DIY-Prefab

Modules play a large role in most any design project but are especially prominent or desirable in prefabrication techniques or methodologies. As I think of it a module is basically a sub-assembly of a larger assembly. While a module could be an entirely unique sub-assembly it is more common and more efficient to design modules that can be used more than one time in a given design. There are two key reasons that come to mind as to why this is true.

1.)    The amount of time that is required to complete a design is directly related to how many unique things the designer actually designs. Designing subassemblies that can be used in multiple places can radically reduce the total number of unique things that the designer works on with the result that design time is saved. My career in the semiconductor industry where I have been involved in the physical design of integrated circuits (or microchips) has really driven this fact home. In that industry it would be impossible to design at the level of complexity that we do without taking advantage of lots of predesigned sub assemblies. While it is possible to design a house without modules it does not really make good practical sense to do so. In fact designers do use repeatable modules almost without thinking about it. It would, for example, be unusual for every door and window in a house design that is at least nominally complicated to be entirely unique.

2.)    Fabrication time for the parts of a given house design is at least to some degree directly related to how many unique modules are in the design. This is because a certain amount of set up time is typically required to build something different than what was built before. There is a bit of an exception to this if you have access to sophisticated computer controlled assembly equipment that is able to quickly re-tool for each new object. Interestingly in the integrated circuit design flow while the use of modules has a very significant impact on design time it has very little impact on fabrication time but that it because all of the modules are essentially fabricated at the same time. That of course is not the case with DIY construction. I contend that being able to make reusable jigs and fixtures to assist with building multiple copies of each module can result in some very significant fabrication timesaving.

One level of modularity that we may take for granted is the use of standard dimension materials and even the use of standard dimensions. It is, for example, standard procedure to use pre-cut 2×4 or 2×6 lumber to frame a standard wall. It is also standard procedure to space members at 16” or 24” center to center. In general though I am referring to subassemblies that are a little more complicated than individual sticks of wood. It is important to point out however that there is a hierarchy of assemblies in most construction projects.

What I mean by the term “hierarchy” is that there are assemblies within assemblies that may themselves contain assemblies. In prefabrication it is useful to take advantage of multiple levels of hierarchy when possible. One simple example of this occurs in the construction of a typical wood wall panel. The sawyer starts with a lumber cut list of the different pieces of 2×4 in the wall. I contend that each size of 2×4 could be considered a subassembly or module. These modules could be put in bins by size. Some walls will have special features within them that require subassemblies of pieces of lumber. For example if the wall has a window there might be subassemblies for the wood members at each side of the window. It would also be possible to make a more complicated subassembly that included all the framing members for defining a given size window. There could be other subassemblies for door openings, wall corners and wall junction points. Taken to its logical extreme all the pieces parts for a given wall panel could be treated as a hierarchy of sub-assemblies and could be prepared in advance.