Ventilation & damp — Sydenham & Forest Hill SE23, SE26
Top-floor flats in Sydenham, and the duct that drips
Hillside Victorian villas, most of them long since split into flats. If you are on the top floor you have a cold roof void directly above your ceiling — and an uninsulated extract duct crossing it is reliably the first thing in the building to start dripping. It looks exactly like a roof leak, it gets reported as one, and it is not one.
Most of the larger Victorian and Edwardian villas along these hills were divided into flats decades ago. Whoever did the conversion had to get extract out of a bathroom that very often sits in the middle of the plan, and on the top floor there is one obvious route: up through the ceiling, across the roof void, and out.
That roof void is not inside the building in any meaningful sense. It is ventilated to the outside on purpose, so through the winter it sits close to outdoor temperature. A duct crossing it is carrying warm, saturated air straight out of a shower room through a space that is near enough freezing.
Warm wet air inside a cold duct condenses on the inside of the duct. The water runs to the lowest point — which on concertina flexible duct is every sag between every fixing — collects there, and eventually finds its way back down. Onto the insulation, through the plasterboard, or straight back into the fan.
How to tell it from a roof leak
This matters because the two get confused constantly, and the wrong diagnosis costs a scaffold. Four things separate them, and you can check all four yourself.
It tracks the duct, not the roof
A leak follows the defect — a slipped tile, a failed flashing, a blocked valley gutter — and lands wherever gravity takes it. Duct condensation appears at the fan, at the ceiling directly under the run, or at a joint. If the wet patch is a neat circle round the extract fan, it is not the roof.
It is worst after showers, not after rain
The single most useful test there is. Keep a note for a fortnight of when it appears. If it follows heavy rain it is the building; if it follows a run of long showers on cold mornings and does nothing on a wet mild day, it is the duct.
It stops in summer
Condensation needs a cold surface. From about April the roof void stops being cold enough and the problem simply goes away, then returns in November. A roof defect does not politely take the summer off.
A roofer has already been and found nothing
Which happens a great deal round here, and it is not the roofer’s fault. They were asked to look at a roof and the roof was fine. If that has happened to you once already, that is a strong result in itself — it is worth looking above the ceiling before anybody puts a scaffold up again.
What fixes it, and the flat-specific bits
- Insulate the run, properly and along its whole length. Every duct crossing a cold void gets a lagged sleeve as standard here rather than as an extra. A run that is insulated for most of its length and bare for the last metre condenses in that last metre.
- Get rid of the sags. Rigid or semi-rigid duct on the shortest route the building allows, supported so it cannot dip. Every dip is a place water stands, and standing water is also a partial blockage — so the fan performs worse and worse through the winter.
- Fall it the right way. Where a small amount of condensation is unavoidable, the run is set to fall back towards the terminal so anything that does form drains outward rather than back into the room.
- Terminate it outside. A surprising number of the ducts we follow across these roof voids simply stop. If yours ends in the void rather than at a terminal, everything above is academic — every shower is going into the roof.
Two things specific to a converted flat, and both are worth knowing before you get a quote. The route usually needs the freeholder’s agreement, because it crosses or terminates on communal fabric, and that is a conversation to start early rather than on the morning of the work. And if the block has a communal extract riser serving several flats, putting a more powerful fan into it changes the balance for everyone else on that stack — so it gets checked rather than assumed.
Lower-ground and semi-basement rooms, which these hillside plots produce a lot of, are a different problem again: part of the wall is below external ground level, so what looks like condensation can be moisture coming through the structure. We measure rather than assume, and if it turns out to be ground water we will say so and tell you what trade you actually need.
Questions we get asked in Sydenham and Forest Hill
Four checks separate them and you can do all four without anyone coming out. Where is it? Duct condensation appears at the fan, directly under the duct run, or at a joint — a neat wet circle around the extract fan is not a roof. When is it? Note it for a fortnight: a leak follows heavy rain, duct condensation follows long showers on cold mornings and does nothing on a mild wet day. Does it stop in summer? Condensation needs a cold surface, so it disappears around April and comes back in November; a roof defect does not take the summer off. And has a roofer already been and found nothing? That happens a great deal here and it is not the roofer's fault — they were asked to look at a roof, and the roof was fine.
Because the roof void is ventilated to the outside on purpose, so through the winter it sits close to outdoor temperature — and your extract duct is carrying warm, saturated air straight out of a shower room across it. Warm wet air inside a cold duct condenses on the inside of the duct. The water runs to the lowest point, which on concertina flexible ducting is every sag between every fixing, collects there, and finds its way back down onto the insulation, through the plasterboard, or into the fan. A lagged sleeve along the whole run stops it. Along most of the run is not enough: a metre left bare is a metre where it still forms.
For anything that crosses or terminates on communal fabric, almost certainly, and it is much better to start that conversation before you get a quote than on the morning of the work. The roof void above a top-floor flat is usually communal, and so is the external wall a terminal lands on. It is rarely a problem in itself — a freeholder generally wants the building dry more than you do — but it is a step with a lead time, and it is the commonest reason a straightforward job stalls. We will put the route and the terminal position in writing so you have something specific to send them rather than a description.
Not without checking, and this is worth taking seriously. A communal riser is a shared system: it was balanced, at least in principle, for the flats connected to it. Putting a noticeably more powerful fan into one flat changes the pressure in that stack for everybody on it — you can pull air out of a neighbour's flat, or push your own extract back into theirs, and the flats at the top and bottom of a stack behave differently again. Where there is a shared riser we look at what is already connected before specifying anything. Sometimes the honest answer is that the riser itself is the problem and it is a building-wide conversation rather than a flat-level one.
Often not, and it matters because the remedies are completely different. These hillside plots produce a lot of lower-ground and semi-basement rooms where part of the wall sits below external ground level. That can be condensation, exactly as upstairs — a cold surface in a room with moisture in the air. It can equally be moisture coming through the structure from the ground outside, and ventilation will not fix that. The tells are worth knowing: condensation is worst in winter, appears on the surface, and eases in summer; ground moisture tends to persist through a warm dry spell and often shows a defined tide line. We take readings rather than guessing, and if it turns out to be ground water we will tell you which trade you actually need.
Usually yes, and in a converted flat it is often the reason a fan gets switched off at the isolator — which is the real problem, because a fan nobody runs ventilates nothing. Most of the noise is not the motor. It is air moving fast through an undersized or restricted duct, a rattling backdraught shutter, or the unit itself hard-mounted to a joist so the structure carries the sound. A slightly larger duct at a lower velocity, a continuously running low-rate fan instead of a loud intermittent one, and mounting that isolates the unit from the structure deal with most of it. A continuous fan running quietly all the time also removes moisture as it is produced, which is what a wet room actually needs.
Not sure where to start?
Send us a message with what you have noticed at home. We will tell you honestly whether you need a survey, a system, or neither.
Or see what it costs — unlike most in this trade, we publish our prices.
Trained on the systems we install
Certificated — and here they are
Most trade sites show you a row of logos. These are the actual certificates, both sat in August 2026. Tap either one to read it, or download the PDF.
- 12-month workmanship guarantee On top of the manufacturer’s warranty, which we register in your name. If anything we installed fails because of the way we installed it, we come back and put it right. What it covers.
- Company no. 12092552 Registered in England & Wales. Trading as Cavalli Ventilation since July 2019, and in ventilation since 2003.
- Airflow measured on every install A calibrated anemometer at the grille, recorded and handed over. The only figure that counts is the one measured in your room.