Hi,

We have a willow tree, possibly 30 years old, about 3 or 4 metres from a stone outbuilding. There is a ditch the other side of the outbuilding and a crack in the concrete floor between the tree and the ditch. We're on a clay soil and if stories about willow roots seeking out water are true, I suspect that the roots are going under the floor of the outbuilding and into the ditch.

I intend to dig a trial trench to 'test the hypothesis' but would appreciate some advice on how deep the trench would need to be.

If the worst comes to the worst and the willow has to come out, when would be the best time to do this as I am aware that taking a tree out on a clay soil can cause as many problems as leaving it in situ.

Many thanks in advance for any assistance.

Best wishes

- Hilary

Hi Hilary

The way trees cause damage to buildings on clay soil is to extract the water from the clay, causing it to shrink. The building foundation then moves down with the shrinkaing soil and if this causes one part of the buioding to move down differentially from another, crack damage can result. This is called clay shrinkage subsidence.
Willow trees are notorious for causing this sort of damage. The general way to solve the problem is to remove the tree (relatively cheap)or deepen the foundations (relatively expensive).
The issue of damage resulting from tree removal is due to 'heave' and is the exact opposite of clay shrinkage. Following tree removal, the clay swells. That's okay if it just returns the building to its original position. The problem comes where if a tree significantly pre-dates a building then the swelling may continue such that the building is lifted beyond its original position and further damage occurs.
So, in short, if the Willow is not older than the building its removal should not lead to further damage.
You can remove the tree at any time but may have to consider some type of stump treatment or stump removal to prevent regrowth.
Finally, after you have removed the tree you should wait for the clay to rehydrate before undertaking permanent repairs - there will be a period of clay rehydration and swelling that lifts the building, so that cracks reappear in cosmetic finishes (plaster, paint work etc).
How long is this period? Where I work in the UK this is a common problem. Mostly full rehydration will occur over a single winter. In some circumstances however, rehydration may take two or more winters.
Hope this helps.

Oscar - many thanks for the reply - I have to admit, I had given up getting a response. The building definitely pre dates the tree, so that's good news. We've started to take down the tree - which has four trunks/stems. It's around 25 years old, from counting the rings, and I guess the outbuildings are perhaps 150 years old. Thanks again for all your advice - much appreciated! Hilary