Best when read in chronological order – see Outline above
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.
If you have given enough care to planning your project you should find that the pieces would all go together nicely once you are ready to assemble them on-site. There is one thing that occurs to me to mention about prefabrication of panels that does impact final assembly that you might want to think about when you are doing your design. It turns out that the more panels you assemble side by side the greater potential there is for dimensional errors. What I am referring to here is that panels are not always exactly the size that you think they are. Wood products might swell, bow or twist a bit between when you fabricate them and when you are ready to assemble them. When you are putting the panels together you might not be pulling them together as tightly as they need to be either. This issue may or may not be a problem depending on the nature of your project.
For one thing the smaller your building is the less likely it is that there will be enough of a difference to bother you. Also there might be some automatically compensating factors about the way that your building goes together. For example if your entire building is made out of panels from floor through the roof then the entire overall size of your building might just be a little bigger than you thought it would be. The assembly of your floor panels might grow a little such that you do not notice that your wall panels grow a little when you assemble them. Also the size variation tends to occur at panel junctions. This fundamentally means that fewer panels means less size variation.
There are two basic ways to deal with this issue. One is to design your building so that you can tolerate it being slightly bigger. The other approach is to intentionally fabricate some part of your design slightly smaller in anticipation of the growth. Remember it is a lot easier to add a thin spacer or shim between panels than it is to cut a panel smaller on the job site. One good place to consider doing this for wall panels is at the corner where walls will connect. One panel will typically have sheathing material that overlaps the end of the other wall. Consider making the wall framing on that panel just a little smaller. It would be a very good place to add a small space if necessary and the sheathing material makes up for their being a small gap between the panels. Refer to the drawing below which shows a top view of a wall corner for where you might do this.
I have already said something about the stacking order of your building panels in my previous post. Let me reiterate the importance of carefully considering this when you are ready to transport your panels to your building site. You will lose some of the advantages of prefabrication if you do not think this out carefully.
What I suggest that you do is think about the last place your panels will be sitting just before you install them. Work backwards from that all the way to your manufacturing and storage approach. For example if you will be moving your panels to the construction site on a truck or trailer and unloading all them before you install them then the relative order of the load on the truck needs to be different than it would if you were going to immediately install each panel as it is taken off of the truck. Again let me stress just how important it is that the next panel you want should be on the top of the pile rather than buried somewhere further down.
You may be able to transport or store your panels standing up on edge rather than stacked flat on top of each other. This can be a very significant advantage if you are able to use a crane to lift your panels into position. It becomes less important that your panels be in the correct order if you can pick a panel out of the middle of the stack. When installing wall panels with a crane it is also easier to lift them by the top edge rather than having them flat on a pile. Of course it should go without saying that the panels should be placed with the top edge up.
Having panel labels or numbers easily visible while they are stacked up can be useful. This is less important if you have stacked your panels in the exact correct order but it is still nice to know that the panel that you are about to move to its final position is exactly the one that you think it is. Of course if your panels get out of order for some reason it is much easier to find the panel you want if you can just look at the edge of the pile rather than having to move all the panels one at a time to find the one you want.
If you are undertaking a DIY prefab project that is going to be built in you spare time and then assembled all at once you will have to deal with storing the various component parts as you build them. This of course suggests that the availability and type of storage space will need to be considered as part of your design process. The good news is that there is a tremendous amount of air contained in your finished building. This fundamentally means that your panels will take a lot less space to store them than they will require when they are finally assembled.
Another good piece of news is that you do not necessarily need to store your prefabricated parts indoors. You do of course need to think about weather exposure and the possibility of pilferage or vandalism. The larger the pieces are that you prefabricate though the harder it would be for someone to walk off with them. Depending on your design approach, the availability of storage space and the means for transportation that you pick you may also be able to store your panels is some sort of bundles or groups that you can load with a fork lift when you are ready to transport them.
One good thought to keep in mind when you are storing prefabricated parts is that you will need to access them later. I mention this because it is important to think about the relative order of access when the time comes to assemble your building. It can be a real pain if the part that you want first happens to be on the bottom of the pile. You should also be thinking about how many times the parts will need to be moved before assembly. For example if you are going to move your panels to a remote site before assembly you might want to store them in the reverse order of need so that when you load them on the truck you can more easily arrange them so that the first panel you need ends up being on the top of the pile.
Perhaps it should go without saying but think about some way of labeling your panels that will help with the assembly process. It is a very good idea to make sure that you can read the labels when the panels are in storage or when they are stacked on the truck. If you discover that the next panel that you need does not happen to be the one on the top of the pile you need to be able to quickly figure out just where it is. Also if you project will be implemented over time labels make it easier to remember exactly what is what. Use some type of label that is suitably indelible too. Years ago I heard of a situation where someone bought a Scottish castle and had it carefully disassembled and labeled. Unfortunately the labels were not waterproof. In the hold of the ship that was transporting the stones to the United States the labels washed off. They left Scotland with a castle and ended up in the United States with an expensive pile of rocks.
