Why are waterpipes run under house concrete slabs?

I am asking this question for a number of reasons, one is that there is a remote chance that of a pipe failure resulting in extensive damage under the slab. More importantly though is the immediate effect of the water pipes being in the ground resulting in huge heat loss on the hot water pipes. I live in Canberra in a three year old house, with external gas HW heater and all water pipes under the slab! This arrangement results in the hot water cooling in the time I take to have a shave. I run the hot water into the sink, with a lot of wasted water waiting for the hot stuff to arrive, fill the sink and have my shave. When I finish my shave I then have to run the water until it gets hot again - this happens even in the heat of summer, just imagine what it is like in sub-zero winter!! I am unable to do anything to insulate the pipes because they are totally inaccessible!
So, to my question: is there a real very legitimate reason for running water pipes under houses? Or is it just that it saves the builder costs, with no consideration of the long term implications of the house owner?
PS: I do not accept that the risk of damage to the house of a burst water pipe is too great if it is run inside the house ceiling. If it did happen, it is evident immediately and any damage repair is straight forward! It is easier to replace some ceiling or wall linings, plus carpets and furniture - it is much more difficult to dig a hole in a concrete slab!

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Firstly I am not a builder but based on my understanding of building construction I think underslab piping is considered to improve energy efficiency as the temperature should fluctuate less surrounding the pipe. This is based on an assumption that the pipe is surrounded by soil or concrete rather than air. Soil and concrete is a much better insulator than air as it is generally >10cm thick.
I would suspect that the problem you describe is due to a long pipe between where it leaves the slab and comes out of the tap. If you had to run all the water out of the pipe between the hot water system and the tap you would be using 10’s of litres everytime.
To really understand the reason why your water is so cold I think you would need to ask a plumber to investigate for you.

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Hi allandorrington, I am a Tradesman myself & most of my friends are from a trade background. I was a Maintenance Manager for a medium size care facility some years ago & I have 1st hand experience with having to rectify this exact problem after leaks developed under a few resident unit slabs.
Short answer from the trade qualified plumber was to dig under the slab to find the leak if it was within about 2 metres
of the outside of the slab. If elsewhere under the slab he would ask me to choose from Jack Hammer up the slab at the leak site or plumb the water through the roof space or outside of the slab underground.
So in answer to your question Yes cost is the reason its done.
The builder wants the cheapest price, his plumber can keep costs down this way by less material cost & more importantly in time to do the job. Usually the crack occurs very close to where the pipe comes through the slab because the ground expands & contracts more the concrete in the slab. This creates a shear point where the pipe comes through the slab.
Hot water pipes should be lagged or insulated to keep as much heat in the pipe for as long as possible weather underground or run in free air.
Hope this is helpful to you.
Regard Pegasus.


Thanks for the insights, @jyrn @Pegasus

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jyrn is correct about the length of pipe being the reason for the length of time that a hot tap delivers cold water before you get any hot water.
That is why well-designed homes have the taps which are used most frequently close to the HWS.
My water pipes run through the ceiling of my slab-on-ground home in Qld and I complain about the length of time it takes to get hot water at a tap that is approx 20 metres from the HWS, but have much shorter time to wait at taps that are only 5 or 8 metres from the HWS.

Having a house that had a leak in the ceiling, not discovered as tenants were away for the weekend resulting in several ceilings falling in, all the carpets ruined and some walls needing repair I can vouch for ceiling pipes being a big issue (but only if they burst - which outs did due to a very cold snap in Ballarat when at least 18 older houses had same issue on the same weekend). We did a lot of the work ourselves but still spent $40,000 in repairs. Only a short time later a friend had a water pipe issue under her slab and had to have it repaired. While this included some drilling into the slab and also one wall, the damage was minimal and her bill was under $2000. So while under the slab may seem odd it is not a potentially more costly issue. However, not having the pipes insulated is an issue and it should be part of building code that all pipes under slabs have to be insulated. In this age of energy use awareness this is a no brainer.


