The Screw Through Central Heating Pipe That Sank a Ceiling in Prenton
- 1 hour ago
- 10 min read
There is a screw in a floor joist in Prenton, on the Wirral, that is going to cost somebody tens of thousands of pounds.
It is about 50mm long. It went in through a brand-new herringbone floor, through the board beneath, and into a central heating pipe that should never have been anywhere near it. The floor layer who drove it in did nothing wrong, because he had no way of knowing the pipe was there.
We were not the heating engineers on that job. We are the ones putting it right.
What we found when the ceiling came down
The house is a Grade II listed property in Prenton, one of a stone-built pair from around 1840 under a Welsh slate roof. The owner had moved in and was living downstairs while her belongings sat upstairs, and that is a sensible way to run a big renovation: finish the ground floor, move into it, then work upwards. She had two separate heating systems going in, one for each floor, and the downstairs work went to another firm.
We cut a square out of the ceiling below, roughly the size of a tea tray, through plaster on timber lath with the old metal mesh still in it. The story was there in about four seconds. Original joists, a run of pipework threaded through them, and the pipe sitting proud of the top of the joist instead of sunk into it.
The room it happened in has been decorated since. Sage green walls, white picture rail, a stack of skirting offcuts propped in the corner where the joiner left them.
That is the whole failure in one sentence: the pipe was higher than the timber it was supposed to be buried in.
Everything after that was inevitable. The floorboards went down on top of a pipe instead of a joist, so the boards bore on the pipe. Then the herringbone went down over the boards, and the herringbone was was fixed with a lot of short screws in a lot of places, and eventually one of them found what was underneath.
Why was the pipe sitting proud
When you run a heating pipe across joists instead of along them, you have to cut into the top of each joist so the pipe drops below the surface. That cut is a notch. Get the notch wrong, and you either weaken the floor or leave the pipe exposed.
There are numbers for this, and they are not vague.
Local authority building control guidance sets the maximum notch depth at 0.125 times the depth of the joist depth, capped at 35mm no matter how deep the joist is. Notches go in the top edge only, never on the bottom or the sides. They are permitted between 0.07 and 0.25 of the span, measured from each support, which puts them near the ends of the joist and well away from the middle, where the bending stress is highest.
NHBC Standards put it slightly differently for joists up to 250mm deep, allowing a notch of up to 0.15 times the joist depth in the top edge between 0.1 and 0.2 of the span. Different number, same principle: shallow, near the supports, top edge.
Drilled holes have their own set of rules. Maximum 0.25 times the joist depth or 65mm, on the centreline, between 0.25 and 0.40 of the span, at least three diameters apart, and never within 100mm of a notch.
Here is the part that catches people out. Those figures are a ceiling, not a target. A 22mm pipe needs around 25mm of clearance once you allow for the clip, and on a 200mm joist your maximum permitted notch is 25mm, so you have no margin at all. On anything shallower, you cannot legally notch deep enough for a 22mm pipe, and the honest answer is that the pipe has to go somewhere else.
Somewhere else means running the pipe parallel with the joists in the void, or dropping it into the ceiling below, or accepting a boxed-in run. All of those take longer. Notching every joist and pressing on is quicker.
That is the decision that got made here, and then it got made badly, because even the shallow notch that was cut did not swallow the pipe.
The bit nobody protected
Even a correctly notched pipe is vulnerable because a notch, by definition, brings the pipe close to the surface. The fix is a steel protection plate over the notch, so anything driven down from above hits hardened steel and stops. They cost about a pound each.
There were none on this floor. Had there been, the floor layer's screw would have skidded off a plate, and the owner would still be living in a finished house.
We fit them as standard on every plumbing and pipework job. It is not clever and it is not a selling point. It is the sort of thing that only shows up in your work fifteen years later, when somebody else opens the floor and finds nothing wrong.
Was the plastic pipe the problem?
No, and we should be straight about this, because there is a post on this site that says otherwise.
We have written before in favour of copper over push-fit plastic, and part of that still holds. Copper takes a knock better, and where pipework will be on show or buried somewhere we would struggle to reach again, we specify it. That post also said we had ripped out plastic pipe because of flooding, which left the impression that the material was the problem. Having opened many floors since, that is not a fair account of what we actually find.
Barrier-type plastic pipe is a compliant, well-proven material used in a large share of UK heating installations, and it did not cause this failure. A copper pipe notched into a joist at the same depth, with no protection plate over it, would have been punctured by the same screw through the central heating pipe. Copper is thinner-walled than people assume, and a floorboard screw goes straight through it. Blaming the material lets the actual mistake off the hook. The mistake was the notch and the missing plate. Somebody in Merseyside will fit copper badly next week, and it will fail in exactly the same way.
We would rather say that plainly than take an easy shot at another firm's choice of material.
For the record, the replacement pipework here is copper throughout, end-feed soldered, with two bundles of 22mm and three of 15mm, with twenty isolation valves, so any part of it can be dropped without draining the lot. That is a decision about this house and this floor. It is not a verdict on plastic.
What does it cost to put right?
This is where a cheap heating installation stops being cheap. Putting this house back means lifting the new herringbone floor, and engineered herringbone cannot simply be prised up and re-laid, because it is glued and clipped, and some of it will be damaged coming out. Then the boards come up, the pipework is redone properly, notched and plated or rerouted entirely, and it is boarded again. Then the herringbone is re-laid, and the ceiling below is pulled down, re-plastered and redecorated. The owner also has to move out of the part of the house she had just finished, and her belongings have to come down from upstairs so the upstairs can be worked on. We are handling that move as part of the job.