As I have already mentioned you might want to have brackets that could help hold framing members in place while nailing them. It might also be useful to have some aspect of your framing table that helps you place the plywood sheathing accurately. In the case of the table in the diagram in my last post the amount of overhang of the plywood at the top and bottom of the wall frame needs to be checked before fastening it in place.
One simple way of providing a movable bracket to help locate your framing members would be to attach brackets that can hinge or otherwise move out of the way when not needed. The following diagram shows one way that this might be accomplished for the left end of the panel table that I have shown in the post about layout table markings. In this diagram the green lines represent the wall panel that is being built on the table. The blue shape represents ½ diameter wood doweling that is used to attach the removable piece of 2×6 lumber. The dowels would be glued into the 2×6 member and would be a slip fit into the top of the table. It would be a good idea to taper or round off the bottom ends of the dowels so that they can more easily slip into the holes in the tabletop. Red is used to highlight handles on the removable bracket to make it easier to place in position when needed. Notice that in this case the wall panel under construction uses 2×4-framing members. It probably would be possible to nail or screw through the top edge of the wall plate without removing the bracket which would help locate everything so that it stays in place after removing the bracket to add the final nails or screws.
One easy modification that might makes sense to make to the bracket would be to add a piece of steel angle iron to each corner of the bracket that would indicated exactly where the corners of the plywood sheathing should come. The following drawing shows one corner of the table with part of the table top shown in orange, the wall frame in green, the removable bracket in red and the added angle bracket in steel gray.
I have not said very much yet about brackets and markings that might make sense to have on your layout table. One thing that I think could make a lot of sense is to make the top surface of your table out of something that is very easy to draw on. How about adding an overlay of some type of surface that you can write on with dry-erase marking pens for example? You could use shiny white plastic laminate or perhaps a layer of smooth white melamine paneling. You could put some markings on your tabletop that were more permanent if you wanted to by using paint or indelible marking pen. You might also consider using colored plastic tape of the type that you can often find in the electrical department of a home improvement store. Different colors might be useful for different things on your panels.
The following diagram shows a table for building wall panels that are a maximum of 4’ wide. The tabletop is made out of a 4’x8’ sheet of plywood. The colored bands might be painted on or could be tape. They indicate where typical framing members would be placed. In this case there is an assumption that there would be some plywood overhang of the frame to connect to a second top plate (on the right) and to overlap floor panels (on the left). The green stripes show the location of the top and bottom plates. The red lines indicate where standard studs would be placed if they were at 16” on center. The blue band in the middle of the panel shows where the middle stud would be if the studs were at 24” on center spacing instead of at 16” on center.
So what should you build your layout table out of? Part of the answer to that question depends on how permanent you want it to be. Another part depends on what types of materials you are comfortable working with. You do want the table to be sturdy. You also want the table to be square and flat so that the panels you build on the table are square and flat. If you were starting up a small factory to turn out a lot of panels you might want to make your table out of metal. For a smaller scale operation wood might be fine. Using plywood (or maybe industrial grade particle board) as a tabletop is a good idea for several reasons.
1.) Plywood sheets are typically fairly square. You might want to confirm that though.
2.) Plywood sheets come in incremental sizes that might be close to what you would want your panel sizes to be. It’s not at all a bad idea to pick modular sizes for your panels that are based on a sheet of plywood or OSB. It does make some sense to pick a panel size that is a multiple of 4’ since it would require less cutting of your sheathing material during assembly.
3.) It is easy to attach brackets at virtually any place you want them on the plywood surface.
If budget permits you might consider using a combination of wood and metal. One approach that I find appealing is to use steel angle iron of at least 1-1/2” on each leg and at least 1/8” thick to create a frame around the edges and along the joints of the table if there is more than one sheet of plywood. The steel might be straighter than a wood framework would be resulting in a flatter table. It would be very easy to attach the steel to the bottom of your plywood with screws through the steel with the result that there wouldn’t be any screws visible from on top. I would suggest that your plywood be at least ¾” thick if you are doing this. It would also be easy to attach legs under the table by screwing through the vertical flange of the angle iron into the legs.
I do think that you could make an entirely acceptable table entirely out of wood though providing you exercise reasonable caution in picking frame members that are as straight as possible.
I suggest that you let the edge of your tabletop extend past the framework for at least 1-1/2”. This makes it very easy to attach a clamp to the edge of your table at any point along its edge.
Here is basic drawing of a 4’ wide table made from ¾” plywood, 2” angle iron and 4×4 legs. The table shown here would be about 16” tall.