DIY-Prefab

Factories that build automobiles and airplanes typically completely prefabricate their wiring into wiring harnesses. To do this accurately requires the building of jigs and fixtures that allow the wires to be placed in the correct relationships to each other. Wire connectors can even be pre-installed at the right locations on the wires. While this level of detail might not be practical for a full house I suggest that at least some pre-cutting and prefabrication of wiring related products could be useful for you DIY project. At the very least if you carefully think out where your wiring will need to be placed you can get a fairly accurate count of what parts are needed. You should also be able to pretty accurately predict where all of your outlets and switches need to be. How about pre-installing at least some of them in your wall panels? I have read about panelized housing approaches where electrical boxes and wiring is largely installed in the panels before shipment. It is very typical for wiring to daisy chain from one box to the next. In these prefabricated panels the piece of wire that connected across panel boundaries was pre-attached to a box in one of the panels. Enough wire to connect to the next box in the adjoining panel was simply coiled up at the edge of the wall panel awaiting connection the rest of the way to the other box. If you do not want to pre-attach your electrical boxes you could perhaps at least mark where they should go on each panel as you build it.

Structural Insulated Panels (SIPS) typically have a solid core of insulation foam between skins of OSB or plywood. Adding wiring after the fact would be pretty hard. SIPS that use preformed expanded polystyrene foam (EPS) typically have wiring channels precut inside of them during the manufacturing process. The design process needs to specify where these wiring channels need to be located. SIPS that are made using polyurethane foam insulation that is foamed in place during the manufacturing process require a different approach. For this type of panel plastic electrical conduit and electrical boxes are pre-placed in the panel before the foam is injected. Obviously this approach also requires predetermination of the locations where the wiring should be located.

So if you know where all of the walls are going to be and where the framing members in the walls will be why not do some prefabrication of plumping parts? You can at the very least make up a fairly accurate parts list to have on hand when it is time to install the plumbing. Some aspects of a typical rough plumbing installation could easily be predicted accurately enough to prefabricate if you wish. One disadvantage is that preassembled plumbing trees could be a little awkward to pack and ship to your job site. On the other hand you could easily consider pre-installing some of the plumbing components into your building panels where they might be taking up otherwise wasted shipping space. Pre-drilling holes in places where you know you will need them can result in some job-site timesavings. You might even be able to do enough pre-drilling that you do not need to do any on the job site.

One significant advantage of building a carefully dimensioned prefabricated structure is that you can accurately predict the dimensions of various subcomponent parts. This of course is one of the key benefits of prefabrication in that more things can be prefabricated. On type of subcomponent that can be prefabricated that I have not yet said anything about are stair parts. I have personally designed a number of stair systems that were at least partially preassembled in the factory. The remaining parts that were not pre-assembled were at least typically pre-cut. Stairs that are not simple straight runs are usually composed of either triangular steps or landing platforms. Sometimes I was able to prefabricated landing levels so that they stacked on top of each other when they were installed on site. I can also remember one curved staircase where we prefabricated the curved handrail system in the factory. We first prefabricated and temporarily assembled the stairs in the middle of the factory floor. We then had an expert in stair railing come in and pre-build the railing in place as though the stairs were in their final location. The rail parts were curved, glued and clamped to the top of the stair treads with special brackets designed for the purpose. There were a few joints in the rail assembly that were only bolted together with concealed fasteners to make sure that all of the parts aligned properly with each other. In the final assembly glue would typically be applied to these joints as well when they were bolted together in their final location. One all of the glue had dried the railing was sanded and then taken a part for shipment. I think that this particular house was sent to Japan.

As I have said before the degree of pre-assembly of subcomponent parts depends on many things not the least of which is the overall size of the component in question. Architectural details such as dormers, pop out bay windows or cupolas can be pre-assembled if they will be small enough to ship and install once assembled. The company that I worked for would sometimes pre-assemble smaller garden window frameworks such as might be found over a kitchen sink. For larger pop out bay windows we would more typically pre-cut the various parts for assembly on the job site just because of their overall size and awkward shape for shipping purposes. I remember once designing and building a small dormer in such a way that it folded up for shipping.

A good example of the prefabrication of some architectural details such as cupolas and dormers can be found at the following web link:

http://pennypincherbarns.com/Accessories.aspx

Whether or not a roof system can be panelized is very definitely a function of its complexity. Of course it is also related to the maximum size of panels that can be built given the means of transportation selected and the means of installation to be used. For example if panels will need to be manually hoisted into place then there will be fairly restrictive weight and size limitations. For a good example of some practical roof panel designs for manual installation take a look at Michael Janzen’s prefabrication plans listed at the top right of this blog. Most of his panels are 4’ wide and less than 12’ long. Even so their overall weight will require some muscle to put them in place.