Hi vke57061.
Your experience is very interesting which, as you indicate, is counterintuitive in that the under slab pipe repair was less expensive than a ceiling pipe failure. It sounds like the pipe failure in your house was fairly major? I wonder whether this was a compounding of an old house with weakened pipes freezing plus the time delay of discovering the pipe failure? (Did you get any insurance?)
My concern follows your experience with an old house, what will happen to houses with under slab pipes when their pipes age. At least, theoretically, the pipes under the slab should not freeze?

I have the same problem. It will cost an arm and a leg to fix. Do what I do when I want hot water in the ensuite wash basin. Buy a cheap electric kettle, bring to the boil and dilute with cold water or time it cooking until you get the required degree of doneness for your shave.

Hi deborah1901: Your solution of having a kettle in the bathroom is quite workable, and actually I did that in another house where I had the same problem but for a different reason. My only concern is having an extra electrical appliance next to a sink with water
and boiling water in a confined space could create a hazard.
This is the crux of the point that led me to post this question. We have houses where people are left with a choice of wasting water and energy (both costing money) or putting themselves at risk of injury. It is time for house construction to focus more on the long term issues that effect the owner more that their short term profit margin!

Freezing pipes is a rare event in Australia although expensive to fix, spare a thought for people in Canada and North America as it is common to leave the house heating on when you go on holidays in winter to prevent pipe freeze up and subsequent bursting.
As far as pipes under the slab it has more to do with looks as well as cost, many new housing estates have developer covenants that require no plumbing must be seen on the outside of the building so builders get in the habit of building that way and it becomes the norm.

Hi tndkemp: I agree that having freezing pipes is no a common event in Australia, but living in Canberra with frosts, i.e. sub zero temperatures for about six weeks it does happen. It is even more common in the ‘alpine’ region.
My concern is that having the water heater outside results in a section of pipe in the open air (there are some versions where the pipes are enclosed). This means that when you want hot water first you have to put up with a blast of absolutely freezing cold water. What happens is that this cold water passing through the pipes makes the pipe cold, cooling the hot water as it follows, so, not only is there a delay for the first lot of hot water to arrive, but then there is a delay as the hot water actually reaches a proper temperature.
This is not an issue for the vast majority of Australian houses which are located on the more temperate coastal region - even less so for the northern regions. What happens is that houses are being built to a common standard with no consideration to the weather zone.
I believe that there is a lot of water and energy wasted all around Australia because of the long pipe runs (under houses). Maybe consideration should be given to locating hot water heaters (yes, plural) next to the hot water taps and not a single heater providing water to wide spaced hot water taps??

We had a strange experience staying in a house in Mareeba. Run the hot water until it is hot and you fill the sink. Ten minutes later you run the hot water again and it’s cold. I reckon it was the pipes in the cold cement. The air temperature while we were there was pleasantly warm. We don’t have that problem at home (Coffs Harbour) even when it’s cold because our pipes are in the wooden floor or walls. When I lived in Mansfield Vic. it was not uncommon in Winter for the water pipe in the ceiling feeding the hot water tank to freeze. I have seen systems where the longest hot water pipe actually has a return to the hot water heater. A very small pump keeps the water circulating. I wonder if the economics of such a device is worth it? It means hot water whenever you turn on the hot tap.

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It’s certainly annoying. Perhaps one solution is to use those on-demand heaters that heat the water close to the individual outlets.

But with the preponderance of solar and reverse-cycle water heaters that cost next to nothing to run, it’s really no more than an annoyance.

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The shorter the run from the HWS (hot water service) to the outlet the better. Up into the ceiling, along and then down again is usually longer, so wastes more water. All HW pipes should be lagged (insulated) from HWS to outlet, whether under slab or in the ceiling and walls. Gauge of copper tube can be a factor – thinner is cheaper, but more likely to fail.
We had a leak in a shower pipe behind tiles in a brick internal wall. It wrecked paint and carpet in adjacent rooms and cupboards. We had a hell of a fight with the insurer until I bit the bullet, took a hammer and chisel to the wall and opened it up to show a pin-hole leak in the copper pipe. The photos helped and we settled for a reasonable sum to make good the damage (we had to pay for the the copper tube repair ourselves). Mind you, it took a lot of nerve to start smashing the tiles!

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