Foundation and Floor: Start and finish there
Walls and Ceilings are this one
Now, if you've exercised a decent amount of perfectionism in square, plumb, level, and centers, the rest is going to be easier. If you were sloppy about it, the building is going to fight you to the bitter end. Your walls are next, and what you decide to use for siding will determine some of how you build them. If you go with metal, you'll want to use firring strips. If you go with vinyl siding, it'll be plywood or osb (or possibly a mixture of that and rigid insulation.) You also have to decide between 2x4 studs or 2x6. If you go with 2x6, you'll be able to fill it with R-19 insulation. If you really want to make it energy efficient, you can go with 2x6, with exterior foam insulation.
You'll also want to have your doors and windows planned out. The first things you'll want to cut are the headers and the jack studs (which support the headers). For a 36" door, your header will be 41 1/2" long. This accomodates the 3" that rest on the two jack studs, the 1 1/2" of door trim (1x4 on each side) and leaves you 3/4" to shim it plumb on each side. Normally, your jack studs will be 81 1/2" tall, which gives you rough opening height of 83" when you set that jack stud on the bottom plate. Some doors are taller than the "normal," so double check the height. It's not a bad idea to use the same length for your jack studs for windows. A 36" wide window gets a 41" header, with the difference being the area used for shims. You're going to want to see out, so plan on at least one window per wall. As long as you aren't building a two story structure, 2x6 uprights and either 2x4 or 2x6 top and bottom plates (same as your wall studs) will suffice for headers. If you think you might add a 2nd story, go ahead and use 2x8 uprights. The headers distribute the weight of the structure above to the jackstuds where the stud is missing. If your window is more than 6' wide, you'll need to add 3" to the header and double up the jack studs.
The first thing you'll want too nail (not screw) together are the headers. After you've built the headers, nail the jack studs to the king studs (the full height 2x next to the jack stud), flush to the bottom of the king stud. Next, you'll need corners. Find 12 pieces of short 2x4 scrap and nail 3 pieces (top, bottom and middle) between two studs. This gives you a place to attach the other walls to, as well as a solid surface in the corner for drywall. If you're building 2x6 walls, you'll need to build your corners differently, so that the inside 2x6 is exposed on the inside of the wall, when the other wall is built. This can be done by using 6 pcs of scrap 2x6 on edge rather than flat between the studs.
If you went with a rectangular structure, rather than square, you'll want to start with the long walls. Chances are that your floor is not perfectly square and straight. On each corner, make a mark 3 1/2 or 5 1/2" for each wall and then pop a chalk line to mark the inside of your wall. Lay down your plates and pull 16" (or 24") centers, making sure the two boards are the exact same length, and flush on the end you're pulling from. I like to have rafters fall over studs over joists, but that isn't a requirement. On a 16' structure, studs will fall on top of joists, naturally, but if you built a 15' wall, this will happen if you pull from the same corner that you did your joist layout. It does add strength to the structure to do this, and it makes layout easier, but the double plate and structural system does not require it. It does add a wrinkle to the roof system.
After you have the plates laid out, the corners, headers, jacks and kings nailed together, you'll want to put the jack studs and headers in place and nail them together, and finish laying out the wall with corners and studs, and nail the top and bottom plates to them. Some framers prefer to nail the plywood to the wall while it's still on the floor and others prefer to do so after all the walls are framed. The plywood holds the structured together and the walls square, so if you do this on the floor, you'll want to pull corners and make sure it is square before nailing plywood to it. I highly recommend that you pull the measurement for the substructure, subtract that from 48" and pop a line that will mark the top of the first run before doing so. By running the plywood down to the bottom of the joist, it will decrease drafts, and tie the two systems together.
After you've built the wall, you'll need to put a second topplate on top of the first. Cut this one 3 3/4" shorter than the first, on both ends. This plate does two things. It helps distribute the weight from whatevver is abovee it and it ties together the corners of the wall.
That wall is going to be heavy (and if you nail plywood to it, it'll be heavier). You can lift it on your own, but you're going to need to leverage it. Once you get it upright, it's easy to hold it there. IF, you nail a few short boards, with a single nail pivot, towards the ends, this will help you lift it. Say, you nail a 5 foot board, 4 feet up the stud, with a single nail and the free end of the board at the top of the wall, as you lift the wall, it will fall free to the floor, and help you keep the top of the wall moving up. If you nail another
Another possibility is that you could push the bottom of the wall off the side of the floor. If you nail a couple of short boards upright from the joists, it will catch the bottom of the wall. Once you have the wall upright, run a short board from the corner stud to the joist on a 45 degree angle (or 30 or 60 degrees) to hold it upright, on each end of the wall. Next, you'll want to make sure the ends are flush with the floor and the inside edge is on the chalkline. Start at one end and nail the bottom plate down, adjusting it to the chalk line as you go. Repeat, the process on the opposite side.
The perpendicular walls are slightly different. The end is a single stud, and the 2nd toppllate is 3 1/4" longer, rather than shorter. This is where you tie the two walls together, nailing through the end stud into the corner studs and down through the top plates into the other one. Make sure the corner and stud are flush on the outside, before nailing. Lifting the wall is going to lighter but if you nailed the 2nd topplate while it was on the floor, you will have to have that an inch and half off the floor to get over the other bottom plate, and lift the wall on an angle since the top plate is longer than the opening between the two walls.
After you have the 4 walls up, plumb the corners. (Plumb is vertical as level is horizontal). Brace them off. Then pull a string from each corner to corner along the wall and brace it straight.
Ceiling joists are next and they're pretty much the same as floor joists but 8 foot above the floor and requiring lumber of smaller dimension. On a 16 ft span, 2x6 is fine for ceiling joists, as the only weight will be the insulation and drywall (or whatever you use as a replacement. It's not a bad idea to run rim joist and a 2x4 plate on top of your ceiling joist, but you can run the rafters beside the ceiling joists.
This is a good place to run out that sheathing, the plywood, on your walls. Pull up from the bottom of your joist 48" at each corner and pop a chalk line to mark the top of the first run. It's a lot like running the floor decking, without the tongue and groove, and veritcally. To make it easier, nail a few pieces of wood to set the first run on while you get it started. Start your run by nailing the outside corner along the 4' side flush with corner. By pulling the 8' length to the chalk line, it will straighten your corner. The strength of plywood is along the 8' length. Some will set it on end, but that is stupid and lazy. Plywood is not just something to nail your siding to. It ties your house and the individual parts of the structure together. If you go lazy and leave a seam over the break between floor and wall, it significantly weakens the whole system.
Thus far, framing is a fairly simple process, but stairs and rafters can befuddle carpenters with a few years experience. If you're working this project with a helper, the foundation probably took you a day, and the rest (so far) a couple days. As soon as framing starts, it's a race to get it "dried in." When you plan the project, you want to plan it top down, with one principle in mind: gravity. You're using gravity to bring all the weight onto those concete pads you poured. With roof, another factor of gravity comes into play: water rolls down hill.
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