The overall design of a roof system may very well determine the feasibility of panelizing vs. pre-cutting. I worked for a while at a wood frame prefab shop in the late 80’s where we shipped quite a few house packages to Japan. The average floor plan in Japan was definitely smaller than in the United States. At that time the average family of 4 lived in about 400 square feet of space. What I observed with the projects that we built was that what these houses might lack in overall size was very definitely made up for by complexity of roofline. More often than not the designs had full hip roof lines. For those that are not familiar with that terminology it is simply a roof where all of the sides of the roof come down to the top of the walls. This type of roof is definitely more complicated in that the rafters that are required to make it are all different lengths and typically have compound angles cut at their top end. Certainly this type of roof could be prefabricated into panels but we found it was generally easier to carefully pre-cut and label all of the pieces and furnish a detailed drawing of their placement. Usually the plywood was not precut but it could have been.

Obviously any building needs a foundation of some sort. Foundations for prefabricated structures have a few requirements that might not be quite as important for stick-framed structures. The one that is probably the most important is that the foundation needs to be more accurate than might be tolerated for stick-built. For prefabrication the foundation needs to be not only the right size but also needs to be as close as possible to the exact shape as the prefabricated building that is to sit on top of it. It also needs to be as flat and level as possible. Prefabricated panels are of course designed to fit together in a specific way. They are not as easily adapted on the job site to accommodate sloppy foundation work. I can remember when I was working for a wood frame prefabrication company that we would sometimes go to the construction site before the concrete was poured and check some of the key dimensions ourselves. Also it was sometimes useful to check the poured concrete after the forms were off to see if any dimensional changes should be made to the set of panels before we finished manufacturing them.

If you are building a DIY project and you are the one that is building the foundation itself then you have somewhat more direct control of the overall dimensional quality of the foundation. If you are having someone else build it for you be sure to stress the importance of the dimensional accuracy. There are some design tricks that you can play though that make for a less critical dimensional relationship between the foundation and the prefabricated building.

One suggestion that I have in particular is that you consider some type of post and beam foundation system. In this type of system the floor framing members or prefabricated floor panels span across and beyond the supporting foundation beams. Consider the type of foundation shown in the following drawing.

I think you can see that the placement of the beam relative to the floor panel that is sitting on top of it is not particularly critical in the left-right direction as shown in the drawing. Of course the beam needs to be level along its length as well as level with the other beams in the foundation. It also needs to be long enough to support the floor panels out to their edges. Otherwise the location of the beam could vary considerably without hurting the overall structural integrity of the building. By the way this type of foundation system is what is used in Michael Jansen’s prefabricated small building plans.

Prefabrication of floor systems can actually be a more complicated problem than prefabrication of walls. This is primarily because there can be such a wide range of sizes and load bearing requirements. It is not unusual for manufacturers of prefabricated wood frame houses to just pre-cut floor systems. While I contend that any amount of factory work done prior to actual construction on-site can be classified as prefabrication this is not as much prefabrication as we have been discussing so far with walls. Another reason that floor systems are sometimes just pre-cut is that floor panels typically have a lot more air in them. This is because floor-framing members are quite often larger in dimension than those in wall panels. For example typical walls might be constructed with 2×4 or 2×6-framing members while floor systems might be built with 2×8 or larger framing members. The members could also be spaced closer together.

One other aspect of floor systems that can contribute to making panelizing more difficult has to do with insulation, wiring and plumbing. Wall panels most typically have at least the outer skin attached. This leaves ready access to the inside of the panel so that wiring, plumbing and insulation can be installed from inside of the building. It is more convenient on the other hand for a floor panel to have the top skin installed. This makes access to the inside of the panel a little more difficult since it must be accessed from below. This might not be a problem if there is enough space underneath the floor. Still it can complicate the overall process of completing the floor part of the assembly. Supplying the floor system in a pre-cut only kit sometimes is worthwhile because many of these other installation problems can be avoided.

There is also potentially a very large difference between the bottom floor of a house and the other floor levels. If the bottom floor is built directly over the ground it is easier to provide support points at predictable increments that are small enough to make building pre-assembled panels feasible. You could easily build floor panels that are 4’ x 8’ in size if you could support the from beneath at least at their corners.

One other factor to consider when you are thinking about pre-cut or panelized floor systems has to do with how you will attach the panels together. Conventionally framed floor systems typically rely on the overlap of plywood sheets to fasten the various sections of the floor together. In a panelized approach some other means is required to fasten the panels to each other. This is easily enough accomplished if you have access to the panels from beneath. If, on the other hand, the panels are fully enclosed them some other technique is required. One approach is the type that Michael Janzen adopted in his pre-fab house plans (more information is available by clicking on the link on this site). In that design Michael uses a plywood spline between the panels and under the top level of the floor sheathing. This is very similar to the way that he attaches his wall panels together. Please refer to the following drawing repeated from an earlier post that shows the spline in red. Of the course the floor panels might be thicker than what is shown here.

Lets talk a bit about wall corners. The style of corner intersection that I favor the most is one where you add a pair of 2x members in an L shape. This gives a solid attachment point for the intersecting wall as well as an adequate backing for interior wall sheathing. Consider the following diagram that shows the outside corner at the lower left of the drawing.

