Facts

5.     In 2007 the owners of the subject property noticed cracking to the brickwork and mortar joints on the walls at the front left-hand corner of the property, and contacted their insurers (the claimant) who, following initial investigations (the obtaining of a report confirming oak tree root activity below the foundations), instructed Marishal Thompson on 8 January 2008 to carry out a vegetation survey and to advise on proposed mitigation works.  Investigations were instigated including, in January 2008, excavating a trail pit immediately in front of the damaged front wall of the subject property, and a bore hole in the rear garden (outside the alleged zone of influence), and obtaining a soil test report, together with the commencement of crack monitoring which would continue until January 2009.  An initial Arboricultural Consultancy Report was however submitted to Halifax GIS on 11 January 2008 which although stating that “no technical investigations are available at the time of reporting”  provided a synopsis that stated:

“5.1.  This report is based upon our understanding at the time of visiting the property that Halifax General Insurance’s engineers are satisfied that damage is due to clay shrinkage subsidence exacerbated by vegetation.

5.2 In our opinion T1 has the clear potential to be exerting influence in respect of the moisture status of soils beneath the insured dwelling.  The removal of T1 subject to heave assessment offers a predictable and effective arboricultural solution. Pruning within acceptable limits will not promote recovery and long-term stability.”

6.     On 29 February 2008, the former owner of Ashlea, Mrs J Hannam, made an application to the council to fell the tree, that being accompanied by documentation that had been supplied by Marishal Thompson – the initial root report, the soil survey and the 11 January Arboricultural report..  The application (ref: 08/0102/TPO) was refused on 24 April 2008 for the following reasons:

“1. The oak (T1) is of important amenity value to Ashburton Road and the wider area. The removal of the tree would result in a loss of amenity to the area.

2. The application does not identify any reasons of condition or health that would require the tree to be removed.

3. The application states that T1 is implicated in subsidence damage. However, the information submitted with the application does not establish the tree to be a significant contributor towards the unspecified damage of 15 Ashburton Road. In order to better identify the tree as a significant source of structural damage, any application for consent needs to include a structural engineer’s interpretation of the mechanism and progression of the damage as well as level and crack monitoring over an appropriate length of time.”

No appeal was lodged.

8.           On 17 February 2009, an application was submitted to the council to reduce the crown of the tree by 25%.  At this time it was agreed that it was approximately 17 metres in height. The application was accompanied by the same documentation that had been submitted with the application to fell, along with a copy of Mr Lycett’s engineering report dated 8 April 2008 and some crack monitoring data. On 20 April 2009 the council issued a notice of refusal (ref: 09/00567/TPO/E2/08/36/A) stating the following reasons:

“1. The proposal may have an adverse effect on the long term health and/or structural integrity of the tree.

2. The proposal will have an adverse impact on the amenity offered by the tree.

3. Engineering opinion from a council structural engineer states that the new data shows very slight movement, and does not indicate that the tree is contributing to damage at the adjacent property.

4. The requested engineering interpretation of the new data was not provided by the applicant.

5. Where a tree is influencing a structure (which does not appear to be happening here), crown reduction is a poor management technique to address the issue.”

No appeal was lodged.  It is as a result of this refusal that the claim has been made.

Issues

10.  The matters for determination are:

1. Whether the evidence establishes that, on the balance of probabilities, the property suffered subsidence caused by moisture take up by the roots of the oak tree, the subject of the Tree Preservation Order, and whether the damage might continue or re-occur.

2. Whether the claimant has demonstrated that crown reduction is an effective management tool to address tree related subsidence, and avoid future problems.

3. Whether the engineering works undertaken were necessary, appropriate or proportionate in all the circumstances, and whether the costs were reasonable.

Claimant’s case

12.        As to the crack monitoring that had been undertaken over a period of a full year, Mr Hart, in respect of the council’s argument that level monitoring (a different test) should have been carried out, said that it was his professional opinion that the crack monitoring produced irrefutable evidence that the damage had been caused by moisture take-up from the roots. Reference was made to a paper published in 2009 by Dr Giles Biddle OBE entitled “Level monitoring for investigating tree root damage to buildings”, in which he said:

“Where vegetation is involved, it produces a characteristic seasonal pattern of foundation movement; subsidence in the summer reaching a maximum usually in September, followed by upward recovery in the winter.  No other cause produces a similar pattern”

He continued:

“If it [seasonal movement] is occurring, there is no need to try to demonstrate shrinkable clay or desiccation – soil drying by vegetation must be involved (unless foundations are less than 300mm). One does not need to demonstrate the full cycle; just sufficient to confirm movement is consistent with this pattern. Monitoring upward recovery in the winter is particularly valuable.”

