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November 2015

Question

My husband and I recently purchased a house. In the process of getting to know our neighbours, one of them informed us that there may be water under the house. This neighbour had recently discovered water under his property and had informed the previous owners of our property. However, nothing was done. Recently I took the carpet out from one of the rooms on the ground floor, followed by a section of the floorboard and indeed it does seem that there is water under the house. From what I can see, the water was quite low down but there was a lot of it. What impact will this water have on my property and what do I need to do to minimise its impact on my property. 

Answer

The level of water in the ground will vary and is generally known as the “water table level”. The water table level tends to be fairly static but can vary between summer and winter conditions and clearly can rise following heavy or prolonged rainfall. In low-lying ground or in areas along the coast, where the water table level would not be that much lower than the finished ground level, the possibility of water underneath the suspended timber floor is in fact quite possible and is something that we have come across on numerous occasions.

When one is building or designing a house it needs to be considered a bit like a boat in that part of the house, ie the foundation/base is below the ground, and this could be below the water table level. In other words this is in a permanently damp location. For this reason, damp proof courses are incorporated in the walls, and damp proof membranes in the floor, in order to prevent water rising up through the house. This phenomenon has been known and recognised for years. It is most likely this issue has been going on for a very long time and no doubt most of your neighbours will be experiencing the same issue.

The good news is that more often than not, this is not a major problem and you could possibly just ignore it.

The potential issues for concern are foul damp odours and/or a risk of decay to the floor timbers due to excessive dampness. However, provided that there are good strong levels of ventilation into the sub floor void (the space between the floor and the ground), which can be provided through air vents at a low level in the walls, these issues are unlikely to materialise.

If the water level is very high, there may be some merit in attempting to reduce this by forming a suitable French drain at a lower level around the perimeter of the house to alleviate the issue somewhat. The other potential issue for concern is that it does indicate that the ground water table is relatively high when compared with your actual ground/floor level and this may be an indication that you will be more susceptible to a risk of flooding, and as we know of late, unfortunately there are many properties which have been built in known and potential flood plains. It might be prudent to seek further information as to the risk of flooding in your area.

Val O’Brien is a chartered building surveyor and a member of the Society of Chartered Surveyors Ireland.