Costs: Off-site manufactures

Off-site manufacture is far from new. The Victorians manufactured corrugated steel churches, which found their way all over the world. In 1906 three days were required for the erection of a prefabricated “tin tabernacle” in Maesbury, Shropshire. Today there are far more off-site choices and crucial lessons have been learned from past failures. Important issues such as progressive collapse are covered by Building Regulations, European and British Standards, and there are good-practice guides and third-party accreditations to give confidence to specifiers and occupants. Additionally the BRE is developing a standard that will cover “novel building products and systems that are intended to be constructed into dwellings”.

Off-site manufacture offers many benefits, but translating benefits into costs can be complex, especially where they relate to risk. The issues that play a role in cost equations generally focus on capital costs, but whole-life cost issues are becoming increasingly important.

Time benefits

Capital cost advantages Shorter site-based construction times lead to the possibility for lower site overheads and financing costs. OSM may also remove components from the critical path and reduce the risk of delay. As fewer staff are on site for a shorter period of time, the risk of accidents is reduced. There are environmental benefits through reduced production waste and these may also reduce capital costs.

Whole-life cost advantages Shorter completion times mean buildings can be operational more quickly. For example, a hospital can be used to treat patients sooner, and the more quickly a hotel can generate income, the sooner it will make a return on its investment.

Quality benefits

Capital cost advantages Component parts and assemblies are likely to be part of a quality-assured process that will aim to prevent their substitution with inferior alternatives. Quality assurance should result in reduced snagging and defects after installation.

Whole-life cost advantages Certainty of quality should reduce risks of premature failure and defects during the design life of the building. The in-use costs can be minimised by integrating maintenance requirements and access as part of the design.

Cost benefits

Capital cost advantages Fabrication plants can be located where building, land and skilled labour costs are cheaper, resulting in lower production costs.

Whole-life cost advantages Where quality and installation standards are met, component performance and life will be improved. Whole-life costs should be less than conventional alternatives as component performance and lifespan should be better, always assuming quality and installation standards are met. Prefabricated buildings frequently have increased insulation levels and lower levels of air leakage, resulting in energy efficiencies with lower whole-life energy costs.

Determining best value

Determining best value should be carried out on project- and component-specific bases as these influence the whole-life cost variables. The many cost advantages for OSM need to be balanced and evaluated against cost risks. For example:

Long lead times and detailed design may be required.

This may cause delays in the development programme and higher capital costs

Transport and cranage costs may be significant for large, prefabricated assemblies, thus increasing capital costs

Option appraisals where a range of OSM solutions and traditional construction is compared can be complex and time-consuming. Whole-life cost assessments provide a comparative measurement methodology on which to base option appraisals

Tolerances are critical for trouble-free site assembly.

If these tolerances are not achieved, rectifying errors is time-consuming and costly. Problems may occur at junctions – for example, if a poorly connected service pipe to a shower pod leaks and causes water damage to the floor. The cost of repairing these could lead to reduced profits. Site supervision and commissioning should minimise these risksl Higher capital costs for OSM components are often a reflection of overheads associated with the manufacturing facility and financing the product without the benefit of interim payments

Whole-life costs may be increased where services or fabric components are not readily accessible for maintenance and repairs during the life of the installation.