The foundations here were more than 300mm deep, Mr Hart said, and crack monitoring produces a similar pattern to level monitoring.  Level monitoring was appropriate where damage might have come from several different sources, but here there was only one – the oak tree.

“Crack monitoring can help to establish the direction, scale and rate of movement (measured in mm) that is occurring within a building that has already suffered movement. It usually takes place over a number of months and is most useful when undertaken over a seasonal period (ie summer to winter) so that it can demonstrate seasonal movement and recovery at particular times of the year.”

This is precisely what had been done in this instance, and Mr Hart said that the results from the 4 monitoring stations (including one at the rear on the warmest, southern, elevation that had been installed at the request of the council) clearly showed that the movement on the front and side wall was due to soil shrinkage and that to the rear was caused by thermal movement. Figure 6 in the statement of case graphically demonstrated the seasonal movement, with the crack at that monitoring station opening during the drying season, and closing again in the winter months as the soil re-hydrated. In his view, the evidence points irrefutably to the oak tree being the causal factor of the damage.

24.        In cross-examination, Mr Hanson was asked to comment on the paper relating to level monitoring that had been prepared by Dr Biddle, and that was included as an appendix to Mr Lycett’s report (bundle p121).  In that, Dr Biddle had said:

“It is now recognised that in most cases underpinning is unnecessary and that foundations can be stabilised by appropriate tree management, usually felling the offending tree or carrying out heavy crown reduction. Site investigations should reflect this change and be aimed at providing the information to allow appropriate decisions on tree management, in particular ….”

It went on to list 7 categories of information and concluded

“Level monitoring provides definitive information on all of the above, and in most situations is the only investigation which should be necessary.  Trial pits, boreholes and root identification do not provide relevant information and are of very limited value.”

Mr Hanson said that whilst he was familiar with Dr Biddle’s work, he did not necessarily agree with it. He said that level monitoring is normally used where the source of the problem is not known, or the whole building is affected. As to why level monitoring had not been provided despite two council requests to do so, he said it was not considered necessary in this case. It was much more expensive to undertake, and would have been overreacting to the problem. 

Compensating authority’s case.

Conclusions

39.        Issues 1 and 3 are therefore resolved in favour of the claimant.  As to issue 2, whether crown reduction would be an effective management tool to address tree related subsidence, little if any conclusive evidence was produced by either party. There seemed to be a paucity of  published research into the subject, and the parties were at odds as to whether, in the longer term, such action would be sufficient to prevent any further damage. Nevertheless, the BRE digest produced with Mr Lycett’s report says at page 8 [Bundle p 120]:

“Buildings near existing trees (Digest 298)

Where the action of existing trees has been identified as the principal cause of foundation subsidence, care and experience are required in assessing the likelihood of progressive movement and hence in planning remedial action.

The following factors should be noted:

Where the trees have reached or a re close to maturity, although seasonal shrinkage and swelling movements can be anticipated, larger movements are likely to occur only in exceptional spells of dry weather. Felling of such trees can lead to worse damage due to swelling of the clay.  Tree pruning may offer an acceptable way of reducing the influence of the tree.” [my emphasis]

In my judgment, based upon the limited evidence available, it was reasonable for the claimant to conclude that crown reduction may well have been sufficient to prevent any re-occurrence of the cracking which was acknowledged to have been slight. Mr Hart made it clear that it was their intention to then monitor the situation, and underpinning works would only be considered if the crown reduction failed to achieve the desired effect.  Thus, it was clear that they were not sure, but the result of the council’s refusal was that it became necessary for other options to be considered.  The option they took was the cheapest and most cost effective, and I am satisfied that the costs incurred were reasonable. 

DATED 7 June 2011 

P R Francis FRICS