Under factory conditions, quality of finish should be guaranteed, but according to Barbour Index and Scott Brownrigg nothing should be taken for granted

Construction projects delivered using an off-site solution are frequently described as being of better quality than traditional schemes. But, bearing in mind that quality control standard BSEN 9001:2001 only ensures that the output of the factory is consistent, not necessarily better, how does the specifier ensure they are getting a higher quality product?

1. General principles

The following principles apply to both conventional and off-site construction.

The project documents should clearly describe the agreed quality standards, as many conflicts are created by misinterpretation of these standards. Once this is established, the details, including the procurement route and the processes can be agreed. This should be regularly monitored to ensure that the project team adheres to the original aspirations.

Ensure that dimensions, performance and life in service of the base materials are all established in the specification. Control samples should be produced and held as references. They should also be artificially aged to demonstrate the performance of the element over its lifetime.

Remember that a rigid process is needed to ensure that the specification is delivered. This is because the building procurement process goes through many stages, particularly on larger projects, increasing the potential for misunderstanding, misinformation, corner cutting and deliberately ignoring the specification. Achieving a high quality result depends on keeping a close eye on all of these issues.

2. Ensuring quality in the factory

The specifier needs to take account of the factory environment and production principles when procuring an off-site solution. Ensure that within the specification there is a requirement for continuous review and improvement.

The specifier needs to have a clear understanding of the methods of assembly at their disposal. If some mechanisation and automation is available, take full advantage of the added quality this can bring to the finished product and ensure that the design does not prevent its use. There may be a need to compromise over some design and performance elements to match the factory processes.

The specification should carefully set out the expected methods being employed by the factory so the production process can be shaped to minimise mistakes and errors. Control of this can be reinforced by ensuring that the production process goes through a series of "gateways" at significant parts of the process, each needing its own review of the quality being achieved and identifying any shortcomings. The work should have to pass all the quality control checks before it can pass to the next section on the assembly line.

The specifier must be very precise when considering the tolerances. The degree of flatness or the precision of fit of the components need to relate to the desired quality but also be practical. However, factory tolerances can be much smaller than site-produced work and are also guaranteed to be repeatable, which will bring much needed confidence and certainty to the finished product.

3. Transport

However, as soon as the components leave the factory, quality control problems can occur. If the means of transportation is not properly considered, a superb quality product could potentially be ruined on the way to site. The specifier should ensure that any work leaving the factory environment is adequately protected. Large items must have all edges securely protected with padding and with sheeting covering all other areas.

Establish that the protection will stay in place until removed by the operatives on site. Ensure that the protection can be removed without damaging the product and that it will not inhibit assembly - too often, protection tape is still visible in the finished building because it was too difficult to remove.

Also consider the route to site, as with large loads road bridges and obstructions may be impossible to negotiate.

4. Ensuring quality on site

The specifier should try and ensure that the offloading operation is sequenced correctly so components aren't damaged. The specification should require checks on arrival to site and, if the components are not going to be immediately installed, a pre-installation check is needed as well.

If components are stored prior to installation, the holding area must be secure and fit for purpose. Once components are installed ensure that access is strictly limited to people who need to work in the area. This is essential if hard-fought quality is to be maintained.

5. Key issues

The key issues to remember if you are considering using off-site manufacture for a project are:

  • Establish the quality standards required prior to manufacture
  • Consider all elements as manufactured components with the ability to specify very precise tolerance levels
  • Maximise machine-made components
  • Always think in terms of the manufacturing principles and processes when establishing all other parameters on the project
  • Consider the processes, machining and assembly methods and agree these with the factory management. Remember sub-assembly tests can be undertaken
  • Minimise any on-site operations and exclude all site operations from the completed modules.
  • And remember, the higher the level of completion in the factory, the greater the opportunity for better quality overall.

Off-site manufacturing