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Planning Advice Note PAN 69
Planning and Building Standards Advice on Flooding
WATER RESISTANT MATERIALS AND FORMS OF CONSTRUCTION
79. The use of water resistant materials and forms of construction can minimise the damage done by flood water but "will not be sufficient to make a development acceptable when the probability of flooding indicates that it should not be approved in principle." (SPP 7 paragraph 32.) They are most likely to be applicable for alterations and small scale extensions and for redevelopment within built-up areas where there is a risk of flooding. They may be relevant even if a flood prevention scheme exists or is anticipated. They are also likely to be applicable when buildings are being reinstated following a flood.
80. Water resistant materials and forms of construction may be a material consideration and a requirement of planning permission. Where it is intended to develop in areas that may be at some risk of flooding, guidance on precautionary measures that can be taken is given in 'Design Guidance on Flood Damage to Dwellings, 1996 published by HMSO. This document describes the likely effects of flooding on materials and elements of the building and assesses various forms of construction and measures to reduce the risk of flood damage in dwellings. Although written primarily for dwellings it is also relevant to all small buildings. Further guidance can be obtained from Preparing for Floods, 2002, available free of charge from the Office of the Deputy Prime Minister ( www.odpm.gov.uk).
81. This section explains how water affects different materials and forms of construction, and indicates the factors to consider in the choice of materials and how the requirements of the Building Standards can be met.
EFFECTS OF FLOODWATER ON BUILDINGS
82. Floodwater can penetrate buildings rapidly, causing widespread damage to floors, walls, finishes and services, and structural damage in more severe floods. The vulnerability of individual buildings is dependent on construction methods and building materials. For example, poor construction techniques and some common bricks are very porous allowing water to penetrate very quickly to the building interior. Inside the building, gypsum based plasters (e.g. most plasterboard) absorb large quantities of water and distort within minutes of contact with water. Even with measures to flood proof buildings, water will tend to find its way through weak points within the wall such as cracks and voids in the mortar jointing, brickwork or rendering. For semi-detached and terraced houses floodwater may also seep through party walls with neighbouring properties, above or below floor level.
83. The processes and pathways by which water enters a building during a flood depend on the characteristics of the flood - specifically flood depth and duration, and water velocity. Groundwater flooding results in water entering cellars and voids beneath floors causing problems of damp in walls. In general terms:
- Shallow floods will penetrate "weak" points in the building such as air vents and cracks in brickwork, and will overtop doorsteps. The use of flood barriers such as sandbags or proprietary flood proofing systems will merely delay the penetration of water.
- Deeper floods and faster flowing water are likely to penetrate the structure of buildings more quickly. Flood water will enter buildings through a larger number of pathways including drainage pipes from downstairs toilets and baths and even windows that may be broken due to the pressure of water or debris.
- Where flood depths exceed 1m there is a risk of structural damage and collapse, particularly if the water exerts pressure on only one side of a wall.
SECONDARY EFFECTS OF FLOODING
84. There are also secondary effects of floodwater on building structure and the health of the occupants. These impacts include:
- Contamination by sewage and the sediments from both watercourses and blocked drains. Watercourse, coastal and sewer flooding can lead to the contamination of flooded properties. In the case of sewer flooding, raw sewage can be deposited on affected sites. Following a flood, external walls will be dirty and may be permanently stained if not cleaned. Contaminated sediments may be deposited on site and these must be removed.
- Damp conditions following a flood may lead to the growth of moulds that can damage the building and present a health hazard. Buildings with excess moisture, poor ventilation and those exposed to standing floodwater can be breeding grounds for moulds. All moulds have the potential to cause health impacts, such as mild to severe allergic reactions and breathing difficulties for asthmatics.
- Coastal or estuarine flooding can lead to salt water damage such as the corrosion of metal fittings including metal ducting and switch boxes, and steel reinforcement within reinforced concrete.
- Flood damage can also result from the impact of debris, corrosion due to chemical contaminants, changing hydrostatic pressure due to waves, pressure from breaking waves, lift due to the buoyancy of the property and scour undermining the foundations.
FLOOD RESISTANT AND RESILIENT MATERIALS
85. Building design can incorporate materials that are either flood resistant (i.e. unaffected by flood water), such as plastics, or materials that are resilient (i.e. they recover relatively undamaged from flooding) . Some materials, such as concrete, can be both resistant and resilient, for example solid concrete floors can provide an effective seal against water rising up through the floor, provided they are adequately designed. In addition solid concrete floors generally suffer less damage than suspended floors and are less expensive and faster to restore following exposure to floodwater.
86. Good workmanship and maintenance will limit the amount of water that can seep through masonry walls. Re-pointing may be needed. Water-resistant coatings applied to porous surfaces will also fill any cracks in the masonry or the joints. To avoid trapping water in the wall however, any such coatings should be micro-porous to allow the wall to 'breathe'.
