Flood mitigation solutions in buildings - Part Three

Barely a year seems to pass nowadays without flooding being a feature of the national news. It is an emotive subject due to the degree of devastation it can cause – not only can severe flooding endanger life, but even at moderate levels can lead to substantial damage and disruption to normal patterns of life.

Water source contamination and inhibited access are significant problems, coupled with the ease with which internal fixtures and decorations can be damaged irreparably. A flooded house can be uninhabitable for months, and subsequent insurance costs can rise considerably as a result. And now the UK government’s plans to overhaul their flood-cover agreement with the insurance industry are coming under scrutiny, over concerns that a significant proportion of the population could find their property not only uninsurable, but also un-mortgageable and, in extreme cases, unsellable. Fortunately there are a number of measures that can be taken to minimise both the risks and impact of flood damage, both for new development and for existing buildings.

Flood mitigation measures for refurbishment and repair (continued)

Mitigation measures, appropriate for a design flood depth of 0.3–0.6m,to be adopted for managing residual flood risk, are aimed either at resistance (keeping water out), resilience (to water damage), and/ or repair (of water damage).

For resistance, barriers or bunds can be considered. These may be either permanent landscaped features or bund walls (including for example around fuel storage tanks), or removable products for installation as temporary barriers across building apertures. These include, for example, flood boards on doors, or airbricks/ service ducts (although the NPPF Technical Guidance document advises that “temporary and demountable defences are not normally appropriate for new developments”). Similarly, temporary, freestanding barriers which are assembled close to, but not in contact with, buildings such as property flood skirt systems can be useful. Fences can be designed to include impermeable materials at the base, such as concrete planks or masonry dwarf walls. Drainage systems can incorporate double-sealed lock-down inspection chamber covers, and non-return valves (to prevent sewage backing-up) to BS EN 13564. Refer also to CIRIA publication C506 Low-cost options for prevention of flooding from sewers (1998). Sanitary and washing appliances should be sited above ground level (i.e. not in basements).

Although it can be argued that resistance measures such as those noted above have the undesirable effect of ‘moving the problem elsewhere’, they can of course be compensated by other measures, such as the creation of flood retention areas in places such as car parks or landscaping features.

Resistance and resilience measures can also pay a major role for:

• Water-compatible development (for example outdoor sports and recreation, or facilities such as changing rooms), and less vulnerable development (for example retail and restaurant buildings) where appropriate flood warning is provided and temporary disruption may be deemed acceptable
• Refurbishment of buildings (where there is no material change of use) that are already exposed to flooding, or are likely to be in the future
• Some material change of use proposals, where it can be demonstrated that no other flood risk management measures are practicable.

Resilience is defined as minimizing the impact that flood water has upon entry to a building, seeking to avoid permanent damage or loss of structural integrity; maintain pre-flood dimensions (e.g. timber swell) and improve the speed and convenience of drying and cleaning to avoid rot or mould decay.

“Flood-resilient buildings are designed to reduce the consequences of flooding and facilitate recovery from the effects of flooding sooner than conventional buildings ...“Resilient construction is favoured because it can be achieved more consistently and is less likely to encourage occupiers to remain in buildings that could be inundated by rapidly rising water levels.” NPPF Technical Guidance document

Floodwater is invariably contaminated in some form, and these contaminants can cause further damage to buildings and services, besides posing a threat to public health. Contaminants can include sewage, hydrocarbons, silt, salt and other biological or chemical substances, depending on the location of the flood and any unsecured hazards that may be present in the vicinity. Hydrocarbons are perhaps most commonly present in this manner in the form of petroleum, although asphalt and wood preservatives such as creosote are other sources for hydrocarbon pollution.

Apart from the obvious routes, water can enter buildings through many places. The wall and floor materials themselves can be permeable; concealed voids such as wall cavities and party walls; at junctions between e.g. suspended timber floors and walls; air bricks and other ventilators; inadequate or broken seals around window and door frames (including thresholds); weepholes in facing brickwork; services entry points; cracks in mortar or render; subfloor voids; inadequate or defective damp-proof membranes or tanking; and through sanitary or washing appliances via drainage backflow.

Resilience measures can take many forms, across many areas of the building:

• Floors – use ground-bearing solid concrete slabs in preference to suspended timber; specify ceramic, stone or concrete-based tiled surfaces to floors and skirtings (with cement-based adhesive and water-resistant grout) ideally draining to a floor sump pump; paint timber skirtings on the reverse before fitting; avoid concrete screeds above insulation as drying time of the insulation is increased considerably; damp-proof courses and membranes should be durable (minimum 1200 gauge for polythene) with particular attention paid to laps, and consider double-layer protection with cavity drains to retaining walls and basements; consider loose rugs in preference to fitted carpets for ease of removal and storage, as well as drying and replacement; specify closed-cell insulation to resist water absorption (but bear in mind that floor coverings will need to counteract the buoyancy of the insulant if submerged)
• Walls – use closed-cell insulation below predicted flood level in external walls; specify water-resistant walling materials such as pressed-face or engineering brick or rendered blockwork, use extended periscope subfloor ventilators or fit removable airbrick covers; fix plasterboard sheets horizontally rather than vertically, or split sheets mid-height with a dado rail, to reduce the extent of replacement; specify lime- or cement-based renovating plasters or renders rather than gypsum-based, with water-resistant paint finishes. The use of water-proof, water-resistant or micro-porous surface coatings on masonry should be viewed with caution; these have been seen in some instances to inhibit the drying-out of the building fabric, leading to further dampness-related problems internally, and their use is currently discouraged by the Brick Development Association
• Kitchens – specify plastic, solid wood or stainless steel for cupboards and housings, in preference to particle board or MDF; mount appliances above the predicted flood height; fit non-return valves to drains from washing machines and dishwashers; seal between and behind cupboards to minimise water penetration; specify low-porosity materials for work surfaces
• Doors and windows – specify PVC-U, aluminium (and aluminium-faced or foam-core door panels) or hardwood frames in preference to softwood; use loose-pin butt hinges to enable easy removal of internal doors for temporary storage above flood level; ensure that all frames are well sealed and gasketed
• Services – avoid (or minimise) any wiring below predicted flood level; fit all switches, socket outlets, service panels, meters etc. above predicted flood level; consider routing electrical ring main at first floor level with drops to ground floor; fit electrical cabling in surface trunking rather than chased-in to wall surfaces; install boilers and other heating or cooling equipment at first floor level (or as close to ground floor ceiling level as possible); protect communications wiring and other services with insulation within services ducts.

