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A concrete solution to flood resilience

A new British Standard has been published providing design guidance for buildings at risk of flooding. Elaine Toogood of The Concrete Centre explains more.

Flood resilient and resistant construction is not currently a requirement to meet building regulations.

But it is necessary for local planning authorities and developers to ensure that development is appropriately flood resilient, resistant and safe for its users for the life of any development that needs to be located where there is a flooding risk.

Raising standards

A new British Standard has been published providing design guidance for buildings at risk of flooding, alongside recommendations that highlight the flood resilience and resistance that can be achieved with concrete construction.

British Standard BS 85500: Flood resistant and resilient construction – Guide to improving the flood performance of buildings was published in November 2015.

It is based upon the Department for Communities and Local Government (DCLG) guide Improving the flood performance of new buildings – flood resilient construction (2007), but also takes into account “additional research into the reaction of modern materials and construction methods to flooding”.

The guidance has been extended to cover both new build and retrofit conditions with a focus on details and guidance for new build scenarios. Recommendations are made for wall and floor constructions as well as other design decisions based upon the predicted height and duration of the flood.

The guidance retains the established strategic hierarchy for design, i.e. for avoidance; delaying a flood; mitigating its effects.

Resistance is not futile

The existing definitions of flood resistance (water exclusion strategy) and flood resilience (water entry strategy) also remain essentially unchanged. BS 85500 explicitly advises that a combination of resistance and resilience should be provided for most flooding situations.

Consideration of doors, windows, fittings and services are included with the bulk of the guidance relating to construction and structural solutions.

Concrete and masonry solutions appear throughout as recommended solutions for construction, irrespective of the type of flood predicted.

In BS 85500, a ground bearing cast in situ concrete floor remains the recommended ground floor construction for buildings at risk of flooding. This comprises a minimum 150 mm concrete slab over a robust, continuous damp proof membrane with appropriate closed cell insulation.

Although the preferred location of the insulation relative to the slab is not stated definitively, the example illustrated features a solution with the insulation above the slab. Suspended concrete floors are stated as an option to be considered where ground conditions or other site specific issues dictate.

Wall and void

Solid wall construction, with external insulation and render is stated by BS 85500 as a recommended external wall construction, with the clear advantage of having no cavity or voids located within the wall.

This type of wall can be simply achieved using masonry, as illustrated in BS 85500, but can also be achieved using precast or in-situ concrete, be it cast into standard formwork or as part of an insulating concrete formwork system.

Cavity walls are also a recommended solution, with both inner and outer leafs constructed in masonry and with external render and internal render or plaster. Specific guidance for material selection and specification of the details are provided including the use of closed-cell insulation.

Timber-framed walls are ‘not recommended for a water exclusion strategy’ and ‘generally not recommended’ for a water entry strategy unless materials are to be stripped from the walls to allow them to dry. The potential for fungal growth, decay, warping and distortion are all listed as potential impacts when timber frame is in contact with water.

BS 85500 also provides advice on hazards and impacts for a range of construction solutions and materials, but the clear message is of the flood resilient performance benefits of concrete walls and floor construction.

While The Concrete Centre has previously provided guidance to designers for designing flood resilient and resistant homes, we are delighted that this new standard, reinforced by the inclusion of flood resilience in assessment schemes such as BRE’s new Home Quality Mark, will now help to ensure that there are more drivers for homeowners and businesses to be provided with flood resilient homes and buildings.

Elaine Toogood is senior architect at The Concrete Centre

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