Trees
There are many species of trees. However, they can be divided into two main groups: broadleaved and conifers.
The majority of movement that occurs to property is caused by the broadleaved varieties, but not always. The mechanisms of damage are varied but root growth and associated drying out of the subsoil are, by far, the most common. The Institute of Structural Engineers say that 80% of subsidence claims on shrinkable sub-soils are due to trees and shrubs close to the property; some believe this to be an underestimate.
The risk of damage by different species:
| Ranking | Species | Max tree height | Maximum distance for 75% cases | Minimum recommended spacing in an area of shrinkable clay soils |
| 1 | Oak |
16-23 |
13 |
1H |
| 2 | Poplar |
24 |
15 |
1H |
| 3 | Lime |
16-24 |
8 |
0.5H |
| 5 | Plane |
25-30 |
7.5 |
0.5H |
| 6 | Willow |
15 |
11 |
1H |
| 7 | Elm |
20-25 |
12 |
0.5H |
| 8 | Hawthorn |
10 |
7 |
0.5H |
| 9 | Maple/Sycamore |
17-24 |
9 |
0.5H |
| 10 | Cherry/Plum |
8 |
6 |
1H |
| 11 | Beech |
20 |
9 |
0.5H |
| 12 | Birch |
12-14 |
7 |
0.5H |
| 13 | Whitebeam/Rowan |
8-12 |
7 |
1H |
| 14 | Cypress |
18-25 |
3.5 |
0.5H |
The above table is designed to assist recognising defects to a property and should not be used to design a planting scheme. Advice should always be obtained from a registered arboriculturalist prior to planting large trees near any structure.
Trees use prodigious amounts of water to live and grow; furthermore trees require substantially more water during the growing season. Fast growing varieties can require over 455 litres (100 gallons) per day.
Tree root damage occurs principally on shrinkable clay soils. As a rough guide you can test your own soil to establish if it is a clay soil by obtaining a clod of earth from approximately 60cm below ground level approximately 8cm in diameter and place in water overnight. If the sample remains intact the probability is that the sample is a shrinkable clay soil. Clay soils will crack in the summer and the soil will be solidly firm (almost rock hard). When moist, the clay soil will stick to your spade; when digging the garden, a clay soil will also feel greasy and will be easily moulded when moist.
I have movement with trees nearby – what should I do?
If your house has settled because of tree related movement you could cause significantly more damage by removing all the trees at once, as you could experience heave. Heave is a form of structural movement, which is the opposite of settlement. A phased removal of the trees is often the only way to proceed. We would advise that you contact a chartered building surveyor and/or a registered arboriculturalist for advice on tree removal; a structural engineer or building surveyor would also be able to advise if any remedial work is required to the building fabric or foundations.
Sick Trees
Trees do cause other damage to buildings through direct contact, where branches and trees fall on property. Any tree that could cause direct damage to a dwelling should be carefully watched. If branches appear weak or if the canopy of the tree is incomplete it can suggest that the tree is diseased. Diseased trees should be dealt with immediately as there is a possible risk of injury and damage to the building.
Rotting of tree stumps and roots can after many years cause the timber to become brittle. If the root system was large, earth may also collapse into a hole once occupied by the roots. Care should be made to remove as much of a tree as possible, as back filling at a later date will be expensive, particularly if access is restricted.
Tree root damage can cause drainage channels to leak and break; the escaping liquids could cause heave in clay soils or cause the erosion of the soil beneath the foundations. Erosion of the soils could cause settlement or subsidence to the main building.
Tree Maintenance
If trees are actively maintained and the growth of the canopy is limited by thinning, pruning or topping, then the moisture requirement of the tree may diminish to a safer level. Care should be taken as a number of tree varieties are susceptible to disease if they are pruned excessively; other varieties grow more rampantly if pruned. Therefore advice should be sought from a qualified expert.
Tree Preservation Orders
Any alterations to a tree covered by a Tree Preservation Order will require the permission of the local Planning Authority. Ignorance of the law relating to Tree Preservation Orders is not a defence, therefore if in doubt contact your Local Authority Planning Department. The costs and fines of damaging or removing a tree protected by a Tree Preservation Order could be punitive.
