Flood defence: building quality and construction

Waterproofing walls

17 March 2016

Building quality and good construction are vital when walls have to act as a defence against flooding, as Jessica Lamond, Colin Booth and David Beddoes found in their recent research.

Flooding seems to be becoming an annual issue for homes and businesses across the UK, as demonstrated by the events in Cumbria, north-western England, last December. As a consequence, advice on flood protection is increasingly sought by property owners and occupiers.

There is a large array of property-level protection available on the market, with many products kitemarked to certify that they protect openings against the ingress of water. But how watertight is the fabric of a building itself? Are there any ways of improving the performance of masonry walls to keep water out during a flood?

Watertightness of walls

Most of the scientific evidence and the performance standards on the watertightness of walls are predicated on resistance to wind-driven rain permeating the masonry. However, the hydrostatic pressure of floodwater is a different issue, as water may then seep through walls and floors, rising inside a property at alarming rates. The resulting internal devastation comes as a shock to households and business owners, who believed their properties were protected.

It is well known in the professional community that water can exploit the weakest component or joint in a building to gain entry, hence the quality of the construction is important.


Seven litres of water can penetrate a metre of external wall per minute, resulting in a flood depth of 1m inside a property within half an hour

Government-sponsored research suggests that seven litres of water can penetrate a metre of external wall per minute, resulting in a flood depth of 1m inside a property within half an hour. In practice, this means that, unless the building is of well-constructed and low-porosity (engineering) brick or protected in some way, the water inside the property can rapidly match the level outside.

Different types of bricks and renders were also tested during the same research programme, and the conclusions were that well-constructed walls of class A or B engineering bricks can be much more water-resistant than traditional construction, and that renders can be helpful, if well applied and maintained. This research contributed to the formulation of advice for new construction in the floodplain.

Retrofit coatings

Many existing buildings also require flood protection, however, and the use of retrofit coatings offers a potential solution by boosting the resistance of walls to flooding. There are many different coatings available; renders have often been applied, or waterproof coatings. However, waterproof coatings can slow drying, causing potential post-flood problems and maybe condensation issues, so new technology developed in recent years aims to allow breathability – letting water vapour out – as well as water resistance.

New independent research, carried out with the support of the University of the West of England, has tested waterproof masonry coating under hydrostatic pressure that simulates a flood.

The work sought to find a low-cost way to waterproof walls, recognising that veneering systems and renders can be very effective, but also expensive and difficult to maintain. Following research and consultation, a silane-based water repellent was selected for mortar admixture and impregnation of the surface.

Silane products have previously been shown to have water-repellent properties but also, importantly, they allow the treated wall to ‘breathe’. This means that the danger of moisture trapped in the wall leading to frost damage is minimised, and it will also not impede drying after a flood.

As these products penetrate the fabric of the wall rather than simply remaining on the surface, they are also thought to be more durable and need less maintenance. Testing was carried out on Shropshire red-clay-brick walls with a 1:6 Portland cement: sand mix; these were recently constructed, but designed to represent typical walls found in many nineteenth- and early twentieth-century buildings in the UK. An innovative field testing method looked at the rate that water was absorbed into walls through the bricks and the mortar joints.


The coatings can significantly slow the ingress of water to masonry walls

The research showed that, when properly applied and allowed to cure under plastic sheeting, and with the inclusion of waterproof mortar additives and well-filled joints, the coatings can significantly slow the ingress of water to masonry walls.

This treatment did not achieve the very low rates of leakage required for barrier products kitemarked under PAS 1188 – 1:2014, but it reduced the ingress rate by two-thirds. This reduction means that the water could then be controlled and expelled by pumps, preventing more serious damage arising. Further work is currently being undertaken to explore whether different combinations of treatments can improve on this.

Jessica Lamond is Associate professor in the Centre for Floods, Communities and Resilience at the University of the West of England

Colin Booth is Deputy Director in the Centre for Floods, Communities and Resilience at the University of the West of England

David Beddoes is managing Director at Drain Angel and honorary Research Fellow at the University of the West of England

Further information

Download a complete “Flood defence: building quality and construction” PDF