As I have mentioned before the wood members in the corner can be prefabricated into a small sub-assembly or module.

What if your floor plan requires an inside corner on the outside wall? Well you could use the same arrangement as in the above diagram but with the outside sheathing on the other side of the panels. You would have to carefully cut back the outer sheathing at the right point for the lower right wall panel to intersect. An easier way to do this type of intersection would to let the outside sheathing of the left hand wall go all the way to the end of the wall panel as show in the following diagram.

If you take this approach then you might not actually need to use an L shaped assembly in the corner of the left hand wall. Instead it might be just fine to use the configuration shown in the following diagram. Whether or not this will work well for you depends somewhat on how you are fastening the panels together. If you are leaving off the inner sheathing so that you can attach the wall panels together through the framing members then this would work fine.

Once your structure gets big enough to have more than one room you may encounter the need to have interior walls. These interior walls will need to be connected to other walls. Typical framing techniques might provide a connection point in a wall for another wall to attach to using an approach something like what is shown in the following two diagrams. Notice that there are 3 studs required in each approach. The only difference here is that the inner skin of the wall continues through the joint in the second example. This might be the case if the interior wall skins are going to be pre-attached to your wall panels.

There are some advantages to using the above type of wall junction. For one thing it does establish a precise point of connection for the intersecting wall – at least for the version where the inner skin is not covering up the junction. The assembly of 3 studs can be treated as a sub-module and can be pre-assembled prior to incorporation in the wall panel. One disadvantage is that it is difficult to insulate. Another disadvantage is that it will often be in conflict with other framing members such as those needed around doors and windows. This typically ends up requiring variations on the basic arrangement.

An alternative approach, which I favor, is considered to be an example of “advanced” framing in some building codebooks. It is considered advanced primarily because it is much easier to install insulation. In this approach a sort of ladder framework is constructed where there are several horizontal 2×4 members connecting between the two studs that are nearest to the location of the junction point. The two next diagrams show in plan view how this would work. As in the previous two examples one of the diagrams shows the sheathing material stopping at the point of intersection and the other shows the sheathing material continuing through the junction.

The horizontal framing members can typically be spaced at about 2’ on center from top to bottom of the wall. This approach can often result in some savings of material depending on where the wall intersection occurs. In the diagrams above for example the horizontal members are placed in a location that is part way between two studs. These studs are at 16” on center. Using the other type of wall junction you would need to add three studs at the junction point. In this approach about ½ of a stud is needed for the horizontal members. That is a savings of 2.5 studs just for this one wall junction. I think that you can see that it would also be easier to install insulation behind the ladder framework that this type of assembly creates. Here is a drawing of what the ladder assembly would look like:

More details coming…

There are plenty of books available on the topic of residential wood frame construction. I do not want to even try to give a detailed description here of all the possible aspects of wood framing. Rather I would like to talk about what might make sense for your framing at the places where panels connect to each other. There are of course a number of types of panels that might be included in any given project. I want to focus on wall panels for this post and perhaps reserve some discussion about floor and roof panels for another time.

Wall panels have a variety of requirements. Of course there are structural issues but in particular I am referring to issues that impact where you might want to have joints between panels, which then imply panel connections. This can be complicated by the fact that you want to have openings for windows and doors as well as places in the middle of a panel where an interior wall might need to connect. There also is a requirement that sheathing material have something solid to attach to at the necessary locations.

When you are framing a wall in a normal stick frame situation the framing members (studs) that are placed at the joints of the sheathing material are usually just centered on the joint such that both sheets of sheathing can attach to a single member as in the first part of the below diagram. It is possible to make prefabricated panels with a joint of this type where the stud is attached to one panel with enough of it sticking out so that the next panel can be attached to it in the field. It is often more convenient to have two studs at the intersection of the wall panels as in the second part of the diagram below. This does require an additional stud but it makes the joint much stronger and the attachment of the sheathing materials more secure.

If your panels have a single stud attached to only one of the wall panels at the intersection point then you will need to attach the sheathing and at least the top plate of the other wall panel to the stud. With two studs at the intersection you will only need to attach the studs to each other. If your panels do not have the inner sheathing installed when you are assembling them in the field then you can easily nail, screw or bolt the panels to each other through the two studs at the panel intersection. This would of course make it more difficult to take your structure apart later if once you install sheathing on the inside.

This is one of the challenges that Michael Janzen faced in the design of his prefab panels in the eBook to which I link here on my blog. Michael wanted to be able to pre-install plywood skins on both the inside and outside of his panels. He also wanted it to be possible to take the structure apart without having to remove the inner plywood skins. To resolve this issue he has detailed an approach where he makes provision for adding a plywood spline that connects the two studs together underneath the inner plywood skin. This is shown in the following detail where the solid red portion is the spline. Screws can be driven through both the inner skin and the spline from inside of the structure.