The role of surveyors in dealing with damp in homes

The second article in the series examines the measurement of internal environmental conditions, looks at the case of a flat with significant mould, and considers the role of the surveyor in such situations


  • Mike Parrett FRICS

19 March 2020

In the previous articles, we looked at how to quantify the dryness or dampness of our homes, and how to determine whether an occupancy is classed as dry, moist or wet. But how do we measure humidity?

The best form of measurement for internal environmental conditions is absolute humidity. This is a quantifiable measure of the amount of moisture in the air, irrespective of temperature. However, relative humidity is the amount of humidity in the air in relation to the ambient temperature.

When monitoring a home,  removing temperature from the equation gives a more accurate measurement of moisture in the air. It does not then matter whether readings are taken on a warm or cold day, in summer or winter. Absolute humidity will give you a clearer picture of whether the occupancy is dry, moist or wet according to Table B.3 in BS 5250, as shown in the previous article.

For example, a bathroom in a fourth-floor flat in an old block constructed with solid external walls has a bath situated against an internal party wall. The internal face of the outside wall is covered in black mould; on the external face of this wall is a cracke, cast-iron rainwater pipe that is leaking water on to the surface. The tenant says that the mould gets worse during periods of rain.

A surveyor or other expert witness needs to be able to determine whether the cracked rainwater pipe is causing the mould, particularly on the external wall. If so it would be a breach of repairing covenant under section 11 of the Landlord and Tenant Act 1985, and an abrogation of a landlord's duty of care under section 4 of the Defective Premises Act 1972.

Mould outbreak

The outbreak of mould could then lead to further claims under the more recently introduced Homes (Fitness for Human Habitation) Act 2018 and the Housing Health and Safety Rating System (HHSRS), part of the Housing Act 2004, or a prejudicial to health or statutory nuisance claim under sections 79-82 of the Environmental Protection Act 1990.

However, if the tenant caused the mould to occur then the landlord is not responsible for any repairs. Understanding the true cause of the dampness can therefore clearly have major implications for the landlord and tenant.

The first step when measuring moisture would be to take a core sample from the external wall and send it for laboratory analysis, gravimetric test or calcium carbide test. If the found moisture content at the core is greater than the hygroscopic moisture content, then this would confirm that the solid wall has been penetrated from an external source.

If the hygroscopic moisture is greater than the found moisture content, then this would indicate that the mould is the result of an internal problem, meaning that we need to understand whether the growth is symptomatic of the use and occupation of the dwelling. To do this, we need to determine whether it is a dry, moist or wet occupancy.

Related article

In context: Mould in buildings

Read more

Investigation of moisture and its effect on traditional buildings

RICS has collaborated with Historic England and the PCA to produce a joint position statement: Investigation of moisture and its effects on traditional buildings: Principles and competencies.

The document has been additionally supported by Historic Environment Scotland, Cadw, Historic Environment Division Northern Ireland, SPAB and IHBC.

For the time ever, these organisations have come together to agree on the set of principles and competencies that should be used anytime there is a need for an assessment of moisture within a traditional building.

Determining cause

Truly understanding the cause of damp and mould is crucial before a surveyor can determine who or what is responsible for the problem and make recommendations for repair. It is easy to arrive at the wrong conclusions because temperature and relative humidity can give very misleading measurements.

Many surveyors use a hygrometer and thermometer to measure relative humidity on a single day, which means they are at the mercy of temperature. If during a visit the environmental conditions all seemed fine - the air temperature was good, within World Health Organization guidelines, and the moisture content of the internal air was also low or normal - then the measurements would indicate that at this particular time there was no risk of condensation. However, the mould on the walls and around the windows suggests otherwise, indicating that longer-term monitoring ofthe internal conditions is required, especially during cooler months.

It could be that on the day, or even those preceding the visit, there has been little or no moisture activity in the property. I have seen cases where knowing a surveyor was due to visit, an occupier would turn up the heating and open windows to improve the internal conditions and show there was no risk of condensation. In doing so, they would try to emphasise that the moisture results from a defect to the building - such as the cracked external rainwater pipe in the case mentioned above - because this could enable them to make a disrepair or statutory nuisance claim against a landlord. In contrast, mould and damp caused by use and occupation can,  if proven, support a landlord defending such legal claims.

Occupants' activities

In most properties, if construction or design faults are not causing dampness and mould, then occupiers and their activities are creating the problem. They cause poor ventilation by, for example, not opening windows, taping over gaps keeping trickle vents closed, switching off extractor fans or keeping curtains closed all day.

Excess moisture is also created by overcrowding, poorly vented appliances such as tumble dryers, and the constant activity often associated with a young family such as cooking, washing, bathing and drying clothes. Such conditions mean that BS 5250 is extremely useful in helping to determine whether an occupancy is dry, moist or wet.

It is critical to know the mass of moisture in a volume of air, irrespective of temperature, to determine accurately what kind of occupancy it is. Getting this wrong could mean that unnecessary and expensive remedial actions are taken. Part of the solution to damp should be educating building occupiers on adequately heating and ventilating their homes to minimise condensation. Underheated homes and fuel poverty are a huge challenge to the NHS in reactive treatment, and the BRE estimates that HHSRS category 1 hazards cost it an annual £1.4bn in first-year treatment costs.

Understanding whether a property is a dry, moist or wet occupancy has bedevilled the legal system due to poor or inadequate expert knowledge, and has often been compounded by inappropriate evidence from surveyors investigating claims. BS 5250 is a crucial part of understanding that we often have a systemic failure - not of the building but of the way people live.

'It is critical to know the mass of moisture in a volume of air, irrespective of temperature, to determine accurately what kind of occupancy it is'

Michael Parrett FRICS is a building pathologist, founder of Michael Parrett Associates and lead author on the Damp section of isurv 
Contact Michael: Email

Related competencies include: Building pathology, Legal/regulatory compliance, Maintenance management

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