Two insurers are now involved, the builder's and the heating engineer's. That is worth sitting with for a second, because the saving on the original quote has been wiped out several times over, and it has been paid for by professional indemnity cover instead of by anybody's goodwill. We would not have been the cheapest quote on that job.
What are we doing on this one?
She has now handed us the whole project: the heating, the reinstatement, the bathroom, the kitchen, the lot.
That is a good position for us and a terrible one for her, because she is paying for the same work twice in every sense that matters, even with insurers covering the remedial cost. The disruption, the weeks out of her own house, and the fact that a finished floor is being torn up are not things a cheque puts right.
The heating work itself is a straightforward job done properly: pipework routed to avoid notching where we can, correctly sized notches and protection plates where we cannot, and a system designed to heat the rooms instead of being quick to install. If you want the details on how we size radiators for a house like this, our piece on BTU and heat output covers the arithmetic.
Listed buildings bring their own constraints, because the aim is to disturb as little original fabric as possible, and we have written before about working in a Grade II listed Liverpool home where the routing mattered more than the kit.
How to check your own floor
If you have had heating work done recently and a floor covering laid over it, there are a few things worth knowing.
Lift a board at the edge of a room, near a wall, and look at how the pipe crosses the joists. The pipe should sit below the top of the timber, not level with it and not above it.
Look for steel plates over the notches. If the pipe crosses a joist with nothing over it, that is a future leak waiting for a reason.
If the floor bounces or creaks in a line instead of in a spot, the boards may be bearing on pipework.
If you already have a hard floor down and you would rather not lift it, an in-depth plumbing survey with a thermal camera and a pipe detector will map the runs without pulling anything up.
And if you can hear water and cannot find it, do not wait. The pipe in this house had been screwed for some time before anyone worked out why the ceiling was staining, and that is the normal way this goes. A small penetration weeps instead of pouring, and it soaks a ceiling from above for weeks before it shows.
Buying heating work on price
Nobody sets out to buy a bad installation. What happens is more ordinary than that.
You get three quotes for the same list of parts, one of them is meaningfully cheaper, and nothing on the page tells you the cheap one has priced fourteen hours of first fix where the others priced twenty-six. The cheap one is going to notch every joist and get out. The others are going to route the pipework properly and protect it.
The parts list is identical, so the labour is where the job is won or lost, and the labour stays invisible until somebody drives a screw through it.
The only real defence is to ask what happens under the floor. Ask how the pipework crosses the joists. Ask whether protection plates are included. Any competent installer will answer both without hesitating, and the answers will tell you more than the price does.
While you are asking, ask about the controls too. Boiler Plus has been in England since April 2018, and a combi replacement needs time and temperature control plus one of weather compensation, load compensation, flue gas heat recovery, or smart controls with automation and optimisation. A quote that does not mention any of them is a quote from somebody who has not read the standard.
Frequently asked questions
Can you notch a joist for a central heating pipe?
Yes, within limits. Building control guidance permits notches in the top edge of a joist only, to a maximum of 0.125 times the joist depth or 35mm, whichever is smaller, and only between 0.07 and 0.25 of the span from each support. Outside those limits, you are weakening the floor.
What happens if a screw goes through a central heating pipe?
You get a slow weep instead of a burst because the screw partly plugs the hole it makes. That is why the damage is extensive by the time anyone notices. Water tracks along the joist, soaks into plasterboard or lath and plaster, and shows up in a ceiling some distance from the actual puncture.
Is plastic pipe acceptable for central heating in the UK?
Yes. Barrier-type plastic pipe is a compliant material and is widely used. It needs closer clip spacing than copper, and it should not be used where it will be exposed to UV or to high surface temperatures. Correctly installed and protected, it performs. The failures blamed on plastic pipe are nearly always failures of installation.
Should heating pipes have protective plates over them?
Where a pipe is notched into a joist beneath a floor covering, yes. It costs very little, and it is the only thing standing between the pipe and every nail and screw that will ever go into that floor.
Who pays when a heating installation damages other trades' work?
The installer's public liability or professional indemnity insurance, and often the main contractor's alongside it, where the sequencing of trades contributed. It is not a quick process, and it does not compensate the homeowner for time or disruption.
Do you take on remedial work from other installers?
We do, regularly, alongside boiler repairs and power flushing on systems that have been neglected. We will survey the existing installation, tell you honestly what can be salvaged and what cannot, and put it in writing so it stands up if there is an insurance claim.
How do I find out what is under my floor without lifting it?
A plumbing survey using a pipe-and-cable detector and a thermal camera will map most of the runs. It is not perfect over insulated floors, but it will tell you where the pipework crosses joists and whether anything is running where it should not be.
Related reading
Talk to us before the boards go down
DD Wilson Gas and Heating Engineers have been installing and repairing heating systems across Liverpool, Wirral and the North West since 1998. Family-run, Gas Safe registered, and with 2,498+ reviews across TrustATrader, Google and other platforms, including a 4.96 average on TrustATrader.
If you are planning heating work, or you have had heating work done and something about the floor is bothering you, we would rather look at it now than after the flooring goes down.
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Sources: Local Authority Building Control technical guidance on notching of floor joists; NHBC Standards, internal services sitework.
