The usual reason for needing underpinning is subsidence, but the signs aren’t normally as obvious as huge holes appearing in the ground. The evidence for subsidence requires more attention to spot.
What Is Subsidence?
There are many reasons for subsidence, but essentially it’s when the subsoil draws away from the foundations, creating a cavity the building subsides into. This is different from settlement, where the building’s weight displaces subsoil, or heaving, where expanding subsoil pushes the building up.
The most dramatic types of subsidence come from old mine workings or other excavations under the building, but more commonly it’s caused by faulty drains or tree-roots sucking the moisture from the soil, or by drought or a change in the level of the water table.
What to Look For
The most obvious signs to look out for are cracks in the walls. Not all cracking comes from subsidence — for instance, a newly plastered wall may show small cracks, which are quite safe, or minor cracks may appear in older buildings.
On the other hand, substantial cracks in a wall, especially on the outside, are danger signs. They’re most likely to be around the door or window frames, which may also have pulled away from the surrounding walls. Doors are likely to stick, and in extreme cases the building could be visibly leaning to one side.
What to Do
If you spot any of these signs, it’s important to get the house looked at as quickly as possible by a Chartered Surveyor, who’ll be able to tell you whether there’s been subsidence or heave, and how serious it is. In some cases, repairing faulty drains or removing the trees that are damaging the ground can be enough to correct the problem.
However, if the instability is more permanent, or if the fault has damaged the ground too much for it to return to normal, it may be necessary to strengthen the existing foundations with underpinning. This is especially true with older buildings, which often have very shallow foundations. The procedure can be expensive, but in many cases will be covered by your insurance.
There are various forms of underpinning, the main ones involving either digging out sections below the foundations and filling them with concrete or driving mini-piles into the ground. Whichever is used, it’s a highly specialist procedure, and not one to be undertaken by a general builder. Before you hire a company to carry it out, make sure they specialise in underpinning and have a good track record. You need to be certain you’re in good hands.