Q. I am buying a period property and had a full structural survey carried out and the report has come back with ‘foundation failure’. What does this mean, and is there anything to do to rectify the problem?
A. Foundation failure is usually noticeable by cracks in the walls above ground level and may be caused by either one of the following:-
Settlement – the downward movement of the soil due to the load of the building
Subsidence – the downward movement caused by a weakening or change in the conditions of the soil
Hogging- an increase in the moisture content of the soil under a building
Settlement
There is usually a degree of settlement when a building is first built, but this is normally accommodated within the structure. Problems may arise with extensions if the foundations for the extension are not at the same depth of those for the rest of the property. In this case, the settlement is on-going and is not uniform. Settlement failure is also common with bay windows in Victorian terraces where the foundation under the bay is inadequate.
Subsidence
Clay soils swell when the level of moisture increases and shrink if it reduces, causing cracks. A very common cause of foundation movement in sandy/gravel sub-soil’s is leakage from underground drains close to the building.
Hogging
Hogging or heave can take place where the moisture content of the clay increases under a building. It can also occur because of soil shrinkage around the edge of the foundation. Or by an overload of the soil due to the design of the elevation. This happens especially with eighteenth century properties where the foundations rely on the corbelling of the brickwork and the elevation contains large window areas.
In any of the situations described above a property may require underpinning. This is where a new foundation is constructed at a greater depth under an existing building. It may be necessary where considerable movement has taken place or the movement is on-going. There are a number of systems currently available from constructing a new strip foundation to complex piling systems using stakes or posts. I would suggest going back to your surveyor to get further information of the exact problem and then you will be in a position to get your builder to give you a quote for the works and see how bad things really are so that you are able to make an informed decision whether to progress with the purchase or not.