The following diagrams are adapted from tables 6.1 and 6.2 respectively, in the Communities and Local Government publication Improving the flood performance of new buildings (2007).

Flood resilience characteristics of building materials (based on laboratory tests)

Flood resilience characteristics of walls (based on laboratory tests)

Reference to the Communities and Local Government publication Improving the flood performance of new buildings (2007) is strongly advised in all cases.

Water entry

Where predicted flood depths could exceed 0.6m above the ground floor level, a ‘water entry’ strategy needs to be adopted, whereby water is allowed uninhibited access into (and out of) the building. This is because the structural integrity of cavity-walled masonry buildings in particular can be jeopardised, potentially leading to collapse, if the differential head (i.e. difference in water level between inside and outside) exceeds 0.6m. The resilience measures discussed above are applicable to this approach, the focus being on enabling drying and minimizing consequential repair of the building fabric. New products are now being brought to market including:

• flood-proof kitchen cupboards, made from waterproof materials
• retractable, sectional flood barriers which are permanently installed in critical locations, and which rise and fall automatically in response to water levels
• doors which are flood-proof up to a designated height, then allow water in, should this be exceeded. These have the advantage of combining the strategies and hence covering all bases.

Following a flood event, it is recommended that a thorough survey is carried out on the property to assess structural and services damage; as well as damage to finishes and fittings, and dimensional integrity.

Fortunately for the property owner, help is at hand in the form of extensive guidance and standards for the safe and robust repair of flood-damaged buildings. In addition, there are companies who specialise in such work, in addition to supplying and installing flood protection products and flood-avoidance management services. The Flood Protection Association represent companies and organisations involved in the manufacture, supply and installation of flood protection devices; while the National Flood Forum are a charity supporting the victims of flooding. In addition, the government has pledged help for homeowners affected by flooding, in the form of a £5,000 grant towards the cost of installing resilience and resistance measures to protect against future recurrences (this came in to force on 1 April 2014).

Standards and guidance for the repair of flood-damaged property include:

• Communities and Local Government publication BD2760 Guidance and standards for drying flood damaged buildings (2010)
• Communities and Local Government publication Preparing for floods: interim guidance for improving the flood resistance of domestic and small business properties (2003)
• PAS 64:2013 Mitigation and recovery of water damaged buildings. Code of practice
• CIRIA publication C623 Standards for the repair of buildings following flooding (2005)
• Flood Repairs Forum Repairing flooded buildings: an insurance industry guide to investigation and repair (2006)
• White I, O’Hare P, Lawson N, Garvin S and Connelly A Six steps to property level flood resilience – guidance for property owners
• Dhonau M, Wilson G, McHugh A and Burton, R Homeowners guide to flood resilience – a living document
• RICS A clear guide to flooding for property owners

Property owners’ buildings insurance policies should also be checked and claims submitted as a priority; depending on the finalisation of the Flood Re insurance scheme, this may have a significant bearing on the choices (and budget) available for repair. In addition, and perhaps most crucially, repair from flooding can present an ideal opportunity to upgrade with flood-resilient measures, although the balance between insurance and personal financing will need to be established at the initial claim stage.

There are also a number of standards which cover the specification of flood protection products:

• PAS 1188-1:2009 Flood protection products. Specification. Building aperture products
• PAS 1188-2:2009 Flood protection products. Specification. Temporary products
• PAS 1188-3:2009 Flood protection products. Specification. Building skirt systems
• PAS 1188-4:2009 Flood protection products. Specification. Demountable product

Looking to the future

As the pressure on land is only likely to increase, more innovative approaches to construction may have to be considered. Low-lying countries such as Holland have taken a more imaginative approach for a number of years now, and ‘floating’ structures are not uncommon. While tethered or anchored in position, they are able to rise and fall in response to water levels. At least one UK example is currently under construction on a riverside site in the south of England, and a prototype flood-resilient house is being developed at the Building Research Establishment (BRE).

While rare examples are starting to appear in the UK, they remain largely at the theoretical level at present due to their highly unconventional technology and consequentially-higher construction costs. In addition, detailed consultation with both the Environment Agency and the relevant LLFA will be needed from the earliest conceptual stages of a project. But as the pressure on land for development is only likely to increase, we are going to have to evolve new solutions to deal with flood risk.

This article was adapted from a series on Climate Change Adaptation in Buildings.

Parts in this series

This article is Part Three of a series - 'Flood mitigation solutions in buildings'. Links to other parts in the series are provided below: