(1) Precut machinery
Development of machinery for post-and-beam timber houses improved the lead time, quality and cost. Japanese post-and-beam houses used to be built by timber frames being cut and crafted by skilled carpenters. The work to craft holes, pegs, and jointing was labour intensive and the shapes, sizes and speeds were dictated by the skills of the carpenters.
Therefore, cost, quality and lead time for crafting timbers varied, and the cost rose due to the limitations of the carpenters. The precut system resolved these issues. The lead time of the crafting works has dramatically reduced by the system. The time carpenters take to complete crafting timber for a house whose floor area is approximately 130m2 is a month. In comparison, the precut system can do this in only half a day. In relation to quality, the accuracy of size of crafted parts of individual timbers is within 0.5mm in the precut system. With respect to the cost, the CAD systems calculate the exact amount of timbers needed and the combination of the CAD and CAM system means efficiency of materials. Thus the material cost is also reduced.
At the beginning of the 1990s, machines only crafted main structural frames which are posts and beams with traditional types of holes, pegs and joints. The variety of parts which could be crafted were gradually expanded, for example, other frames such as structural braces, floor joists, window sills and roof rafters, by the end of the 1990s. Plywood for roof-base panels and sub-floors was able to be cut by the beginning of the 2000s.
(2) Change in raw materials
The precut system produces accurate sized timber frames. However, the accuracy in the product changes before and during construction due to the nature of wood. Logged woods warp, twist and shrink when moisture evaporates. Although the majority of Japanese wooden houses built by the post-and-beam method used natural seasoning dried solid timbers, the moisture content of these timbers was not accurately measured because the moisture content in timbers is not even throughout.
Reliable quality in the moisture content of raw materials was crucial for accurate production of precut timber. Improvement of the kiln drying system during the 1990s enabled monitoring it during and after the drying process. Thus precut-timber-frame manufacturers now use kiln dried timbers.
Click to photos http://www.sanno-jp.com/lumber/Materials/index.html
Simultaneously, laminated timbers (or Glulams) have become popular to use as the frames after the establishment of the Quality Assurance Act. Part of the act, structural safety, required both accurate size and structural strength. To meet these requirements, the modules of elasticity in timber frames in the main structure needed to be measured in all timber frames in the main structure. The strength of laminated timbers is constant and defects inherent in the nature of wood are unlikely to occur. It is suitable for precut-timber frames.
