Philip Wilson Arboriculture


Tree-related building subsidence

 

Cracks develop in walls if there has been movement in the structure, but this does not necessarily mean that there has been movement of the foundations. Cracks in houses NOT related to foundation movement can be due to:

1. Seasonal changes in temperature (including the freezing and thawing of water) and humidity, initial drying of cement or plaster, loss of volatiles from sealants etc.

2. Chemical changes involving water, such as adsorption by bricks and corrosion of steel, which cause swelling.

3. Over-stressing, often after loft conversions or other structural alterations involving the removal of a load-bearing part of the structure.

4. Vibration associated with earthquakes, road traffic, mine blasting and, in properties over 60 years old, Second World War bomb damage.

 

Foundation movement, resulting in subsidence or heave, also has several causes, including:

1. Shrinkage or swelling in the surface soil due to variation in groundwater level, the influence of vegetation etc., associated with shrinkable clay soils. 

2. Erosion, caused by water leaking from a defective drain or water supply, associated with permeable soils

3. Compression of the ground if naturally soft or 'filled'.

4. Collapse of mine workings or natural cavities.

5. Nearby construction or excavation.

6. Chemical attack on the foundations.

 

The first is by far the commonest in the U.K. Trees and other vegetation types demand water from the soil and can dry it out. On certain clay soils this can lead to subsidence (and rarely to heave). Subsidence related to soil shrinkage/swelling often has the following characteristics:  

 (i) Cracks first appear after a dry summer.

(ii) Cracks open in summer and close (or partially close) in winter.

(iii) The largest cracks are closest to any tree or other vegetation causing the shrinkage.

 

 For more information:

Freeman, T.J., Driscoll, R.M.C. and Littlejohn, G.S., 2002. Has your house got cracks? A homeowner’s guide to subsidence and heave damage. 2nd Edition. Institution of Civil Engineers and Building Research Establishment. Thomas Telford Publishing, London. ISBN 0 7277 3089 4.
Summary of Example Report

 The house was built in the early 20th C. and extensions were added in the 1950s. Some movement has occurred in the overall structure resulting in exterior cracks, mainly in or near the south-east extension. 

The tree at issue, 9m to the south of the east end of the house, is a mature Wellingtonia (Sequoiadendron giganteum). Wellingtonia has a very compact and deep-rooting habit, and the lowest potential of all tree groups to cause damage by root extension below buildings. The soil is derived from the Blackheath Beds and is not strongly shrinkable.

The cracks in the south elevation of the south-east extension are old and non-progressive, and suggest settlement rather than vegetation-related subsidence. A more recent crack on the north side is difficult to interpret at present, but the tree is unlikely to be implicated since its influence, if any, would be seen first on the south elevation.

The progressive cracking should be monitored (to include level-monitoring) to increase understanding of the recent movement. In the meantime, no works to the tree are proposed.  In the unlikely event that a rotation of the south-east part of the house to the south, associated with subsidence, is eventually identified, a case could be made for felling the tree. Note that any tree works would require the prior approval of the local district council since the site is in a Conservation Area.