Off-site prefabrication
Far more common today is the prefabrication of various components: off-site building. Wall parts, floor components, trusses etc. are all built off site at a factory. The components can come prefabricated with insulation, installations, windows and doors. There is a trend towards a higher degree of prefabrication. The advantage of the technique is that the greater part of the building work takes place in an industrial plant in a well-controlled environment with approved qualityassurance. The actual assembly of the building, before the roof is put on, takes one or two days at the building site. At one extreme, entire units are manufactured at the factory and in these units not only are electricity, water and waste pipes installed but kitchens and wet rooms as well. Floors are also laid and walls papered.
Another advantage of building with prefabricated components in wood is that these are relatively light and can be erected at heights of several storeys using simple lifting equipment such as mobile cranes, in some cases with the cranes fitted on the trucks that deliver the components to the site.
It is cost-effective to build with wood. The cost of the wood frame is about 30-35 % lower than that of a concrete frame. The total cost is about 10-15 % lower for wood buildings. Using prefabricated modules, the total cost is 20-25 % lower. The time saving can be up to 80% and during the building phase CO2 emissions can be reduced by up to 85%.
Building on site
The oldest method is to construct the building on site. The building materials are freighted to the site and the various elements – walls, joists etc. – are put together on site and then erected. With the on-site building technique, the wall components are generally assembled resting on the joists or the ground and then erected manually.