DIY-Prefab

Lets focus a bit on the design process as it relates to your DIY prefabrication project. Most of the examples that I will use are likely to be from wood frame construction simply because most of my construction experience has been with wood. I hope that I can convey the concepts in more general ways though so that you can see how they could apply to other construction approaches as well. I hope too that I do not over simplify. It has been my experience that some of the things I take for granted are not taken for granted by other people. With that in mind I will try to be a complete as reasonably possible. So what are some of the high-level or global things to think about that apply to most any design? Here is a list of some things to think about.

1.)    Determining the maximum module size is probably one of the more key decisions that you will need to make. I have designed a fairly large number of prefabricated wood frame structures that had to fit into standard shipping containers for shipment overseas to Japan. This provides some challenges in that it is simply not possible to fit in a wall panel that is 8’ tall unless its width is small enough to fit into the container. The inside of a standard container is something like 90” wide and 92” tall if memory serves.

2.)    The method of placement and assembly at the construction site can dictate aspects of your module design too. A pre-framed wall panel can be fairly heavy per lineal foot depending on the types of material used and the level of completeness of the wall panel. Use of cement based products such as Hardie Board brand siding for example can add a lot to the overall weight. Not too long ago my youngest son and I built an 8’ x 14’ storage shed in my yard. We used 4’x8’ Hardie Board sheet siding which was only 3/8” thick and framed up each wall in one piece on the floor of the shed. We were just barely able to tip the 14’ wall panels up into position. It would not have been practical for the two of us to move that size panel very far. I have also designed panels for jobs where we could anticipate a crane being on location. Some of the panels I have designed were in the range of 24 long with windows pre-installed. I once collaborated with a couple of friends to build a Spec house where we used a combination of prefabricated wood frame wall panels along with SIPS for the roof and floor. I got to operate the rented crane that we used to lift the panels into place. As I recall I designed the wall system to have a maximum panel size of about 16’. If your project is really remote you might need to design panels that are much smaller. The prefabrication panels that Michael Janzen published in the design book that is referenced at the top right of my blog are designed for transportation in a pickup truck.

3.)    Determining how you will fasten the pieces together during the final assembly stage is an important consideration. This is especially true if the wall panels will arrive at the job site fully enclosed. This might require special attention in the prefabrication stage to make sure that the necessary preparations have been made to allow assembly. Again I refer to Michael Janzen’s plans were he uses a plywood spline between panels. This approach requires that the necessary slot be included in the panels when they are prefabricated. There also are various types of special fasteners that are designed for attaching panels together that would need to be built into the panels during construction.

4.)    The degree of completeness you intend for the prefabricated modules can have a huge influence on the design. As mentioned above the more complete a panel is the heavier it will be. There is also a potential issue relative to inspection of the structure if that is required. Prefabrication companies that make modules that are completely enclosed have to make special arrangements with the appropriate government agencies to pre-inspect their products before they get to the job site. This is one reason that wood frame panelizing companies most typically leave the inner skins off of their panels.

5.)    Permanence and/or the potential need to relocate a structure should be considered. One extreme example of this is found in the modular home industry where the entire home might need to have wheels mounted on it so that it can be moved. This dictates certain things about the maximum size of the individual module – namely that it has to fit on the road. This also dictates certain things about the base frame, which is typically built out of steel. Other material choices are made with the thought of moving the structure in mind. For example overall weight and material flexibility is considered. You might want to use construction adhesive in the assembly process if you do not anticipate relocation.