Building Construction

When in good condition, a building's external facade can prevent water ingress from flooding and improve resilience to wind-driven rain. It is, therefore, important to ensure masonry is kept in a good state, with minimal spalling or cracking, and that appropriate lime mortars and renders are used for repairs. If the masonry is particularly historically significant, an assessment of the building's vulnerability to flooding should inform appropriate repairs or adaptation.

In both modern and traditional constructions, the junction between materials is often the failure point, rather than the materials themselves. For example, cracked render is a weak point for water from rainfall or flooding to track between materials. However, solid permeable walls in good condition, with appropriate lime mortars, can slow floodwater from passing through the substrate. They can also accelerate drying due to the superior durability and intimacy of the bond between the traditional mortar and masonry unit.

Where services such as water and waste pipes penetrate external walls, it is important to ensure they are adequately sealed using compatible and appropriate materials, such as sleeves, oakum or lime mortar.

Draughts can help identify potential entry points for water. Where air is able to move freely, water can too.

Doors and windows are liable to warp, shrink and move over time due to changes in seasonal and environmental conditions. Ensuring they are in good condition and edge-sealed will improve energy efficiency and prevent water from entering via any gaps. Old solid timber doors have a degree of resistance to water ingress because the tight grain system slows the passage of moisture. Younger modern timbers can become saturated quickly due to the more open grain system.

Weak points that impact both heat loss and moisture ingress can be identified using a thermal imaging camera.

The immediate external ground level around a building will change its vulnerability to flooding. Historically, buildings were constructed with their floor finishes raised above the external ground level. However, we now often find that external ground levels have risen, due to lack of maintenance or because they have been altered over time with no consideration to the transfer of moisture from the outside environment to the inside of the building. The difference may also be caused by environmental conditions, such as changes in the landscape, or human action (for example the introduction of services, paths or roads). Decay or water ingress can occur as a result of raised ground levels or where the landscape falls towards the building.

Where possible and appropriate, altering the ground level can increase flood resilience. Reducing its height by as little as 150mm can reduce the amount of water entering a building. However, there may be barriers to lowering the ground level. Examples include owner consent, building services, archaeology, exposing limited or no foundations, and building type.

Alternatively, water can be slowed. Displacement of water can be improved by using permeable surfaces, natural solutions such as sustainable drainage systems and good drainage. Where hard impermeable surfaces are present, they may act as a funnel and direct the water towards a vulnerable area of the building.

In this section, drainage refers to rainwater goods, underground drainage, culverts, septic tanks, soakaways and French drains.

Faulty or inadequate rainwater goods are a common cause of water ingress in a building. Where blocked, or where joints or seals are faulty, water will not be able to flow freely away or into an underground drain. Due to an increase in heavy rain events, upgrading or adding capacity to the rainwater goods should be considered.

The role of underground drains is often overlooked, but the consequences of failure can be devastating. Failure can be caused by a heavy load on top of the drain, blockages, breakages from tree roots and lack of maintenance. A CCTV inspection can identify any issues that need attention. Clearing out underground drains will improve their resilience to surface water flooding and may prevent potential structural issues.

Perimeter land drains, encased in a membrane and fines-free shingle and known as French drains, are suited to agricultural use. They are common where external ground levels cannot be lowered, but their effectiveness is not always guaranteed. When installed close to a building's foundations, they may concentrate water at the base of the walls. This may increase the risk of water penetration or undermine the stability of the foundations. High water tables can interfere with the effectiveness of French drains and make them redundant, as can having a heavy clay soil content. French drains are also difficult to maintain and keep free of blockages.

Street drainage, trench drains or private culverts can significantly impact the flood resilience of a building. If they are not kept clear, they can cause basement flooding and increase the likelihood of surface water flooding.

In this section plumbing means the above-ground waste pipes from toilets, baths and sinks and the pipes that supply water for drinking, washing, and heating and hot water systems. This guidance focuses on flooding from the environment, however it is worth noting that 60 per cent of insurance claims are made because of faulty plumbing.

Plumbing should be kept in good condition. Pipes should be insulated, and repairs done as soon as a fault is found. These measures will reduce the likelihood of rot and decay occurring as a result of long-term leakage.

Non-return valves should be fitted, where appropriate, in existing chambers to prevent water surging back through the drainage system during a flood. An automatic gate will be deployed by the hydraulic pressure of the water. Otherwise, valves will be fully open and will allow solids to flow freely when the building is not at risk of flooding.

Hydrostatic pressure is the vertical or horizontal force generated by water at rest on the surface of the building it is in contact with. It can cause serious structural damage to a building and its foundations. Other examples include the horizontal pressure exerted on a building's masonry, doors or windows, or on the plinth base of a cob or timber-framed structure.

Basements may experience external hydrostatic pressure on walls that are higher than the natural groundwater level. This could cause the walls to fail if they are retaining the ground behind. However, the heavier the building above, the less likely failure is to occur, because the vertical dead load provides a restoring force. It is more likely that the floor slab will be affected and break up.

Hydrodynamic pressure is the force of the water moving around a building. These loads include the frontal impact (direction of flow), the drag either side and the suction on the downstream side.

Buoyant forces are the vertical uplift of a structure due to the displacement of water.

