Kieran McCarthy: Fixing cracks in the foundation in Cork

Mon, 29 Aug, 2022 - 14:00

We’re looking to buy a home in a settled area near national schools in Cork City’s southside, so despite the incentives to buy new, our choice is more confined to older houses.

Lots that we have looked at have acknowledged issues with subsidence and leaking drains, some of it has been addressed, more not touched, often where the house owner had been elderly.

We’d be prepared to buy if the purchase price is right (and the cracks not too worrying!), and tackle it later on when we extend, but may not get a mortgage for this. We’re not cash buyers.

How big a deal is it in Cork, and why are remediation costs so high?

Regards, Meg and Andy.

Hello Meg and Andy. Thank you for your question. As a previous resident of Cork’s southside and having renovated countless houses in this area, I am more than familiar with this issue.

Subsidence is certainly a significant issue in the older housing stock and didn’t really taper off until the 1980s, causing many a homeowner sleepless nights and worryingly large bills, stress, and inconvenience.

So what causes subsidence, what can be done to alleviate it, and how does this apply to your future plans in terms of cost and timing?

Subsidence is caused by a deterioration in the bearing capacity (strength) of the ground under the foundations of a house. It may be that a house was built on weak or rebuilt ground or, more commonly, it may be that the drains or a water main may be leaking into the ground locally thus changing its composition and causing a local weakness in strength.

In new homes, subsidence is rare because uPVC drains are used. These are long pipes that have rubber gaskets (seals) at the ends. They offer a little flexibility and so are quite forgiving in terms of slight movement.

In the past, however, these pipes had not been invented yet and in most cases clay pipes were used. These were much shorter, had push-home joints (no rubber seals) and were brittle in nature (just like a terracotta flower pot).

Filling the cracks is the last thing you do. Picture: Domnick Walsh.

What tended to happen was that you would get a slight weep of effluent from one joint which would cause the ground locally to deteriorate, causing the pipe to move a little.

This then caused another joint to open and then further movement would eventually see the pipe subside and crack, thus sending the effluent material into the surrounding ground and beginning a marked deterioration in the bearing capacity of this ground local to your foundations, causing foundations to subside and crack in your brittle masonry and plaster.

So now that we know the principal cause, how do we fix it?

Well, filling the cracks is the last thing you do. You need to start with the cause of the issue: The old clay drains. These need to be dug out and removed from your house and garden. A builder and mini digger will dig these out and remove any of the affected subsoil locally.

What needs to happen next is that the ground under your foundations needs to be stabilised to stop further movement. This is typically done by underpinning where a liquid concrete (grout) is pumped into the affected area to reintroduce the required compressive strength.

Next, a new drainage aggregate is spread in the line of your new drains and new long-length uPVC drains are installed and tested. Then finally the area is reinstated locally.

Work in progress to fill in the subsidence on the Blackrock Road. Picture: Richard Mills

The very last thing that is done, usually a minimum of six months later, is the repair of the cracks on the external and internal of your building. The reason for this delay is that the building must be allowed time to re-settle onto the newly reinforced foundations.

So, now that we know how to fix it, why does it cost so much? This is a very labour- and machine-intensive process. You need mini diggers, jack hammers, grouting drills, and a grouting rig as well and trucks to remove the old waste soil, import the new aggregate, concrete, and of course a team working round the clock to bring the whole operation together.

Groundworks, due to the nature of the heavy work involved and working outside in all kinds of weather, is expensive. This work is generally carried out by a small number of specialist contractors with specialist equipment and tradesmen which forms a niche in the market with an associated price.

Much of this work is paid for by insurance companies which in the past drove the price up even further but the monies sanctioned for these works have been reduced post-Celtic Tiger, and overall rates in the market have reduced a bit.

My view on buying an old house with possible subsidence is to fix it straight away. It may be as simple as removing the old clay drains before they get a chance to leak much further or it may be more invasive, with underpinning required — either way the longer you leave the issue, the more expensive and invasive the repair works will be.

If you were a cash buyer you might be able to net a bargain in need of subsidence repair as the demand would be lower.

I am not in favour of clay drains being in place near an old building so I would remove them in any renovation process, even if a building does not suffer from subsidence.

This may not be exactly the news you were hoping for, but I do feel it is my best advice.

Clearly, if you were a cash buyer you might be able to net a bargain in need of subsidence repair as the demand would be lower for such a property but what I could suggest, is that if you align yourself with an expert engineer in subsidence repair and — if you could — a property that looked very dishevelled in appearance but that didn’t in fact suffer from any significant settlement, perhaps you could turn this frog into a prince.

Building Engineer Kieran McCarthy is design and build director with KMC Homes, serving Cork and Limerick. He is also co-presenter of the RTÉ show Cheap Irish Homes. Tune in to Kieran’s new podcast coming soon.

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