What insurance companies fear is — of course — the unknown, as that is where their risk lies.
I am living in a city suburb that is regarded by insurance companies as an area of high level subsidence. I am fortunate to have a very large garden as I am on a corner site. I recently successfully applied for planning permission to put a second home on a portion of the site. I intend to build a new home for myself and to sell our old home (I’m not setting a precedent, there are other homes in the area that have done likewise). This time, I would like to be in a position to take out comprehensive home insurance — it’s currently only available to me with a subsidence exclusion. I spoke to another homeowner who also built on a corner site and they said they overcame this problem by using a floating foundation. I don’t know what this involves. Is it costly? Is it complicated? Can you advise?
Many thanks,
Dorothy O’Neill.
Thank you for this great question. So, you are fortunate enough to have a suburban site available for your new home and the benefit of being in a position to sell your existing home to help with the new build costs: lucky you. But, as you say, there is the question of subsidence, prevalent in the area, so what does this mean and how would this affect the building process and indeed the insuring of your new home once complete?
To answer these questions let’s look at what causes subsidence in the first place.
There are two types of subsidence prevalent in suburbia. Firstly you have subsidence as a result of old clay drain leakage. Essentially, clay drain pipes are very short so they have a multitude of ‘push-home’ joints that are liable to leak and the resulting effluent leakage causes the ground local to your house to lose its structural bearing capacity and thereby results in cracks appearing in your home as your foundations move.
The second type of subsidence occurs when the original house was built on ground that had poor bearing capacity to begin with. Essentially, the ground wasn’t strong enough to take the weight of your house from the strip foundations of your house.
So, how do we remedy these issues? In the first case, where the drains are leaking, the old clay drains are dug up and removed and new pvc drains are installed in their place. These pipes are much longer with more secure joints so they are much more resilient than the old clay drains.
The foundations are then underpinned where the ground below the foundation is strengthened by injecting a concrete grout into the ground to reintroduce structural stability which had been lost due to the organic effluent leakage.
A version of this underpinning process can also be used to remedy subsidence in a house built on poor ground, though the pins will need to be driven deeper to get to a ‘strata’ or layer of good ground lower down where firm support exists.
What insurance companies fear is — of course — the unknown, as that is where their risk lies. To be fair they have had to pay out vast sums of money in the recent past to repair subsidence in old buildings and at this stage, covered by pre-existing subsidence cover in these policies and hence they have no appetite to reintroduce this cover in new policies.
So, how does this relate to your new build? Well, when you build your new home, you will have a digger on site to dig out your new foundations before the concrete and steel mesh are introduced.
When the digger begins to dig, your engineer will arrive down to inspect the excavations and in particular the solid and ground conditions underneath.
If he is happy he will allow standard footings to be constructed. If not he may opt for a raft foundation (the floating foundation you mentioned) where, rather than your entire house load resting on the ground floor wall strip footings, this load will be spread further to a concrete slab across the entire ground floor of your house (this greater area reduces the pressure on the ground underneath).
Another (more expensive) option available to your engineer, if the ground strength is really poor, is a piled foundation. Here your new home is built on concrete piles which again will be driven down to stronger ground at a lower level to support your home.
Once the foundations are complete, your new drainage is installed and again the longer pvc pipes are fitted.
So, to sum up, your new house will have newly-engineered foundations, designed to suit the ground condition under your home, as directed by your engineer on site and a new ’modern’ drainage system thereby eliminating both causes of subsidence as described earlier and with the additional benefit of a sign-off and certification from your engineer.
This should help you, and your new insurance company, sleep better at night.
Kieran McCarthy is a building engineer and director of KMC Homes bespoke A-Rated new home builder, serving Cork and Limerick. He is also co-presenter of the RTÉ property show Cheap Irish Homes.
Follow Kieran on instagram @kierankmc for more home building information, tips and Q&A advice.
You can also follow Kieran on the Built Around You Youtube channel and @kierankmc on TikTok.
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