It is unlikely that floodwater will lift a traditional masonry building, but it could raise a timber-framed building off its plinths. Stone slab or suspended timber floors could also be lifted. And concrete floor slabs with no top reinforcement may crack, allowing water in and causing the slab to corrode and ultimately fail.

Impact loads can be very destructive because their force is concentrated. Examples include the direct force associated with waves or the impact of debris in the floodwater.

New basements and the use of an existing basement is restricted within Section 14 of the NPPF depending on the Flood Zone they are within. Basements in Flood Zone 3a and 3b cannot be used as habitable spaces.

Those in Flood Zone 2 require an 'exception test' to be carried out before being adapted for residential occupation. Flood Zone 1 allows basements to be used for residential purposes where appropriate mitigation is in place.

When a basement is being used as a habitable space or is being proposed to become a habitable space, the building regulations apply to fire escape routes, ventilation, ceiling height, damp proofing, electrical wiring, water supplies, and risk to life.

Where a basement is being converted into a habitable space it will need to comply with The Party Wall Act 1996 and the building regulations. Items such as fire escape routes, ventilation, ceiling heights, electrical wiring, protection from moisture, drainage, water supplies and risk to life will need to be considered. Further guidance and recommendations are covered in more detail under BS 8102:2022 - Code of practice for protection of below ground structures against water ingress.

Non-habitable basements are likely to be storing archives, valuables, mechanical and electrical equipment or, in situations such as places of worship, may contain caskets and human remains.

Where possible, valuables and archives should be stored above the flood level, to prevent loss and the need for repairing and drying after the incident. Waterproof boxes can be an alternative option for storing items. Where your flood risk assessment has identified a high risk of flood occurring and/or increasing in frequency, removing the archives and valuables to another area or to an offsite store might be the most appropriate option.

Regarding burial crypts, ensuring coffins cannot be displaced via the force of the floodwater will both ensure that remains are preserved, and prevent health and safety concerns after a flood. Older coffins were often lined with lead, which can present a health risk. There might also be anthrax spores, airborne viruses and other illnesses that could be spread should the remains be exposed.

Depending on their intended use, non-habitable basements do not necessarily need to be tanked or have cavity drainage systems installed. If their walls and floors have been maintained with materials compatible with the original materials, they remain able to manage moisture. Ventilation should be provided above flood level to facilitate continuous air flow, this alone can manage any excess moisture and ensure that the basement is in a good state of repair.

Routine maintenance and repair of existing drains, installing land drains uphill from the building or taking back the earth are methods that should be considered prior to tanking.

The most effective way of dealing with lateral penetration of water to the interior of the basement is to remove the earth away from the exterior wall. However, this will not be possible in every situation as it can cause structural issues, could affect the significance of the building, or there might not be surrounding space to accommodate this due to adjoining structures.

Where conversion into a habitable basement is occurring in an area susceptible to flooding, compared to one with low groundwater issues, then mitigation measures may need to be put in place. There are two main ways of achieving this, via tanking or a drained cavity system.

Tanking involves installing a waterproof barrier to the internal or external surfaces of the basement structure to block the passage of water. Installing externally is less likely to be possible in existing basements due to access issues. It also has limitations as it is not possible to fully test if the system is working until the surrounding ground is backfilled. Waterproof coatings are unlikely to have long lifespans when buried, as they are exposed to high quantities of water and salts from the ground.

In addition, the use of waterproofing systems that could significantly increase the hydrostatic load on existing structures should be undertaken only after it is proved that this will not compromise the integrity of the building.

For these reasons any method of tanking for a traditionally constructed building or one of heritage value is not recommended.

A cavity drainage system, also known as drainage protection, manages water ingress via drainage channels and a specially designed sump and pump.

Cavity drainage systems will use waterproofing membranes applied to internal walls, floors, and in some circumstances ceilings. They are designed so that a cavity is created between the waterproof membrane and the internal surfaces of the basement structure. This cavity creates a free space for water to travel in and be directed, via channels, to a sump pump system, which then transports the water externally away from the building. Cavity drainage systems are reported to last a minimum of 30 years. However, proper installation and adequate maintenance occur will impact this.

Issues arise when these systems fail and allow water into the basement resulting in rot and mould. To prevent this maintenance is extremely important, to ensure the system is working effectively and continues to do so. Any impermeable membrane can potentially displace moisture to adjoining material.

The efficacy and longevity of cavity drainage systems in flood zones is impacted by the ability for the systems to still be functional after the flood event. Where these systems are applied to the inner surfaces, they rely on adhesion to resist the water pressure. This means that, where excessive pressure occurs, this can cause failure of the combined multilayer systems, which will impact the ability for the drying contractors and insurers to confirm that the system has returned to pre-incident condition, likely resulting in the need to remove and replace the whole system.

Ventilation should not be restricted. Where passive ventilation cannot be achieved, appropriately designed mechanical ventilation should be installed. What will be deemed appropriate will depend on the intended use of the space and should be in line with Approved Document F of the building regulations. The location of mechanical ventilation plant and its associated electrical supplies should be above the flood risk level and have a suitable IP rating if they need to operate during a flood and remain usable afterward. More information on building services in flood zones is covered in Building Services.