[2025] UKUT 053 (LC)

A notable feature of the expert evidence adduced by the parties is that both the engineering experts and the arboriculture experts have had to speculate on causation beyond what can be determined from the facts available. This is not to criticise them, because each was very experienced in their field and able to use the benefit of that experience in analysing the somewhat limited technical evidence. Their reports have been an invaluable aid to interpreting that evidence but, in making my determination on causation, on the evidence available to the authority when they made their third refusal on 2 February 2018, I must exercise caution in choosing whether to rely on any element of speculation.

The first and key factor to be considered in determining a causal relationship between T1 and the damage at the property is the evidence on desiccation in the subsoil, which would indicate removal of moisture through the roots of vegetation, leading to subsidence. Much has been said about desiccation in all the expert reports but, in the end, the engineering experts agreed that there was no evidence of severe or very severe desiccation in the subsoil at TPBH1 when tested in June 2015. There was speculation as to what might have happened in the subsoil in the period of over two years after the drains were repaired, during which movement of the structures was recorded, but no further site investigations were carried out to establish whether desiccation had occurred. The absence of evidence of desiccation is a significant weakness in the claimant’s case for causation in respect of damage to the extension, for which the evidence from TPBH1 is relevant. As the authority has pointed out, there was no site investigation at all behind the garage to establish whether there was desiccation of the subsoil either initially or subsequently which might have suggested a causal relationship with T1.

The second factor to be considered is evidence of a seasonal and cyclical pattern of movement to the structures, which would indicate root-induced shrinkage and recovery. The engineering experts agreed that the movement at points 10, 11 and 12 around the garage did display a seasonal pattern of movement, but they were not agreed on the mechanism causing the underlying progressive downward movement. Mr Freeman’s theory was that this was a result of sub-surface erosion caused by water running down the slope under the foundations as a result of drainage failure. Mr Allen’s response was that such a mechanism would not have commenced in 2014 as a result of drainage failure and, in any event, the drains were repaired in September 2015. However, Mr Allen’s only alternative explanation for the continuing downward movement, which he called a reduced amount of upward movement, was the development of a persistent moisture deficit in the clay subsoil. Dr Dobson had postulated that the roots of T1 proliferated while the drains were leaking and then continued to extract water after repair, causing progressive downward movement and desiccation. Again, without any subsequent site investigation evidence this remains speculation. Dr Dobson also commented that limited upward recovery may have been attributable to drier than average weather conditions over winter 2016-17. Mr Pryce did not disagree that roots would have proliferated as a result of the leakage, but he considered that they would have become redundant after the drain repair and since T1 and T2 were mature trees their rooting zones were unlikely to have altered appreciably in recent years.

I conclude that whilst there is obvious evidence of seasonal shrinkage and some recovery around the rear of the garage, there is only speculation as to what might have happened in the subsoil to cause the progressive downward movement during the period of monitoring. Without any factual evidence the causation argument for movement of the garage is weak. For the extension, the only significant evidence of seasonal shrinkage was at point 6, and there was barely any evidence of winter recovery afterwards. Again, Mr Freeman relied on his sub-surface erosion theory to explain the progressive downward movement and Mr Allen relied on the theory of root proliferation to explain it. Whilst neither theory on the cause of downward movement can be relied on, the evidence is clear that there was no pattern of seasonal shrinkage and recovery to the extension. This is another significant weakness in the claimant’s case for causation of damage to the extension.

Finally, I must consider whether there was evidence in the documents submitted with the 2017 application to indicate that T1 and its roots would extract sufficient moisture to cause subsidence. The evidence of identified oak tree roots in BH1 was indisputable, but the arboriculture experts were not agreed that this was sufficient evidence. Only four roots were found, all of which were juvenile. T1 was at a distance of 29.15m from the garage and 31.65m from the extension. Dr Dobson considered that research confirmed these distances were within the potential zone of influence of an oak tree, and that it was likely that roots were plentiful in areas outside the borehole. Mr Pryce did not dispute the research, or the possibility that the oak roots could spread sufficiently to be the cause of damage, but he remained firmly of the opinion, based on his long and extensive experience that it was unlikely at these distances. He was also firmly of the opinion that the roots identified could have been either from T1 or T2, which were so close as to be likely to have a combined root system. Dr Dobson did not dispute this possibility and in their joint statement the arboriculture experts expressed the situation as: “The oak roots were from an oak on golf course land, most likely T1, but possibly T2 also.”.

My conclusion from the evidence and the expert opinions is that it has not been demonstrated that T1 and its roots extracted sufficient amounts of moisture to cause subsidence to the garage and the extension. The distance from the tree to the structures is at the margin of the maximum distance where damage has been recorded in research. The roots identified in June 2015 were few and juvenile and only found to a depth of 2.0m. It is speculation that roots were plentiful outside the borehole. It also requires speculation to infer that roots which had proliferated during the period when drains were leaking continued to extract water after the repair sufficient to cause progressive downward movement of the structures. Without evidence of further site investigation during the monitoring period this speculation is not persuasive. The fact that the identified roots might even have belonged to T2 is a complicating factor in causation, but I consider that the evidence was insufficient without that factor.

I conclude that the evidence on desiccation, on seasonal movement and on the presence of oak roots in TPBH1 does not establish on the balance of probabilities a causal link between T1 and subsidence damage to the property at the date of third refusal.

Having decided the factual issue of causation against the claimant the remaining issues do not arise.

For these reasons I dismiss the claim.