis does not explain the pattern, it is not the diagnosis.
The other flat problem: it may not be yours
This is the point that changes who pays, and it comes up constantly in Central London.
Damp inside a flat very often originates outside the flat's demise. A roof or parapet above a top-floor flat. A failed communal soil or rainwater stack in a riser. A leaking pipe in the flat above. Defective stucco, pointing, render or windows forming part of the external fabric. Gutters and downpipes serving the whole building. In most leasehold arrangements those elements are the freeholder's responsibility to repair and maintain, funded through the service charge — but the boundary between demise and common parts is set by your specific lease and varies between buildings. It has to be read, not assumed.
So the sequence matters. Before you commission internal work, establish where the water is coming from. If it is coming from outside your demise, internal remedial work is money you should not be spending, and the correct first step is a written report to your managing agent. If you are a leaseholder and the building's response is a large communal repair, that may bring Section 20 consultation into play. A porter, incidentally, is often the fastest route to knowing whether the same corner of the building has done this before — our guide to porter access covers why that conversation is worth having.
Landlords and rented flats
If the flat is let, damp and mould are not just a maintenance question. There is a legal framework governing hazards and condition in rented homes, and the requirements and expectations in this area have tightened. Treating mould in a tenanted property as a lifestyle issue to be raised with the tenant is not a safe position. Do not take the detail of your obligations from an article, including this one — take current advice on where they sit for your tenure and circumstances.
The practical position is straightforward: investigate promptly, record what you find and what you do, fix the cause rather than repainting the symptom, and give the tenant the means to ventilate and heat the flat without it costing them a fortune. A tenant with a working extract fan, openable windows and affordable heat is a tenant with less mould. Our landlord maintenance checklist covers the visits that catch this early.
What actually fixes condensation
- Extract that works, at source. A properly specified fan in the bathroom and kitchen, ducted to outside on a short run, with enough capacity for the duct it is on. Bathrooms engage Part F for ventilation, and getting this right is a real intervention, not a token one.
- Background ventilation. A means for air to move through the flat continuously, not just when a window is opened.
- Warmer surfaces. More consistent heating, and where it is achievable and consented, insulation of the cold wall. In period and listed stock this is constrained and needs care — breathable build-ups, not sealed ones.
- Remove the moisture at source. Somewhere to dry clothes that isn't the bedroom. Lids on pans. Doors closed while showering.
- Then treat the mould — last, once the cause has gone, or you will be doing it again next winter.
How we approach it
We diagnose before we quote, because on damp the diagnosis is the value. That means establishing the pattern, checking whether the fabric or the ventilation is at fault, testing whether extract actually works, and being clear with you when the source appears to sit outside your demise and is therefore not your bill to pay.
Where the fix is ventilation or electrical work, or tracing a leak through plumbing, our work starts from £80, with maintenance on the same basis and bathroom refurbishment from £2,800. You get a fixed quote before anything begins. Send us a photograph of the affected area and where it sits in the flat, and we will tell you what we think it is.