Prevention is still better than cure

Last winter was dry compared with previous winters, but that’s no reason to be complacent about flooding; Safeguard Europe’s Hudson Lambert says that when designing new homes it’s essential to prioritise prevention.

It could be thought that, flood-wise, the winter of 2016 was something of a let off, being relatively warm and dry, given the events of 2015 and 2014. 2014 saw Somerset and the Surrey and Thames river valleys under water; while the following year it was Cumbria, north Lancashire and Northumbria that were worst hit.

Despite last winter’s reprieve, the reality is that extreme weather events are increas- ing – look at the Caribbean and southern United States recently – and, according to the Environment Agency, there are over 5 million homes in England and Wales that are at risk of flooding.

Design solutions to tackle flooding have been described as either ‘flood resistant’ or ‘flood resilient’. However, use of these terms had become muddled and occasion- ally used interchangeably. Consequently, less ambiguous terms are now being employed, namely ‘water exclusion strate- gies’ (i.e. resistance), and ‘water entry strategies’ (i.e. resilience).

A ‘water-exclusion strategy’ house is built so that water cannot get into the building and cause damage. A ‘water-entry strategy’ means constructing the house so that although water may enter, the impact is lessened, structural integrity is maintained and drying and cleaning is made easier.

The likely depth of flooding will impact on the choice of design. For low water depths, a ‘water exclusion strategy’ is typically recommended. A ‘water entry strategy’ is needed for higher water levels: a difference in water level of over 0.6 metres between inside and outside can cause signif- icant structural damage to standard masonry buildings.

In practice, a pragmatic combination of both is taken because it is often either prohibitively expensive or impractical to provide a completely flood-resistant building – especially in cases where flood protection is being retrofitted to existing buildings.

Designing in flood resistance

In a new-build situation, the best way to stop water penetrating is to ensure that all concrete used is waterproof and to seal all construction joints with specifically designed proprietary tapes. Services coming in through walls and floors should be designed to be watertight.

As a second line of defence, and if budget and circumstances allow, a cavity drainage membrane can help with the management of flood water. These types of membrane direct the water down the walls into a perimeter drain with the water running to a sump, where it can be evacuated using a pump. This approach is also popular as a retrofit measure to existing buildings, used in combination with measures to upgrade the resistance of the existing masonry such as tanking and repointing.

Designing in flood resilience

When designing for flood resilience, the goal is to make it as easy as possible to clean the interior of a house after a flood. Floors and walls should be finished with an easily cleanable surface such as tiles. If the floor surface isn’t water compatible, it should be easy to take up and replace.

Water compatible doors and woodwork should be considered. Electrical circuits and sockets should be situated above the expected water level and in a kitchen, appli- ances can be raised up off the floor.

Apart from floor coverings and joinery, interior decoration can take the biggest hammering – especially where walls are finished with gypsum- or lime-based plasters. A new report from the University of Portsmouth (‘The Effect of Moisture on

Plaster Performance’) supports the use of damp-resistant plasters instead. This cost- effective system utilises breathable, moisture-resistant plasters to allow the fabric of a flooded building to dry out while providing a durable finish that will be resilient to future flooding. Moisture-resis- tant screeds have also been developed. After a flood, you can – for all practical purposes – simply hose down and sterilise the wall. The make-up of these plasters means they have large pores and a high pore volume. This allows salts to form within the plaster rather than on the surface; and the high pore volume results in high water vapour diffusion (breathability) and higher thermal resistance, reducing condensation risk.

Vital details

When considering the right combination of measures and products, it may be necessary to get specialist advice from a consultant or from the technical team of a supplier. As with any system designed to protect build- ings from water, it is vital to get the detail right at the design and installation stages.

Hudson Lambert is the director of Safeguard Europe