Resistance Measures

Resistance measures should only be introduced where they will not cause structural issues during a flood. This is particularly relevant where floodwater is estimated at 600mm or more, or where timber-framed or earthen buildings are set on low-level plinths.

Before any resistance measures are installed, make sure the building is not at risk from groundwater flooding. Resistance measures could end up acting as a bund, keeping the water inside the building for longer and causing more problems.

It is essential to consider the impact, benefit and potential to fail of all resistance measures.

Flood barriers

Flood barriers can be effective where a building is continuously occupied and the height of the water does not exceed 600mm. If barriers are to be installed above 600mm, a structural survey of the building should be carried out. If a building is not continuously occupied, such as a holiday cottage, the barrier would need to be permanent or there would need to be a plan in place to install the barrier when a flood is forecast.

For timber-framed buildings and earthen structures, barriers should not be installed above the plinth line. This is because the junction between the plinth and the building above is a structurally weak point, and in some circumstances will have minimal or no ties.

Flood barriers can be positioned in front of doors or windows, and along boundary walls and gates. They can be temporary or permanent. However, they will only be effective if the external drainage and internal plumbing are protected by a non-return valve, otherwise water will enter via these routes.

Flood doors

A flood door is a custom-designed door to protect a building from floodwater. It offers a more permanent solution because it does not require the building to be occupied.

The Environment Agency's guidance states that flood doors have a designated maximum water depth, specified by BS 851188-1 Flood resistance products – Building products: Specification.

As previously mentioned, the hydrostatic pressure exerted by floodwater can cause sudden or long-term structural damage to a building, unless it has been specifically designed to withstand such pressure. Typically, most residential buildings are not structurally capable of preventing floodwater above 600mm. Where it is predicted that floodwater will exceed 600mm, an appropriately qualified structural engineer should inspect the walls and give advice about which flood resistance measures can be used.

Installing a flood door may mean replacing a historic door. The potential impact of a flood on significant historic fabric, fixtures and fittings should be assessed against the harm of losing a historic door. If a historic door is in poor condition and requires substantial repair, it may be more sustainable and cost effective to replace it with a flood door of an appropriate style. Alternatively, if the existing door and its aperture have the scope to include a secondary door – to prevent loss of the original historic door but improve flood resistance – this option should be discussed with the property owner and conservation officer.

Where an original door is hardwood and well maintained, with a close fit to its frame, it can be as effective at withstanding a flood as a flood door. However, without testing, it is not known to what depth and duration the door is able to provide protection.

Composite, UPVC or softwood doors are more susceptible to failure because their materials are highly absorbent – and they will struggle to dry after a flood. This means the material will eventually disintegrate and need to be replaced. In this situation, a flood door is the best option.

If no original door is present, a flood door is recommended to improve the building's resistance to floodwater. The style, material and colour will need to take into consideration the age of the building and, in some situations, neighbouring properties. The design should be discussed with the local authority to determine appropriateness; it should be noted that presently there are limited suitable materials and products that comply with BS 851188.

Anti-flood airbricks and airbrick covers

Smart airbricks are a passive measure that can replace existing air bricks. They will shut off the ventilation holes when floodwater rises but will require being opened again once the flood event has passed.

Alternatively, airbrick covers can be permanent or demountable, and they can prevent water entering underfloor voids. After a flood, demountable covers can be removed to enable proper ventilation. Permanent ones will need to be actively opened and cleaned.

Non-return valves

These valves are fitted on outlet pipes to prevent the backflow from wastewater and sewer pipes. They are easy to install, but they can get blocked.

If a building is built on an area of silt or mud, which particularly occurs around the mouth of an estuary, the non-return valves are more likely to get blocked. In this situation, consider installing open or easy access drains and internal fittings such as toilet pan seals or bungs.

Damp proofing

Damp-proofing products should not be used in buildings of traditional construction because they have been shown to cause decay. Such products contribute to damp, mould and decay of building fabric via rotting timbers and spalling masonry. Sometimes, the harm is not obvious until it causes structural damage or health issues. In most situations, damp-proofing products will have to be removed eventually, once the harm they have caused is discovered.

Sump and pump systems

Installing a sump and pump system in a basement or under a timber suspended floor – to pump water back outside – is an effective way to remove floodwater.

The system needs to be sized appropriately to make sure it can cope with the volume of water. It should be maintained regularly and checked after each flood event.

Furthermore, the power supply to the pump needs to be rated to withstand the ingress of water, and there needs to be a back-up power supply above the flood level so the pump can still operate if the mains power supply is lost.