Table 2: Summary of the potential effects of exposure to flood water
Material | General Effects |
Masonry, Concrete and Brick | In general masonry and concrete are unlikely to be severely damaged by contact with floodwater. In the case of coastal flooding, salt water may cause surface powdering and flaking of soft brickwork. Lightweight concrete may expand and contract depending on moisture content so wetting and drying may cause some cracking. |
Timber | Timber swells and may distort on wetting. In timber framed buildings, swelling of immersed members could cause damage in other parts of the structure, e.g. through stresses on external cladding. Timbers that become wet and cannot dry may be at risk of decay in the long term. Guidance on the selection of timber preservatives is provided in BS5268: Part 5:1989 for structural timber and BS 1186 Part1:1991 for joinery. |
Wall finishes | Renderings containing cement are unlikely to suffer damage. Lime based plasters are preferable to gypsum which softens when wet. Similarly, following flooding, any plasterboard will probably be damaged beyond repair and require to be removed. |
Metals | Metals are affected by the corrosive effects of sea water so resistant metals rather than mild steel should be used in coastal areas where flood risk is an issue. |
Insulation | Close cell insulants will not absorb water but may restrict drying out of a cavity wall. Mineral fibre and other absorptive insulants will retain water and can lose their insulating properties or disintegrate over time. |
Source - Design Guidance on Flood Damage to Dwellings, The Scottish Office, 1996
87. Regulation 8 of the Building (Scotland) Regulations 2004 relates to the durability, workmanship and fitness of materials:
(1) Work to every building designed, constructed and provided with services, fittings and equipment to meet a requirement of regulations 9 to 12 must be carried out in a technically proper and workmanlike manner, and the materials used must be durable and fit for their intended purpose.
(2) All materials, services, fittings and equipment used to comply with a requirement of the regulations 9 to 12 must, so far as reasonably practicable, be sufficiently accessible to enable any necessary maintenance or repair work to be carried out.
88. Regulation 8 requires that materials, fittings and components used in the construction of buildings should be suitable for their purpose, correctly used or applied, and sufficiently durable, taking account of normal maintenance practices, to meet the requirements of the building regulations. For example, external timber cladding for low-rise buildings that is readily accessible and replaceable need not be as durable as that which is to be used at a higher level on medium rise buildings.
89. It also implements the intention of the Construction Products Directive, that specification of construction products should not be used to effectively bar the use of construction products or processes from other European countries. The relevant countries are those in the European Union, and those who in the European Economic Area Act of 1993 agreed to adopt the same standards
90. The Technical Handbooks also provide guidance on fitness of materials and workmanship. Traditional constructions, when supported by adequate technical descriptions, may also be appropriate.
91. The EC is introducing durability requirements into European Standards (ENs) for construction products. Durability has been defined by the EC as the ability of a building material, fitting, component, or part thereof to perform its required function over a period of time and under influence of agents. 'Agents' are factors that may affect the durability of a product and include: exposure conditions, temperature, humidity, water, UV radiation, abrasion, chemical attack, biological attack, corrosion, weathering, frost, freeze-thaw, and fatigue.
92. The ABI has recently prepared guidance on flood resilient homes and the cost and effect on future insurance claims of installing flood damage resistant measures. Although prepared in an English context, this information is likely to be relevant UK-wide, and is available at www.abi.org.uk/Display/File/Child/228/Flood-Resilient-Homes.pdf and www.abi.org.uk/Display/File/78/Flood-Resistance-report.pdf
CONSTRUCTION TECHNIQUES
93. There are two basic approaches that may be appropriate for the protection of buildings against the effects of flooding:
- Dry proofing aims to prevent flood water from entering a building. It relies on the use of waterproof barriers integral to the structure, across entrances and non-return valves on drains. Dry proofing a building however can be difficult and is unlikely to work if buildings are subject to flooding for long periods. Simple measures are unlikely to prevent water penetration for more than a few hours while more complex solutions may protect the building for a day or two.
- Wet proofing assumes water will enter the building and is based on the use of water resistant and resilient materials within the building and the raising of electrical wiring and sockets above the maximum flood level. This is the most practical approach and there are a number of ways to limit the damage from flooding. Wet proof construction should also allow water to drain easily from the building following a flood and not retain it in walls, floors and air pockets within the building footprint.
94. Further advice on the suitability of different materials can be found in Preparing for Floods, February 2002 published by ODPM. Some general comments on suitable and less suitable materials are provided in this Advice Note but the appropriateness of specific measures will depend on site and building characteristics and developers should seek expert advice when flood risk is an issue.
