Home Buyers Survey

Some reflections on Japanese Knotweed, Overestimation, Damp Valuation

Getting in a knot about Japanese Knotweed

Japanese Knotweed or Reynoutria Japonica is a native plant species to East Asia which, in Europe, is classified as an invasive species and pest, has caused many surveyors, including this one, sleepless nights and provided yet another avenue for negligence claims, in a world obsessed with assigning blame.

According to Wikipedia its flowers are valued by some beekeepers as an important source of nectar and in Japan is foraged as a wild edible spring vegetable with a flavour similar to rhubarb.

In Japan, the plant does not appear to cause alarm, in part, obviously because it is native but also because it is kept in check, by fungus and insects.

The images below show Japanese Knotweed in early growth with similarities to asparagus, in fully bloom in summer, dying back in winter and having been chopped down.

So why does it cause so may headaches for surveyors?

The World Conservation Union consider the plant one of the world’s most invasive species, with an invasive root system and robust growth, which can damage foundations, flood defences, roads, paving and retaining walls. In the UK Japanese Knotweed has its own Gov.UK homepage www.gov.uk/guidance /prevent-japanese-knotweed-from-spreading which, includes detailed instructions on how to deal and dispose of Japanese Knotweed.

However, a joint study by Aecom and the University of Leeds undertaken in 2018, found little evidence the plant causes structural damage. The study looked at 68 pre-1900 properties located on three streets in northern England, each property had been abandoned for at least 10-years and was in a state of disrepair, representing the ‘worst case’ scenario in terms of susceptibility to damage from unchecked plant growth.

JK was identified with seven metres of 18 properties and was linked to less damage than trees, climbers and shrubs such as buddleia. (Also non-native to the UK).

A separate survey provided results of 81 excavations showing that JK roots or rhizomes rarely extended more than 4m from above-ground plants and was generally circa 2.5m.

Co-author of the report Dr Karen Bacon said the plant remains a concern for biodiversity and flooding risk but is where JK damages, built structures, it is usually because of an existing weakness or defect.

The Dangers of Overestimation

More years ago, than I care to remember, I attempted to author a dissertation on ‘The Application of Experts Systems of Property Valuation’ it was rubbish. Alan Turing I am not, I had no idea what I was writing about, that being said, few people did, it was a very long time ago.


The dissertation, necessarily morphed into something else and tried to address the continued relevance of Chartered Surveyors and future proofing. It seemed to me at the time, and still does, to some extent, today, that many disciplines covered by Chartered Surveyors require specialist knowledge but too much specialisation would inevitably lead to redundancy when markets change.

I do not think I was alone in sharing those concerns and according to the RICS there is likely to be a shortage of surveyors when my generation retires. Part of that may be the inevitable conclusion of litigiousness, surveyors have been and continue to be an easy target, as Heart v Large attests. Despite that, I do carry slight concerns though that there is a tendency within Building Surveys and Homebuyer Reports to over classify every defect requiring urgent repair and further investigation. At some point, it seems to me, the question may be asked, ‘What’s the point?’, If everything is going to be Red, then we may as well refer every element of fabric and services to specialist contractors before completing the purchase. This, I think, would lead, inevitably, to buyers being mis sold and so the wheel turns.

Energy, Ventilation, and damp

It seems to me, as energy prices climb, Building Surveyors are likely to be finding more dampness in buildings, particularly, if the trend of very wet winters continues.

Occupation equates to high humidity and in turn condensation when internal temperatures drop. The moisture generated by people, bathing/showering and preparing food must go somewhere when the air temperature drops below the dew point. If homes are poorly heated resultant of high energy prices, condensing damp is likely, especially if the external air is, addled with moisture.

The concept of Thermal Mass is difficult to grapple with. It would seem counter intuitive that it is more economical to turn heating off when one is warm enough. When heating is, switched off, the fabric begins to shed heat, thus requiring considerable heat energy to restore lost heat. High insulation levels are good at for retaining heat and for keeping things cool. An unheated highly insulated home is likely to be chilly.

Understanding thermal mass, is going to be important as more homes are, retrofitted with heat pumps. Heat pumps work best with thermostats set to trickle, in conjunction with high surface area radiators and with mechanical heat recovery ventilation systems.


The covid crisis should by now, have impressed on everyone, the importance of ventilation. Maintaining heat at a low setting via a thermostat retains heat in the fabric, enabling stale air to be, purged, even in winter, without significant heat losses.

Valuations and Covid

Today, my 1990 dissertation, would have had the title ’The Application and Use of Algorithms in Property Valuation’ but I knew little of Alan Turing, much less the machines he developed to decrypt the enigma codes or further development led by IBM. The idea that there would eventually be algorithm led Applications like Zoopola, Right Move and even our very own SFY Reporting App would have seemed very fanciful. I concluded that Expert Systems had little application in Domestic Property Valuation, that view has not changed.


Here are two examples of why:

An Expert System may have concluded, as early pioneers of Energy Performance in Buildings Assessments anticipated, Energy Performance Certificates would create a two-tier marketplace, however, even following the introduction of Minimum Energy Efficiency Standards, Energy Performance appears to have had minimal or no effect on the domestic market. This is because homebuyers are not rational actors.

The effect of Brexit and the Covid outbreak, it seems to me, could not have been, meaningfully forecast, there are too many, irrational, variables. The societal change that has occurred following, ‘stay at home orders’, has seen many people reappraise their home needs to accommodate home working. It seems unlikely people will, at least not soon, return to working from offices five days a week, and City living is no longer so attractive when there is limited opportunity to escape the confines of an apartment.

Computational valuations require definable factors, such as proximity to High-Speed Rail and direct benefit incentives for energy efficient homes. It will be interesting to see if Government introduce direct mortgage incentives, in addition to grants, for the installation of heat pumps.