Once the Mud Sills are in, you have something to attach the house to. Now the Rim Joists must be put in and the Floor Joist pattern decided upon. Twelve feet is not an enormous span for a Floor Joist. As a result, I will show you a very simple pattern of Floor Joists. The issue becomes the Sub floor. If Floor Joists in a parallel pattern are installed, there is a place to nail down all the edges of the 4x8 foot Sub floor plywood, but going across the joists, there will be no place to screw down the edges of the plywood, so the joints between the sheets can move when walked on. There will be flexing, squeaking etc. as the seams are just floating. This might be fine for a shed, but to live with that, would be unacceptable.
You can purchase tongue and grooved plywood.
You can add short lengths of 2x stock just under the seams.
I will address each of these.
All of the joists and rim joists will be made from 2x 8 inch by 12 feet.
Dimensional lumber is always the length quoted, but the width and thickness of the wood is not. Wood is cut to the dimensions that you order. A 2x8 will begin its life measuring 2 inches by 8 inches. The lumber is then planed so as the give you a uniform and splinter free surface. The final dimensions will be 1 1/2 inches thick, by 7 1/2 inches wide. In general no matter what you are ordering, the final dimensions will be 1/2 inch less in both directions.
Plywood is exactly what you order. If it says 1/2 inch, it really means it. So it is with all "sheet goods".
When I was a child, most of what I saw for construction had joists and wall studs 24 inches apart from the center of one to the center of the next. But now, all work on anything important will be 16 inches "on center". This divides evenly into the 48 inches of the width of sheet goods. One on each edge, and two spaced in the middle.
We are going to run into a little problem here because while the 12 foot wide building we are making will perfectly fit our flooring, trying to keep the nailed edge on center will mean that joists closest to the rim joists will have to be a bit closer together than 16 inches. But that is just planning, and we can certainly do that. This may not seem clear right now, but I will take you through it.
Cut two of the 12 foot boards to 11 feet 9 inches long.
Next, choose two 12 foot boards whose ends seem to be in best condition. Avoid boards that have little cracks and splits. If this is not possible at the lumber yard, buy the next longer boards and cut off each end to make a nice clean end. You could use pressure treated wood, but examine them carefully to make sure the quality is there. This might be a good option where snows get deep and water is an issue.
Nail metal joist hangers onto the long boards at 16 inches on center. Use galvanized nails that are recommended by the hanger manufaturer as they will vary. The hangers at each end will be in a different location, so refer to the illustration for the correct placement. Measure to the center measurement given. That will be where the middle of the joist will eventually be. Nail one side of the joist hanger through the holes into the rim joist. It would help to use a combination square to draw a line 3/4 of an inch on each side of the center line. Once the first side is nailed down, put a scrap of joist (cut from the short boards) into the hanger, and push the second side into place tightly against the scrap. At the top of the hanger, there is a little raised prong. Keep the hanger snug against the scrap by pounding this into the rim joint. Now you can nail the second side to the rim joist, hopefully very close to your line. At all times, make sure that the bottom of the hanger is flush with the bottom of the rim joist, and that the top of the floor joist is going to be flush with the top. These hangers are wiggly little suckers, and it is easy to nail these in place with the bottom of the hanger not quite square. Your joists will probably end up in the same spot, but you may not have quite the vertical support you want. Install all of the hangers before proceeding.
Place one of the longer boards with the metal hangers already nailed in place, along the outside edge of the sill, standing on edge. Make sure that the rim joist is flush with the outside of the sill. An extra pair of hands will be a good idea here to hold it steady and flush. Nail through the bottom of the rim joist at a 30 degree angle, into the sill using 8d(8 penny box or common nails, galvanized is best) nails every 6 inches. Resist the temptation to nail many extra places. Each time you nail into wood, you cut fibers and split it a little. Eventually you will weaken the wood with too many nails. If for some reason you have a gap in the boards you are nailing into; you might put an extra nail there so it is nailed on both sides of the seam.
Next, stand one of the shorter boards up on the sill in a similar fashion, butted up against the inside of the first board. Nail this board into the sill as well(I do this first because you will be pounding into the corner next and it is best if the two boards are both secure before you do this.
Using 16d nails(you can purchase specially coated nails for this that will literally glue themselves into the end grain of the short board) nail four or five evenly spaced nails through the long board into the end grain of the short board. Nailing into the end grain of any board is a weak joint. I also like to move my nails just a bit farther from the edge, say 1 1/4 to 1 1/2 inches from the end rather than 3/4 of an inch. Nailing at a slight angle, gets you a bit farther from the end of the board, thus reducing the likelihood of splitting the end. Don't over do this though, as the distance increases and the angle increases, you have less length to enter the end grain of the second board.
