I’m gonna try and make this vocabulary list as exciting as I possibly can.
Most homeowners (or people who happen to live in a house) recognize basic terms related to home construction- I’m assuming you all happen to know what a roof is. But do you know what what soffits or fascia are? You may know what a stud is, but do you know the purpose of Kings, Jacks, and Cripples? Read on and learn more than you’ve probably ever wanted to know!
The base of a house is perhaps the most important part of its structure. In homes, you usually have concrete (not cement! Cement is the binder in concrete) going down several feet as a foundation wall, landing on a footing.
I won’t go into detail about foundations and damp-proofing and drainage, since they don’t really relate to building a tiny house on a trailer. However, building off of a trailer isn’t so different than building off of a foundation. I still need to start with things like sill gasket, sill plate, and a subfloor (see diagram). My tiny house will be built on a steel frame trailer. As far as heat transfer goes, metal acts in much of the same way as concrete. It holds in cold for a long time and transfers it easily through conductivity. If I were to put down flooring right over a metal frame, I would have freezing floors all the time. Especially on a trailer, when there’s about 30″ of air between the floor and the ground. So to start my flooring, I would first put down a sill gasket. A sill is the bottom edge of your wall, and the sill gasket is a thin layer of insulation that separates your house from your foundation to prevent heat transfer. In the case of a tiny house, I’m putting my subfloor directly on top of my trailer. This requires two things: I need to make the trailer waterproof from the bottom, and I need to make sure every place where the trailer touches the subfloor is gasketed/covered with insulation.
To waterproof, the trailer is covered with aluminum flashing, or sheets of metal. The gasket goes over this, which is then covered with CDX. CDX is just a fancy name for exterior-grade plywood. If you go to the plywood aisle at Home Depot, there are different types of plywood for different prices: OSB, CDX, MDF, and just plain old plywood. I may get into that in a different post, but not here. No time.
Plywood comes in 4’x8′ sheets. Plywood designed for subfloors often comes as T & G (Tongue and Groove) so that they can butt up against each other nicely without the use of fasteners. Especially with your bottom layer of construction, space to expand and contract with the weather is important. This layer of plywood gets less fasteners (i.e. Nails) in the field than plywood that would go on a wall. In the field means in the center of each sheet, as compared to along the edges. After the next step, it will get nailed upward into the floor joists.
The floor joists are 16″ O.C. This means that there are 16″ between the center of each joist. This makes it so that when I lay down the top layer of subfloor, I don’t have to do math to figure out where each joist is- the plywood sheets will start and end exactly on a joist, and construction plywood comes with nailing grids printed on them. All I need to do is sink a nail at each cross. Also, insulation batts (pieces of pre-cut insulation) come designed to fit into these cavities. And the floor will definitely need the insulation to keep me from having icy floors.
It’s also important to note that the floors in your home are generally built from 2×6″ joists. This is to allow for more space for insulation. But with a house on wheels, I have a limit as to how tall I can build without exceeding DMV regulations. So in order to save some much-needed headroom, I went with 2×4’s.
Onto the next layer of subfloor. This layer of plywood doesn’t need to be CDX, since it’s less directly in contact with the exterior. I’ll likely use OSB (oriented strand board) since it is far cheaper. This completes the floor construction. Flooring comes far later.
Next are my sill plates. These will become the bottoms of my walls. At this point in the construction, I will drill through my last few layers of construction and bolt my floor directly to the steel of my trailer. Those bolts will serve double-purpose later on as the bottom of my strong-ties, also known as hurricane ties.
Next come the walls. In theory, these should all go up in a day. In practice, it may be a little more complicated than that.
In the floor, we called the 2×4’s joists. Joists run horizontally, like the joists supporting the loft pictured above. They were spaced at 16″ O.C. In the walls, the 2×4’s go vertically and are called studs. The studs in the walls are spaced at 24″ O.C. to save weight. They also make batts of insulation for this size. There are several schools of thought as to whether 16″ or 24″ centering is better. My main consideration in the matter was weight. 24″ O.C. simply means less studs, and less weight. Another consideration is that wood creates a thermal bridge between the outside and inside, transferring cold air in. The less studs, the less thermal bridging.
Now it’s time for a quick lesson on studs!
Stud walls are pretty simple as long as there are no windows or doors- generalized as ‘openings’ in the wall. You make a rectangle with a top and bottom plate (top plate not shown above) and place studs accordingly. When you need to include an opening, some changes need to be made.
Anatomy of a Rough Opening. When you buy a window or door, there are generally two dimensions to it. One is the actual dimensions of the window/door, and the other is the RO dimensions. This tells you how much space you need to frame out for the window/door to fit. The RO above is for a window, but the idea for a door is very similar, just eliminate the sill and cripples below.
There are three kinds of studs: King, Jack (a.k.a Trimmer), and Cripple. On either side of an RO, these studs butt up against each other. A king stud is any stud that goes from top to bottom plate. The jack stud/trimmer is directly next to the king stud, and goes from the bottom plate to the bottom of the header. This stud, along with the header (usually two 2×6’s placed with the long side up, over the RO) transfers the load around the RO and down to the bottom plate. Without them, your window or door has the weight of the roof and wall pushing down on it, and this can seriously compromise wall strength. The final step is the cripple and sill. Cripples go on either side of the window sill to hold it up, and they also go above and below the RO where the stud would go if it weren’t interrupted by the window or door. This makes completing your nailing pattern easier and adds strength.
Next is the roof joists! Note that since they go horizontally, they’re joists and not studs. I plan to use 2×6 joists, like is typical in homes. This allows for more insulation and strength. Especially in Vermont, roof integrity is important.
Now a lesson on roof construction!
When we talk about roofs, the important descriptor is generally pitch. Instead of angles, like in some awful geometry word problem, builders think of roof pitch in “x in 12”. For example, there are two roof pitches in my tiny house. The steeper angle is 7 in 12, and the more shallow angle is 2 in 12. Those numbers simply mean rise in run. As someone who has taken math classes for the vast majority of their life, one wants to convert that into a slope and angle. But from a practical viewpoint, this is difficult to deal with on the worksite. Builders can use a roofing triangle to quickly draw angles with linear measurements, while working with degree angles force a lot of math and trigonometry that we will, sadly, never use after high school.
This is an image from an online tool I used to calculate my gables. Individual roof joists are just called joists, but 2 joists viewed from the gable end become a gable; a triangle in several pieces working together in tension to hold up the roof. Thus, the walls on either end of the gables (in the case of my tiny house, the short walls) are called gable end walls. These are load bearing walls; they hold up the weight of the roof. Thus, I may feel the need to add extra studs into this wall at some point; perhaps making these wall 16″ O.C. rather than 24″.
This post is getting pretty long, so I guess you’ll have to wait for part two to find out what soffits are! Hang in there, we’ll get there.
If I didn’t explain anything well enough, let me know!