This week in the Examiner, Kieran McCarthy of KMC Homes provides advice on building in an area that is prone to subsidence.
Q. 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?

A.

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.

View full article in The Examiner here.