Guide to Conversion of Traditional Buildings

3.3 Flooding and groundwater

Mandatory Standard

Standard 3.3

Every building must be designed and constructed in such a way that there will not be a threat to the building or the health of the occupants as a result of flooding and the accumulation of groundwater.

3.3.1 Application of standard to conversions

In the case of conversions, as specified in regulation 4, the building as converted shall meet the requirements of this standard in so far as is reasonably practicable, and in no case be worse than before the conversion (regulation 12, schedule 6).

Where there is a known flood risk, it is good practice to design property flood resilience (PFR) measures into a conversion proposal. Additional consideration should be given where a basement conversion is proposed due to the potentially higher risk and impact of flooding on the building and its occupants.

3.3.2 Commentary

Flooding is one of the most severe environmental hazards Scotland faces, having the potential to pose significant risks to buildings and their occupants. Flooding is diverse, often site-specific, and can occur due to a range of factors. The impacts of flooding on a traditional building can include significant damage to materials, services and structure. Contamination from a range of sources is also likely to be present in the floodwater. Traditional solid wall construction inherently offers greater flood resistance, resilience and recoverability. The use of compatible materials and approaches can maintain and enhance these in order to achieve compliance. It is important, therefore, to understand the existing resilience of a building, the specific flood risks and the need to balance adaptation with protecting the building’s cultural significance.

Greater intervention may be necessary where flooding is likely to result in the loss of historic fabric, or impact on a building’s cultural significance. In this circumstance, archaeological recording is also recommended. The conversion of traditional buildings can result in maladaptation and increased risk to historic fabric and the health of occupants. To avoid these issues and inform best practice when developing a conversion scheme, the standard offers a level of flexibility to utilise solutions outside of the Technical Handbooks. Flooding cannot be prevented, and therefore sensitive adaptation of buildings of cultural significance will be necessary with consideration of appropriate compatible materials and suitable property flood resilience (PFR) measures.

Buildings and settlements of traditional construction, especially vernacular buildings, were in some cases sited to reduce flood risk, whilst others along rivers and in coastal locations have historically flooded and recovered. Some buildings, such as watermills and net sheds in fishing villages, were built and used in a way that accommodated periodic flooding of the lower floors.

Alongside changing climate, various factors can increase the risk of flooding to a property. These might include increased areas of hardstanding, changing land management or poorly considered alterations to the surrounding built environment that have occurred over time. In some cases where regular flooding and recovery has occurred, the natural moisture and vapour permeable materials used in buildings of traditional construction have been seen to safely tolerate and recover from extreme wetting conditions. The building standards highlight that both internal and external solid wall construction can provide an effective method of minimising water penetration.

Significant damage to materials, services and structures can occur during and following a flood event. This is especially the case where the drying process is inhibited or is not carried out with an understanding of fabric performance (for example, overly rapid drying or lack of ventilation). Buildings of traditional construction offer greater flood resistance and resilience when well maintained and where lime plasters and renders are in place. The standards assert that the correct detailing and material selection for external walls will assist in making the building more resistant and recoverable to flooding. Conversion work may provide the opportunity to reinstate systems, materials and surfaces that will increase this resilience and recoverability. Annex 3.B of the Non-Domestic Technical Handbook and Annex 3.B of the Domestic Technical Handbook indicate the preference for lime plasters over gypsum to meet the standard and the use of lime mortar, plasters and renders has been shown to aid drying of the construction and increase the recoverability of a building at risk of flooding.

a) Flood risk assessment

Any site-specific risk of flooding to a building or its occupants should be identified and assessed to allow sustainable design mitigation. Flood risk assessments (FRA) are an integral part of the design and construction process with the appraisal also considering the effects that development may have on adjoining ground. The FRA is an opportunity to understand the risks to traditional building fabric and building contents in service of their care and protection.

Building standards advice on Flooding in Annex 3.B of the Non-Domestic Technical Handbook and Annex 3.B of the Domestic Technical Handbook provide further detailed advice on flooding and demonstrating compliance with Standard 3.3.

