Subsidence in Ireland: What it is and what to do about it

Subsidence occurs in most ageing houses and will need some level of treatment, but don't let that deter you writes building expert Kieran McCarthy
Subsidence in Ireland: What it is and what to do about it

Subsidence causes cracks, blocked drains, doors that won't close and needs to be dealt with.

Hello Kieran, 

My partner and I are looking at buying an old house in the suburbs of Cork City. One thing I have heard is to watch out for is subsidence. What actually is subsidence, what causes it and what can be done to fix it?

James and Fiona, Glanmire

Hello James and Fiona, 

Thanks for this great question. Subsidence is one of those words that has put a shiver down the spine of many a home owner in the past. 

Essentially, subsidence occurs when the ground under a building no longer evenly supports the weight of the building above it. It may be that one section of ground deteriorates in terms of structural capacity relative to another section of ground and, in this instance, this part of the building drops a little. 

subsidence occurs when the ground under a building no longer evenly supports the weight of the building above it.
subsidence occurs when the ground under a building no longer evenly supports the weight of the building above it.

Even a very small drop in height here can cause a crack in your walls and could even mean that an internal door may not close properly. 

Let's dig a little deeper here (no pun intended). In most cases (but not all), subsidence is caused by drain leakage. In the past, sewer drains were built using clay pipes, which are short and literally pushed together. 

Given that these pipes are so short, any slight leak from the many (unsealed) joints causes the ground under the joint to degrade structurally. Then the pipe slips a little more and another joint opens. This process continues until there is a significant deterioration in the structural capacity of the ground local to your foundations. 

This causes uneven settlement of your foundations on this ground and you get cracks, blocked drains, doors that won't close and a process that will keep progressing until it is comprehensively remedied.

To fix the problem, you will need the assistance of a specialist engineer and subsidence contractor. Firstly, the building will be surveyed and analysed, and an engineering solution will be designed. 

The old clay drains will need to be removed. 

In the affected areas as determined by the engineer, new local foundations will need to be installed. This may be by digging out locally and placing new concrete in sections or, indeed, using an underpin — effectively a ‘pin’ of reinforced concrete grout that is drilled and pumped into the affected area to re-introduce a measure of structural integrity into the ground beneath these foundations.

Kieran McCarthy: Even a very small drop in height here can cause a crack in your walls and could even mean that an internal door may not close properly.
Kieran McCarthy: Even a very small drop in height here can cause a crack in your walls and could even mean that an internal door may not close properly.

Once the building has been stabilised, new uPVC drains are installed (which are much longer than the clay drains and have sealed rubber gasket joints). These drains are surrounded by a special pea gravel to protect the pipe. 

The ground is then reinstated and, after a period of say six months to a year to allow for any latent settlement to dissipate, you can fill any cracks and get a carpenter to make good any doors that are not closing properly.

In truth, most suburbs in Irish towns and cities where houses were built in the early 1970s or before will likely have clay drains fitted. While they may not all be leaking now, they will eventually leak. 

If it is the case that you buy such a house, it would be essential that you remove these pipes local to the house (and indeed any lead water pipes) and replace them with the modern uPVC equivalent. 

If you are viewing such a house, a look at the manhole covers or gully drain heads will tell you a lot. If they are not plastic, I would certainly lift them at some stage and look at the pipe below to see if it is plastic. 

Another key check is to look at the top corners of a variety of windows inside and out. If you see cracks running at 45 degrees, there is likely some movement that needs to be further investigated.

In summary, most houses of from the 1970s or older will need some level of this treatment. If it is still under the ownership of the party selling the house, it may be that their insurance company will pay for the subsidence repair works (which is a very considerable sum). 

However, if you buy the house without prior advice, it becomes your problem and unlikely to be covered by your insurance, so buyer beware. Don’t be deterred though, these ageing houses, with some TLC, can make great family homes in long-established neighbourhoods, so happy hunting!

  • Civil engineer Kieran McCarthy is founder and design and build director with KMC Homes. He is a co-presenter of the RTÉ show Cheap Irish Houses.

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