When booking surveys I always ask my clients to let me know if they have any questions about the house they are buying or if they are worried about anything in particular. Damp is the most common one but next up is worries about subsidence.
In this blog post I will tell you about what I, as an experienced Surveyor, look out for when identifying possible subsidence along with how I can assess the risk from any cracks to the building.
First up it is important to distinguish between structural movement generally and subsidence specifically. Structural movement refers to any form of movement to the building, usually that caused by a failure of defect in the structure. Subsidence means movement with a below ground cause.
Structural movement can happen for all sorts of reasons, some serious and some not so serious. I may talk about some of these in future blogs but today I want to focus on subsidence because it can be very significant and usually very expensive to resolve.
To understand subsidence you first have to understand how foundations work. Put simply the weight of a building wants to push down, if you built a house without foundations it would sink and then collapse. Foundations stop this by transferring the load of the building into the ground in an even way, effectively they equalise the pressure of the building pushing down with the pressure of the ground pushing back. If those pressure stay equal the house stays up. If you increase the weight of the house or reduce the upwards force of the ground it will sink. This is called subsidence.
Generally speaking people don’t increase the weight of their houses enough to make much of a difference so we can rule that out as a cause. Much more likely is some form of change to the ground, there can be lots of reasons for this with the most common where I am being shrinkable clay soils.
If you have every done any pottery (or watched Great Pottery Throwdown!) you will know that clay shrinks when it dries out, for some types of clay this can be as much as 20%. That’s fine if you are making a pot but what about if the ground under your house has a large proportion of clay as is the case for large areas of the country? If we get a particularly hot dry summer then the clay in the ground dries out and shrinks just like a pot in the kiln. As it shrinks it reduces the support to the foundations and subsidence occurs. This risk is increased if there are large trees near the house as these also pull water from the ground, adding to the problem.
So how do you know if it is subsidence? As I said there are lots of reasons but one feature that is common to most of them is that they don’t affect all parts of a house equally. Most forms of subsidence effect only a particular area or a larger area but unequally. This means one of the most common signs of subsidence is a diagonal crack running down towards ground level and wider at the top than at the bottom.
Once you have spotted such a crack the next step is work out how serious it is right now. The good news is you don’t have to be a structural engineer or spend weeks waiting for the cracks to move, a good Surveyor can make an immediate assessment of the risk and we can do that because of a really useful document called “BRE Digest 251”.
Back in the early 1990’s the Building Research Establishment (BRE) looked at information about cracks to 140 houses and as a result they came up with a way of categorising cracks on a scale of 0 (hairline) to 5 (over 25mm). They also found that only cracks of category 3 (5 to 15) and above actually justified further investigation. They determined that anything less than that could be caused by lots of things but it was not worth the cost and disruption to investigate. This does not mean that you can ignore any crack of less than 5mm, especially if you have lots of them, but it does give a reasonable guide as to how concerned you should be.
Once you have established how severe the current cracks are the next step is to work out how likely it is that the movement will be ongoing. That crack might only be 3mm wide right now but what’s to say it won’t suddenly get wider?
In some ways it is reasonable to say a Surveyor would be able to answer that as part of a Home Survey because to fully answer that question you would need to start digging holes, taking samples of the ground and doing all sorts of things which are outside the scope of a Level 2 or Level 3 survey. However, there is still a lot a good Surveyor can tell you just by looking around the property and doing a bit of desktop research. They should be able to identify the most likely causes of the movement and whether there is a reasonable chance that the movement is ongoing. For example, the Surveyor will know, either by local knowledge or checking publicly available maps, whether the property is on clay soil. They should also know whether there has been hot dry weather recently along with being able to spot any large trees that are within the sort of distance where they could affect the house. If these factors are present the risk is high and further investigations are required, if they are not then this can be ruled out. The same is true of other causes like problems with the drains, if the drains run close to the affected area of the house and they have collapsed or a leaking then this could affect the foundations.
By following this approach, it is possible for a Surveyor to work out quite a bit about what might be going on when they see cracks, they might not be able to confirm everything you will need to known but they should be able to give you a very good start.
If you are worried about any cracks at what could be your dream home then do get in touch about booking a Home Survey.