The Scottish Environment Protection Agency (SEPA) provides flood risk information on their indicative river and coastal interactive flood maps on their website.

b) Resilient conversion in flood risk areas

Where site-specific flood risks are identified that can affect a traditional building, it is important that the existing fabric’s resilience to flooding and its capability for recovery is fully understood. This should inform any construction and conversion proposals which should aim to design in greater resistance and resilience for the future. Different forms of flooding, individual construction variables and existing defects specific to the building to be converted will all contribute to its vulnerability.

Property flood resilience (PFR) methods; protection, mitigation and adaptation for traditional buildings require careful consideration. Some measures are visually intrusive or use incompatible materials which can slow or prevent drying and the building’s recovery. Practitioners should be aware that the use of traditional materials will enable compliance to be achieved. Where a building is culturally significant, a balance needs to be struck between protection and adaptation and retaining significant historic fabric. Flood events can lead to extensive loss of fabric and therefore a practitioner should consider the appropriate balance of risk and adaptation. Elements such as timber floorboards, panelling and plasterwork can be affected by flooding and are sometimes damaged, removed or destroyed during the clean-up and drying phases.

3.3.3 Issues to be considered

Typical risks in the conversion of historic/traditional buildings

1. Non-traditional materials and historic alterations

Historic damp-proofing works, installation of services and the use of hard cementitious mortar for repointing or render will make a traditional building more vulnerable to flooding and reduce recoverability.

Applying impervious render, or other waterproofing material to the internal or external face will trap moisture behind the surface and reduce drying capabilities.

Applying impervious insulation materials and systems, internally or externally will trap moisture and could impact drying and recoverability. Gypsum plasters will often crumble or disintegrate; lime plasters are softer and will normally recover when dried.

Gypsum plasterboards are rarely recoverable after flood damage, whilst modern external paints can inhibit drying out of fabric, can blister or peel following saturation.

Failed or poorly installed French drains can increase risk from ground level moisture which is then exacerbated by flooding. Historic watercourses, storage or drainage routes such as culverts, moats, streams, or reservoirs can increase vulnerability to flooding events.

Penetrations through walls and poorly installed modern services can all increase vulnerability and likelihood of loss to historic fabric.

Organic materials used in traditional construction can be more vulnerable to rot if left damp for long periods.

2. Flood resistance and recoverability measures harmful to significance

Raising floor levels may have a significant impact on historic character and fabric and may require additional permissions if a building is listed.

Flood gates, doors and barriers can be visually intrusive and require fixing into historic fabric. These should be well designed to be effective and reduce risk to the building. Excluding flood water up to 600mm to meet the standard may cause flooding elsewhere or increase damage to historic fabric.

In some flood conditions, opening doors and windows to allow water to flow through and reduce structural pressure on a building will potentially offer a more sensitive and low risk solution. Allowing water (especially ground water) to enter a building which has been designed to be flood recoverable may reduce wider risks to the building. However, this approach may require alteration to timber panelling and/or floors. Allowing for historic timber to be demountable or easily removed for drying should be considered to manage risk of total loss.

The installation of sump pumps has the potential to impact on buildings of greater cultural significance. The potential positive impact of removing water quickly minimising damage to the wider building should be balanced against the impact of installation.

3. Physical damage and loss

Flooding can cause serious structural instability in buildings where significant washing-away of supporting ground occurs, or where the property has been subjected to heavy storm seas or fast-travelling heavy flotsam. Fast-flowing water from hillsides, landslips and breaching of sea walls can all cause significant harm or loss. Where fabric is of cultural significance it should be assessed for likely harm, erosion or total loss from flooding and subject to recording if this is likely.

Where there is a substantial flood risk leading to loss or harm to culturally significant building fabric, it may be necessary to increase the resilience of the building to ensure its long-term use and viability. An understanding of a building’s cultural significance and the natural resilience of its construction should be used to develop mitigation and adaptation strategies.

4. Drying out

Remedial work can be more damaging to traditional construction than the flood itself. Ventilation and heating should be designed to aid sensitive drying of the property. Very extensive decontamination and cleaning work may result in the loss of historic fabric.

Overly rapid heating and ventilation to aid drying can result in harm and failure of materials and historic surfaces. Timber elements such as floors, doors and paneling can deform and expand when wet and split when drying. Stone and plaster work can be vulnerable to the migration of salts creating blistering and exfoliation, painted surfaces can also peel and flake.

