DO I NEED TO WORRY ABOUT SUBSIDENCE?
QI’m about to buy a 1960s bungalow to renovate. There’s a storm drain which serves the house and the neighbouring property; it’s approx 2m3m from the front of house and is very close to two medium-sized conifers that I would cut down.
The building survey highlighted a potential problem with a blockage due to tree roots, and stated ‘leaking drainage is the primary cause of subsidence’. We’ve since had a drain survey, as recommended by the building survey, which has revealed the presence of roots in one section. This survey suggested ‘remedial work might be required’. I’ve also hired a structural engineer, who has said that there’s unlikely to be movement, given the condition of the wall, but unless we excavate the area adjacent to the house it would be difficult to be 100% certain.
Obviously the vendor is not going to agree to excavation work. Perhaps I’m being a little overcautious, but I can’t seem to get a definite answer before buying. Help!
TASHA, WARWICKSHIRE
One common criticism of survey reports is that they often include a lot of precautionary advice about generic threats, which can sometimes appear quite worrying. So it’s worth bearing in mind that if a defect is not given a ‘red’ condition rating by the surveyor it means they don’t consider it to be either ‘serious or urgent’. Where there’s evidence of significant structural movement to the main walls of a property this would normally be flagged up in the report, so it sounds as if the main walls were given a clean bill of health in your survey.
Insurance claims for suspected subsidence are fairly common but the vast majority end up being rejected because the movement turns out to be due to some other cause. True subsidence is most prevalent in areas with clay subsoils following long periods of drought. Some species of notoriously ‘thirsty’ trees close to buildings can exacerbate shrinkage in clay subsoils thereby undermining foundations, but conifers are by no means the worst offenders. While it’s true that tree roots entering drainage pipes can lead to leakage that can eventually weaken the ground, if the contractor carrying out the drainage survey had serious concerns they presumably wouldn’t have been shy about quoting for remedial works. Drainage surveys commonly use CCTV cameras so clients can see for themselves the extent of any root penetration.your drainage contractor should also be able to confirm liability for this shared drainage system, which may be down to your water company.
Although 1960s foundations are deeper than those in most pre-war houses, they will not be as deep as in most modern buildings and will therefore be potentially more vulnerable to ground movement. However, your structural engineer’s opinion sounds reasonable, particularly where trees are located further away from the house than the mature height of the tree. If the trees are professionally cut back as you propose, that should be sufficient to restrict future root spread.
IAN ROCK, CHARTERED SURVEYOR