The floods in December 2015 in Cumbria, Northumberland and Southern Scotland, continued to destroy people’s lives and property across the region. Possessions, homes, and businesses were destroyed in a 48 hour period, and as the flood water receded the full extent of damage was seen for the first time.
Infrastructure was destroyed, bridges structurally unsound, homes and businesses washed through with the flood waters. Some damage however can be hidden, in particular in older houses, and in this post we will explain some of the effects it can have on buildings.
Effects of floodwaters on buildings
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.
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 floodwater
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.
In subsequent posts, we will address the issue of appointed a contractor to undertake remedial works, insurance and monitoring of your buildings health post floods.