Overly slow drying out can increase likelihood of fungal attack, rot and mould. Insect activity can also be reactivated or newly introduced into building elements that remain at high moisture content.

3.3.4 Recommendations to meet the standard

Compliance with this standard can be achieved using solutions that are particular to traditional buildings and differ from those set out in the Technical Handbook. Flexibility within this standard allows compliance to be achieved with compatible and appropriate measures and materials.

a) Flood resistance and flood proofing measures

Flood resistance measures need to be considered in relation to the nature of the flood risk including their regularity, duration, speed, cause and source. For example, surface-water flooding can occur very rapidly with little warning, allowing no time to put temporary measures in place, whereas the likelihood of river and coastal flooding is much more predictable.

There is a growing range of flood protection products available, and the National Flood Forum Blue Pages scheme managed by the British Standards Institution awards kitemarks to approved proprietary products. Alternatively, purpose-made solutions can be considered, which can perhaps be incorporated more sensitively into older buildings.

b) Temporary barriers and permanent barriers

There are many products on the market which can be used to temporarily protect a building against flooding events. Sandbags are the most basic, but more specialised products including air-brick covers which simply clip into place, and plastic or metal boards used to seal openings such as doors and windows.

These barriers typically slide into a frame which is permanently fixed to the openings. On a larger scale, flood skirts are wrapped around the walls of buildings to prevent water seeping through porous materials.

Barriers can be fixed at a distance from a property to hold back water or divert it away from the building. However, these will not prevent seepage of groundwater into the foundations, cellar or basement or prevent backflow through overloaded drains.

Backflow can be prevented by fitting non-return valves and bungs to drainage systems and by sealing plugholes and toilet bowls. These measures all require adequate warning if they are to be effective. Maintenance of drainage around the property is important; drainage systems should be regularly checked to ensure they are not defective or obstructed.

Adapting traditional buildings for flood risk should be approached carefully and using the whole building approach and guidance in this document. The replacement of timber doors or floorboards with waterproof alternatives (e.g. plastic and concrete) can impact on resilience, performance and cultural significance. Conversion may provide the ideal opportunity to rewire a building with flood resilience in mind. Standard details for services and plant located in flood zones should be considered. This should include locating services higher up walls and choosing plant and equipment locations carefully, including the use of plinths where higher locations are not feasible. Permanent flood barriers and sump pumps can be installed and, where appropriate, boundaries such as walls can all be utilised for protective measures.

Ventilation and heating should be designed to aid sensitive drying of a property.

c) Flood alarms

For properties close to rivers or watercourses, private flood level alarm systems can provide an audible alarm warning when water levels are raised or flooding is likely to occur, providing time to install temporary PFR measures.

d) Sustainable Drainage Systems (SuDS)

These improve the drainage of rainfall and surface water that accumulates around buildings to reduce possible flooding. There are various SuDS options you can install on your property, including water butts and retention ponds.

3.3.5 Further reading

Additional guidance on flooding and groundwater can be found in the following documents:

• Annex 3.B of the Non-Domestic Technical Handbook – Building Standards advice on flooding.
• Annex 3.B of Domestic Technical Handbook – Building Standards advice on flooding.
• British Standards Institution (2021) BS 5250: 2021 Management of moisture in buildings – Code of Practice
• British Standards Institution (2013) BS 7913 Guide to the conservation of historic buildings.
• British Standards Institution (2022) BS 8102 Protection of below ground structures against water ingress – Code of practice
• British Standards Institution (2025) BS 85500: 2025 Flood resilient construction.
• CIRIA Code of Practice for property flood resilience: C790.
• Defending Scotland's Heritage – Climate facts
• Department for Communities (2025) Flooding Guidance for Historic Buildings
• Historic Scotland (2014) INFORM: Flood Damage to Traditional Buildings
• Pickles, D (2015) Flooding and historic Buildings Historic England
• RICS, Historic England & PCA (2022) Investigation of moisture and its effects on traditional buildings
• Ridout, B, Wood, C, & McCaig, I. (2017) A Preliminary Study of Flood Remediation in Hebden Bridge and Appleby, Historic England.
• SPAB (2018) Technical Advice Note: Control of Dampness.