Continue with the second long board with hangers, and finally with the second short board, Nailing into the sills and butted ends as you go.
Cut all of your floor joists to the same length, 11 feet 9 inches, dropping them into the hangers and nailing through the hangers into the joists as you go. But first, nail in one 10d nail through the top of the joist at an angle into the rim joist at an angle to make sure it stays seated and steady in its hanger. There are two types of hangers available. In one type 1 1/2 inch joist hanger nails drive straight into the floor joists. The second type, double shear joist hangers force you by the shape of the holes to nail at a 45 degree angle through the hanger, through the joist and into the rim joist. A much stronger joint to my mind. Use 10d galvanized nails for this. Pound in any of the little prongs that are on the hangers before you nail.
I find the very idea of putting diagonal braces in between the floor joists extremely tedious. You can make braces out of scraps of lumber(called herringbone struts) or buy metal ones that are a bit easier to work with. You can also buy an extra piece of 2x8 and cut a number of them 14 1/2 inches long and nail them between the joists as you install them. (Again, the end blocks will be shorter because of the different On Center placement.) I would install two rows of the blocks at the four foot(from the outer edge of the rim joist) and at the 8 foot point from the same spot. This will mean that besides reducing the twist of the timbers as pressure is put on them, but also giving support to the seams of the plywood sub floor. The metal ones do not perform this second function. Staggering the blocks so that you can nail through the joist to the block without the previous block being in the way is convenient, but putting them right on the 4 and 8 foot line will provide that support. It just means that you have to nail through the joist at an angle to bypass the previous block and into the end grain of the one you are working on. (see the illustration.) Three nails on each end will suffice.
Now you may begin to lay the sub floor.
Use 3/4 inch CDX tongue and groove sub floor. Some people say that 5/8 inch will do, but you have a good long span here, and every millimeter of material will help with support.
Install small scraps of wood to the side of you rim joists. They will be flush with the edges of the rim and rise about an inch above them. This will allow you to assemble your entire floor before nailing, without the sheets slipping off the sides of the floor.
Go around the rim joists and mark on the outside of the rim where the center line is for each floor joist and each row of blocking.
You should plan to lay the sheets in a brick pattern. That means you will need three full sheets of CDX and three half sheets(Don't worry, you will use the extra somewhere, unless they let you buy a half sheet at your lumber supplier.). You can use regular 3/4 inch plywood, which is a blessing if you have a pile of it to use up, but the floor will not be as secure.
Lay a full sheet in one corner, then the half sheet to end up at the other end. Starting at the same side, the second row will begin with a half sheet and then a full sheet. The third will be full sheet and half.
When all of the flooring is laid in place, the tongue and groove firmly locked in place, or the plain joints firmly butted together, snap a chalk line from the marks you made on the outside of the rim joists to the opposite side, and along the location of the blocking in the other direction.
An extra, but a very good step is to run a bead of adhesive caulk made for sub floors on the top of every joist and block, but you will have to place the sheets more carefully so as not to smear the caulk. In this case you definitely need a second pair of hands.
A chalk line is a string that is coated in(usually blue) chalk. One person holds the end of the line against the mark at one end. The other person takes the spool end to the other end of the floor, trying not to drag the line across the plywood floor as you go. You will end up with big smears of chalk if you drag it.
Pull the line fairly tight.
Either a third person, lifts the tightly drawn line and lets it snap back into place, or the person holding the end tacks it into place with a small nail and snaps it.
A person working alone might tack the free end, and wrap the line several times tightly around a second nail, friction holding it in place. He then snaps it himself. Move across the floor and snap on every joist center and on the blocking lines.
Using a screw bit on your drill, screw 2 1/2 inch decking screws straight into the joists every eight (or less) inches along the chalk lines, and on both edges of the seams at a slight angle into the joists below. The screw heads should just disappear into the surface of the flooring or at the very least, be flush with the surface. Put considerable weight on the spot you are screwing into, so that you have excellent contact with the joist. Also screw into the Rim Joists in the same manner making sure to put a screw into each side of any seams, and try to hit the mid point of the joist.
If any of the seams are raised for some reason, check to see if the screws are screwed in all the way. If it is still raised, use a belt sander to sand it flush.
You may seal the entire floor with a roller or a pad(like a mop) using a sealer recommended by your dealer.
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