Why late September is the right time
The work below takes an afternoon and costs very little. The reason to do it now rather than in December is that the materials are in stock, a plumber can get to you inside a week, and you can still get into a loft without doing it by torchlight in a cold snap. Every one of these jobs becomes harder, slower and more expensive once the weather has already turned.
What actually freezes first in London housing
London's risk profile is not the same as a detached house in the Pennines, and the pipes that fail here fail for local reasons.
- Loft pipework lying on top of the insulation. This is the single most common one, and it is a side-effect of doing the right thing. When a loft is upgraded to modern insulation depth, the insulation goes over the joists — and, very often, over the pipes and under the tank as well. The insulation now works, which means the loft above it is close to outside temperature, and the pipes that used to be warmed by heat leaking up through the ceiling no longer are. A well-insulated loft is a colder loft.
- The cold water storage tank and its feeds in a Victorian or Edwardian conversion, where the tank sits in a shared roof space nobody has been into for years.
- Outside taps and the short run of pipe feeding them. Predictable, easy to prevent, and still one of the most common calls. There is a separate post in this cluster on draining an outside tap properly, because the reason it splits is not obvious.
- Unheated garages, coal cellars, vaults, external utility rooms and the pipe run across the back of a rear extension. Anywhere the pipe is inside the building envelope on a drawing but outside it in reality.
- The boiler condensate pipe, which is not a water pipe and not a leak, but which will stop your heating on the coldest morning of the year. If yours does it every winter the pipe is wrong, not the weather — see why your condensate pipe keeps freezing.
- The private supply pipe under a shallow front path or driveway, particularly where a path has been relaid over it at a depth nobody recorded.
Two of those are specifically London problems. Conversions put pipework in odd places — through a cold hallway void, across the top of a bay, up the outside of a rear closet wing and back in — because there was nowhere else to run it when the house was split into flats. And a flat left empty over the holidays in a converted terrace takes the pipes of the flat below down with it, because the heat that used to come up through the floor has stopped.
There is no single temperature at which a pipe freezes
You will find a specific outdoor figure repeated confidently all over the internet. We are not going to add to it, because the number that matters is not the air temperature outside; it is the temperature at the pipe, and that depends on how long the cold lasts, whether there is wind moving over the pipe, whether the pipe is drawing any warmth from the building, and whether water is moving through it at all. A still, dry loft at -1°C for three days will freeze pipes that a windy night at -5°C would not.
What that means in practice: do not wait for a forecast number before acting. The Met Office's own winter guidance is not built around a threshold either — it is built around insulating the vulnerable runs, keeping some heat in the building, and knowing where your stop tap is.
The six jobs, in the order they are worth doing
- Find your stop tap and turn it. Not just locate it — turn it off, turn it on again, and do it gently. A stop tap that has not moved since the last century will not move when you need it, and forcing it snaps the spindle, which turns a leak into an unstoppable one. The Met Office suggests checking it works every six months. If yours is seized, get it replaced now, not in January. If you do not know where yours is, find it before the clocks go back rather than after.
- Lag every pipe outside the heated envelope. Loft, garage, cellar, external runs, and the tails to the outside tap. Thames Water's winter guidance puts foam lagging at around £1 a metre, which makes the whole job of a typical house a small amount of money. It is worth doing properly rather than quickly: the gaps are where it fails, and most lagging jobs leave the gaps.
- Drain and isolate the outside tap and disconnect the hose.
- Deal with the loft properly. Insulation goes under the cold water tank, never over it, so heat from the house below can keep the tank above freezing. Lag the tank sides and lid, lag the feeds and the overflow, and check that nobody has tucked the pipework under the quilt where it sits in a cold pocket with no heat reaching it.
- Get the boiler serviced and look at the condensate pipe while the engineer is there. The Met Office lists a boiler service among its winter preparations for the same reason we do: the failure you want is the one that happens in October with an engineer available, not the one on 27 December.
- Set the heating, and set it low rather than off. See below, because this is the one people get wrong.
The heating setting, and the numbers that are actually published
The instinct when leaving a property for a week is to switch the heating off. It is the wrong move, and the published guidance is consistent about it.
| Source | What it says |
|---|
| WaterSafe, winter advice | Keep your heating on at least 14°C overnight and when you are away from home, to stop pipes freezing. |
| Baxi, on preventing a frozen condensate pipe | Maintain a steady temperature inside your home, above 16°C at all times, especially at night. |
| Met Office, frozen or burst pipes | If you are going away, leave your heating on low. Get a neighbour to check inside your home occasionally. |
Those are the only heating figures we will quote, because they are the only ones published by bodies with a reason to know. A thermostat left at 14-16°C for a fortnight costs a fraction of the excess on an escape of water claim, and it keeps the whole building fabric above freezing rather than just the room the thermostat is in.
One caveat that matters more than the number: a thermostat in a warm hallway does not tell you what is happening in the loft. If your vulnerable pipework is in a roof space, the background heat has to be enough to keep the ceiling below it warm, and the insulation above it has to be arranged so that heat reaches the pipes. That is a layout question, not a thermostat question.
Going away for longer than a couple of weeks
This is where the advice changes, because it stops being a plumbing question and becomes an insurance one. Most home policies restrict cover once a property has been unoccupied for a set period, and many impose specific conditions during the winter months, including draining the system. Before you set a holiday thermostat and leave, read the unoccupancy section of your own policy — and read what insurers require of an empty property over Christmas, which sets out what the Financial Ombudsman says about how those conditions are applied.
What is not worth doing
- Leaving taps dripping. Common American advice, and it is aimed at a different housing stock. In a metered London flat you are paying for it, and a slow drip through a waste that is itself in a cold void has caused its own problems.
- Lagging a pipe you have not checked first. Foam over an already-weeping joint hides it and holds the water against the pipe. Dry the run, look at it, then insulate.
- Heat tape or trace heating everywhere. It has a place, mainly on a condensate pipe or a single unavoidable external run, but it is not a substitute for insulation and it needs a power supply that stays on.
- Calling us. Genuinely: none of this is a leak detection job. Everything above is either a competent afternoon's work or an hour of an ordinary plumber's time.
When it is worth calling somebody
Three situations are worth money before the cold arrives rather than after. A stop tap that will not turn. A pipe run that is in the wrong place and needs re-routing out of a cold void, where doing the work in October is a fraction of the disruption of doing it after a split. And a system that already loses pressure or has an unexplained damp patch, because a leak you have been living with all summer becomes a much larger problem the first time the water in it freezes.
If something has already gone, the emergency sequence is here, and thawing without splitting the pipe is here. Our work is charged at £150 per hour, agreed before we attend, and if we are looking for a hidden leak and do not find it you do not pay for the time we spent looking. The total depends on the hours, so nobody can give you a final figure before seeing the job.
