Advanced Framing Techniques
What is it:
Advanced Framing is a method of framing that minimizes weight and materials needed by putting lumber together in very specific ways to make it structurally sound in efficient ways. Advanced Framing is gaining in popularity but is still a less commonly used method of house framing. Generally advanced framing uses 2×6 lumber (generally Douglas Fir, exception to 2×6 below) placed 24″ on center so that sheet goods like plywood sheathing, OSB sheathing and drywall (which come in 4’x8′-12′ sheets standard) can be fastened to the structure on the perimeter, these are the walls ‘base studs’. If the spacing was any less or any more than 24″ on center the studs wouldn’t line up on the edges of your sheet goods and could lead to more cuts and compromising the integrity of the material (sheathing is structural and offers less structurally in smaller pieces). These studs are spaced on top of a ‘bottom plate’ which is how the wall is fastened at the bottom to the trailer. A main difference, aside from stud spacing, between standard framing and advanced framing is that there is generally a single top plate at the top of the wall instead of the double top plate you see in standard framing. Advanced framing is able to happen with a single top plate because it requires more planning and requires that roof joists be distributed in line with wall studs so that roof loads are carried directly down wall studs instead of potentially bearing at a mid point on the top plate between wall studs of standard framing. Generally advanced framing is fastened with 16 penny (aka 16-d) framing nails which are 3.5″ long.
The exception to the 2×6 stud requirement is if you are building one story and only supporting the roof load, in that case 2×4 lumber is acceptable.
It has most commonly been recommended to use sheathing (either OSB or Plywood) that is at least 7/16″ thick in order to offer your structure enough shear strength. There are alternatives to this such as additional strapping and T-111 siding which may be able to save you some weight while not compromising structural integrity.
Roof ‘rafters’ are generally 2×4 -2×8 lumber (Douglas Fir) placed 24″ o.c. depending on dead and live loads to accommodate and length of span required as well as amount of insulation desired. Various roof styles require different size rafters. It is important in advanced framing that your roof rafters line up directly over your wall studs for continuity. It is fairly common for people to take certain aspects of advanced framing and not others. If your rafters do not line up with your walls it is okay to do a double top plate as in Standard Framing so that it is able to carry the roof load over to the nearest wall studs without failing. In addition to less lumber you are encouraged to use much more strapping and clips to reinforce your build with advanced framing.
After you have all of your studs spaced out at the appropriate distances you can start to add your door and opening (window) locations. In the diagram below standard framing is on top while advanced framing is below, you can see the difference in amount of lumber used for the same wall:
You notice there is a lot less lumber in advanced framing. One other thing to note in advanced framing is how you connect interior walls to exterior walls, in standard framing you would tie them in with an extra stud so that you get a sturdy connection, with advanced framing you can use ‘ladder framing’ where you can brace with just a few pieces of blocking to attach to, this helps limit thermal bridging.
Each opening has a ‘header’ which collects the weight that would otherwise be carried by the wall studs and distributes it downward around the window or door desired. With advanced framing you don’t have king studs and trimmer studs like in standard framing, you are able to use less lumber by incorporating the use of metal clips (generally by Simpson) to attach to a base stud. You can add an interim stud to carry the load down if the window/opening is not on a 24″ module. To create your rough openings for your doors and windows you generally take the size of your door/window and add 2″ to the width and height, this allows you room to level and square up your window/door as needed. For windows you have an additional ‘sill’ which defines the bottom of the rough opening for the window. The rough opening is generally about 2″ bigger than the actual window being placed, dependent on the size of the window flanges on the window (you need enough room to level and plumb you window in case the opening is slightly out of square but not too much that you cannot fasten the window flanges on the entire perimeter). The sill is supported by cripples which are aligned with the base studs that are placed 24″ o.c. for your sheet goods.
Generally a corner condition where two walls come together in an advanced framed home looks like this:
You can minimize the extra stud usually present by adding some clips to catch the drywall or whatever interior finish surface you are planning on having. The 24″ dimension would start from the outside corner of the structure to assure that you have a nailing surface every 4′ along the outside perimeter of the structure for your sheathing (which is structural).
There is less lumber used than in that of standard framing techniques which leads to less thermal bridging and a more efficient building structurally and energy-wise. While you may think it would be less expensive than standard framing because of the lack of lumber it is pretty much a wash because of the extra metal clips and strapping suggested. Advanced framed buildings are slightly lighter weight than standard framing and so may be desired for tiny homes on wheels.
This is less commonly known and understood than standard framing and has some critics that go along with that because it is less widely understood. There is a significant amount of planning that has to go into the design of your structure to insure that all of the members are attached and located appropriately, there is less room for error in construction.
Tiny House Specific and Regional Considerations:
Advanced framing by nature incorporates a lot of the additional structural measures you would want to take when building a house on wheels with any kind of framing (strapping, hurricane ties, etc.). In most cases it is most desired to use 2×4 lumber rather than 2×6 lumber to maximize your interior space in a tiny house, the requirement of one story can cause issues if you intend to have a loft space (since that is now supporting more than just a roof). You can add extra structural measures, such as using rigid or spray-in insulation which actually adds structural value to your walls (making them up to 150% the strength of just a stud wall) to help accommodate a loft while still using 2×4 advanced framing techniques. Additionally you will need to secure your walls solidly to your foundation using a minimum of 3/8″ lag bolts (recommended at 24″ o.c.). It is also recommended to add Simpson Hold-downs at the front corners (minimum).
Wall, roof and floor thicknesses may vary based on the amount and type of insulation desired. Typically they are 2×4 construction but in some cases more depth may be desired for a greater R-Value.