Building pathology: Off-site manufacture
- Date: 27/03/2009
Building Magazine: Off-site construction techniques have moved on quite a bit since the post-war prefab days, but this method is not without its problems. Peter Mayer of BLP Insurance explains what to look out for
Off-site manufacture has certainly been around since the Victorian era and arguably started in medieval times, when timbers of cruck houses were cut to size and numbered before being carted to site and erected. However, our perception of off-site manufacture is heavily influenced by the prefabrication period after the First World War. It is unfortunate that off-site is associated with this “prefab” period and the failures of their steel and concrete elements – more than 170,000 pre-cast concrete houses built at this time have been designated defective. Yet the BRE concluded that “non-traditional houses built since 1918 have performed overall just as well as their traditionally built counterparts”.
Whether the off-site building systems of the noughties will suffer similar failures remains to be seen, but regulations, standards and scientific understanding have moved on considerably since the seventies. Hopefully, lessons have been learned from past failures. Modern off-site building is a far cry from its predecessors. Indeed, off-site construction is often sold on the promise of higher quality.
So what are the problems?
Theoretically there shouldn’t be any. As with on-site construction, a correctly designed and installed building, built using components selected to meet the requirements and withstand the expected environment and use levels, should not confront the user with unforeseen problems.
However, real life is not perfect. Typically problems, if spotted, are resolved during manufacture or on site. For example, quality control systems should exclude panels that are not bonded with sufficient adhesive, and proper storage of timber sections at the appropriate moisture levels should ensure they remain stable.
Many problems are not associated directly with off-site manufacture. Rather, problems arise in co-ordinating this with on-site preparation. Prefabricated modules or panels require accurately placed foundations, ground beams or floor slabs. Levels can be made up with shims, blocks and expanding concrete, but these should be corrosion and rot-proof materials, sized and positioned to ensure the prefabricated components are erected to the tolerances required. If a 10m-long panel is only one-fifth of a degree off level it will result in it being 34mm higher or lower at one end, with a gap of just under 10mm at the wall. On a large building these errors tend to compound and a structural engineer would be required to implement corrective measures.
Defects in off-site manufactured buildings give rise to a couple of additional issues. First, there is the question of whether the fact the building was manufactured off site caused the defect. If so, would the investigator be able to tell that the building was manufactured off site? Recognising an off-site manufactured building is not always easy as they often look like conventional ones. Investigators may have to rely on documentary evidence, prior knowledge or opening up the structure.
Problems with weathered envelopes are not unique to off–site manufactured buildings. Cladding is just as likely to fail in conventionally erected buildings.
The issue with off-site construction is that the failure may be systemic. Problems typically arise if the off-site elements do not include the external cladding. Defects tend to be related to co-ordination of cladding fixings and the off-site structure, maintaining the integrity of moisture control and insulation layers and ensuring provision for fire control if needed.
Current off-site building systems typically have high levels of insulation and airtightness so the building uses less energy. This is fine if key design concepts such as airtight switches, power sockets and integral vapour control layers are maintained. But there is a risk that defects may result or thermal performance may deteriorate when services are modified or replaced, if the vapour control layer is damaged and allows moisture to penetrate external walls. This may occur on site during construction or during the life of the building.
Off-site manufactured building systems are generally constructed from the same structural materials as on-site buildings: namely timber, steel and concrete. Decay, corrosion, carbonation and chloride attack are the principle material deterioration processes. These risks can be minimised by ensuring that structural components are adequately protected from the effects of moisture and agents of deterioration. The analysis of defects as well as getting the design correct is aided by advances in building science, especially modelling software, which allows the risk of such issues as interstitial and surface condensation, cold bridges, moisture loads and driving rain to be predicted.
BLP provides building latent defect insurance for dwellings and commercial buildings www.blpinsurance.com