Tree Root Damage

1. Is species selection the best way to minimise root damage in the built environment?
Species selection is undoubtedly the most cost effective way of reducing the potential for damage. Most tree-related building damage occurs where the structures are built on expansive clays, and soil type is the major factor influencing the likelihood of damage occurring.
Damage is most commonly caused by the effect trees have on the moisture movement in the soil. Where tree roots extract large amounts of water, the clay sub-grade may shrink and this can cause structures to crack. The damage is not often caused by direct physical pressure from the roots themselves. However, damage to footpaths, curb and channelling and bitumen is frequently caused by radial growth of tree roots in the immediate environment. Often these provide excellent conditions for root growth - a gravel layer, warmth e.g. from a bitumen car park which vaporises moisture in the soil below to create high humidity. Aggressive tree species soon take advantage of the enhanced conditions to increase their root growth and lifting and cracking occurs.
Some trees are very adept at extracting water and the effect on structures will be particularly noticeable in dry years. Structures which rely directly on the sub-grade for support e.g. brick fences and retaining walls, brick and stone houses, are vulnerable to changes in soil moisture levels which can cause differential movement within the subgrade which can cause cracking of the structure. Timber houses built on stumps are rarely affected.
People should realise that tree removal can also alter the soil moisture. While it might be imagined that removing a tree removes a problem, the truth is that it can create another whole set of problems as the moisture dynamics within the soil are altered.
The Institute of Architects have a list of trees that should be avoided and also provide an inspection service for home buyers who seek professional advice about the potential structural hazards.
2. How effective are root barriers?
Root barriers can be very effective or quite ineffective depending on the circumstances. The effectiveness depends on the site conditions, the type of barrier and how well it is installed.
If the barrier is not installed well and openings are left unsealed for service utilities, then the roots will move through the barrier. While it may be aesthetically desirable to bury the top of the barrier, if you do, you are wasting your time. Most of a tree’s functioning roots are in the top 150mm of soil and almost all of them are in the top 300mm. The roots will simply go over the top of the barrier. This will also occur if the top of the barrier is covered by a concrete slab - the roots will go in between the barrier and the slab.
If the barrier isn’t deep enough the roots will go underneath. The bottom of a barrier needs to be treated with a chemical growth retardant. Positioning of root barriers is also vital. Placement of a barrier close to a tree may remove up to 40% of the tree’s roots , and thus cause the tree to be seriously destabilised. It could die or fall over. Another concern that I have with root barriers is that they interfere with the movement of water through the soil which may have other serious repercussions.
3.What is the best way of dealing with problem tree roots coming from neighbouring properties?
It is a tricky problem. Communication with your neighbours is essential. It is very foolish to sever roots which can destabilise trees. A tree through your or your neighbour’s house is a real danger.
There is also the problem of identifying the tree that the roots are coming from. There is a serious need for funding for research to create a tree root identification data base on trees commonly grown in Australia. We would be building on northern hemisphere data bases that have already been compiled.
If the problem of the offending tree roots cannot be worked out in a mutually satisfactory way with your neighbours, then you need to get the professional advice of a qualified arborist and then seek legal advice.
4. Can root damage be prevented by changes in building practices?
Certainly houses can be engineered to withstand problems caused by tree roots. The footings can be made larger and stronger using different materials, but many engineers are reluctant to design larger footings than those that have been traditionally used. It seems to be viewed as a horticultural problem. Certainly it would add to the cost of the building, but it is desirable to look at landscape and architecture in a holistic way. For the moment, species selection remains the most economical way of dealing with the problem.