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Geoff Wilkinson’s Regs: Design details

Don’t risk building failure through poor detailing

Modern design tends to encourage architects away from traditional detailing (projections at eaves, drips on sills and the provision of movement joints, for example) because it detracts from the visual impact of the building. Similarly, contractors are under pressure to use alternative, lower-cost materials or to cut corners in order to save time. The combination of these two pressures will tend to expose the building fabric to the potential risk of premature failure.

A 2008 report by the National House-Building Council’s research arm, the NHBC Foundation, looked at this risk and provides excellent guidance for architects and students of architecture. The report, Learning the Lessons from Systemic Building Failures, looks at a number of causes of past building failures, and points in particular to the need for good detailing to prevent damage from moisture.

With innovative construction systems and materials, detailing has to be developed that allows the particular design to be built. In many cases this leads to poor solutions, such as a reliance on sealants where, for example, a properly detailed flashing might be more appropriate.

The NHBC report also notes that architectural training appears to be placing less emphasis on ‘traditional’ detailing. Many of these details, such as projections at eaves and verges or drips on window sills, were historically developed to protect the structure from the weather by providing a degree of shelter and by directing rainwater away from the building.

Omitting a drip from window subsills, for example, increases rain absorption in the wall below, while removal of the sill completely increases the likelihood of rainwater streaming down the window, over the wall below and penetrating the building. This risk was greater before 1960, when solid masonry construction was the most common form of construction. However, the need to prevent moisture ingress through detailing became less important in the 1970s and 1980s, due to the common adoption of a clear drained cavity in cavity wall construction. As a result, many architects trained since then have got used to detailing which excludes projections and drips and is more aesthetically pleasing.

The 2010 amendments to Parts L and F of the building regulations have put pressure on designers to make increased use of cavity fill and future changes will prompt a move towards innovative construction systems that could replace cavity masonry completely. As a result, the control of moisture is becoming more critical. At the same time, the need to provide greater levels of airtightness in dwellings means that architects are increasingly having to look towards mechanical ventilation to deal with moisture generated by everyday activities, such as cooking and bathing. If the ventilation strategy is deficient in any way (poorly specified, poorly maintained or prone to breakdown), then humidity levels rise, increasing the risk of condensation, damp and mould.

The NHBC report recommends that designers should consider a risk assessment methodology regarding design details. This is the same approach recommended by the Department for Communities and Local Government in its Future of Building Control report, when it became important to refocus limited inspection resources on the most critical elements of design and construction.

One such system has been developed in New Zealand and considers:

• wind zone;

• number of storeys;

• roof/wall intersection design;

• eaves width;

• envelope complexity; and

• deck design.

Each external wall elevation is then assessed and given a risk rating (from low to very high) and a corresponding risk score for each of the six risk factors.

The total risk score is used to determine what types of cladding are permissible. For example, if the overall risk score is greater than six, external insulation systems must incorporate a 20mm drained cavity.

Such a system could be equally relevant in the UK, although the actual level of risk would differ because of differences in climate.

For example, steel frames may be more vulnerable in coastal areas, because the presence of chlorides can accelerate decay in galvanised steel.

The NHBC report concludes that designers should:

• Assume that water will find a route behind cladding;

• Recognise that errors will be made during the construction process – defects will be built in;

• Remember that materials deteriorate with age – especially sealants;

• Adopt the ‘four Ds’ principles (Deflect; Drain; Dry; choose Durable materials) when designing dwellings; and

• Anticipate future work that may compromise the integrity of the cladding.

The report concludes that they should not:

• Rely on sealants to keep water out indefinitely;

• Assume that regular inspections of cladding and sealants will be undertaken; nor

• Casually experiment.

Commenting on the report, Nick Raynesford MP said: ‘At a time when UK house building is facing perhaps the fastest and most pronounced changes (in terms of output, construction methodology and regulatory impact) in its history, this review is a timely reminder of the need to keep sound design criteria at the fore.’

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