Building components
Component | Most suitable | Suitable | Unsuitable |
Flooring | Concrete, pre-cast or in situ | Timber floor, fully sealed, use of marine plywood. | Untreated timber
Chipboard |
Floor covering | Clay tiles
Rubber sheet floors
Vinyl sheet floors | Vinyl tiles
Ceramic tiles | |
External walls (to maximum flood level) | Engineering brick
Reinforced concrete | Low water absorption brick | Large window openings |
Doors | Solid panels with waterproof adhesives
Aluminium, plastic or steel | Epoxy sealed doors | Hollow core plywood doors |
Internal partitions | Brick with
waterproof mortar
Lime based plasters | Common bricks | Chipboard
Fibreboard panels
Plasterboard
Gypsum plaster |
Insulation | Foam or closed cell types | Reflective insulation | Open cell fibres |
Windows | Plastic, metal | Epoxy sealed timber with waterproof glues and steel or brass fittings. | Timber with PVA glues and mild steel fittings |
Box 7 BUILDING AND FLOODING CHECK LIST: Factors to consider before building in areas where flood risk is an issue 1. Background information - If flooding is an issue:
- Is the source of floodwater from watercourses, coastal waters, groundwater or sewers?
- If the source of flooding is from watercourses or coastal waters, is the annual probability of flooding greater than 0.5% (1:200)?
- What is the maximum flood level in the 0.5% event?
- Would the site be inundated rapidly, for example due to a breach in a flood defence, or slowly, for example in the case of groundwater flooding?
2. General issues (adapted from ABI, Assessment of the cost and effect on future claims of installing flood damage resistant measures) - Records of previous flood levels.
- Ground conditions - e.g. permeability and provision of field drains.
- Floodwater pathways into the building - e.g. poorly maintained masonry, ventilation grilles, doors.
3. Floors - Where possible use dense concrete screeds on solid concrete floor slabs.
- Use treated timber to protect it from rotting if exposed to standing water.
- Use steel joists and wall plates rather than timber.
- Use a damp proofing material around the ends of floor joists.
- Use a sump and pumping system in buildings at risk of groundwater flooding.
- Replace expensive flooring, such as Oak floorboards with treated timber boards.
- Raise floor levels.
4. Walls - Install air bricks above expected flood level and duct down to solum.
- Use close cell insulation.
- Replace gypsum plaster with more water resistant materials.
- Fix plasterboard horizontally.
5. Interiors - Replace door hinges with butt hinges that allow door to be removed and placed in a dry area prior to a flood.
- Fit kitchen units with extendable plastic or stainless steel feet so that they will not be damaged by shallow flooding.
- Use raised fitted ovens and fit above highest expected flood level.
6. Services - Move service meters at least 1 metre (preferably more) above the 0.5% (1:200) flood level if this is known.
- Consider completing electrical wiring from the first floor of the property so it will not require replacement if the property is flooded.
- Can one way valves be placed in drainage pipes to prevent water backing up pipes into the building?
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Timber Floors and Insulation
95. Timber joists and floor boards can be treated with preservative to ensure they are more water resistant and to permit them to dry out faster after having been immersed in floodwater. Should joists need to be replaced they can, subject to professional advice, be supported on hangers rather than being built into walls. This can help to minimise the risk of future distortion following wetting and drying.
96. Many suspended timber floors in modern buildings have mineral wool insulation between the timber joists. This material has poor flood resistance closed cell insulation should be used.
Drying out
97. If suspended floors are used, some form of access for inspection and drying out of the underfloor area should be provided. The use of a sump and an identified drainage point where a pump can be installed to drain any flood water should also be considered.
Solid concrete floors
98. In flood prone areas solid concrete floors are generally considered more resistant because they prevent water accumulation beneath the floor level. The concrete has to be of good quality and the damp-proof membrane fully sealed and connected to the damp-proof course rather than just lapped. It may be advisable to raise ground floor levels above the expected flood levels and this could affect other issues such as accessibility. Early discussion with the Planning Authority and Building Standards is encouraged for any site where there may be conflicts between planning and building standards requirements.
TEMPORARY FLOOD BARRIERS
99. Temporary flood barriers will help to reduce the rate at which floodwater comes into a building but will not offer complete protection from flooding and are not an alternative to robust building techniques. Temporary free-standing barriers typically hold water back from a group of buildings or a detached building. In most cases additional pumps may be necessary to remove floodwater that may seep around a barrier. Temporary removable household flood products fitted to individual buildings help prevent water entering through doors, windows, air-bricks and vent covers. Floodwater may be substantially prevented from entering through building fabric with a flood 'skirt' which is designed to wrap around a building. The maximum height for a temporary flood product fitted to a building should be no greater than 1 metre above the ground surrounding the building. Temporary barriers installed across gates or driveways may need planning permission.
100. It is important that temporary barriers are fitted properly and in good time in preparation for a flood. In upland areas the lead time between flood warning and the flood event may be short so it is important that any barriers are available for rapid deployment and are simple enough to be installed by the home owner. Several temporary flood protection products have successfully achieved national standard performance criteria (BSI 'Kitemark' standard). For information see the SEPA website www.sepa.org.uk/flooding/protection/index.htm
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