Most residential framing is done with wood. Depending on where you live in the country you will be using southern yellow pine, pine, or douglas fir etc. When it is cut down the wood is full of water, when it is cut into framing lumber it has to be cut in larger sizes than what you buy. Then its kiln dried to remove the moisture. The drying process is very important, without it the wood you use to frame your house, addition, shed etc. would shrink and warp. After the wood is kiln dried it is milled to the size of lumber that you buy at the lumber yard. The wood can still absorb moisture from the air, so once unbundled, it can warp and shrink as it would have if it were never dried, only not near as much. Why is this important to your building projects?
When you are framing floors, walls, ceilings and roofs, all of the lumber will usually have a natural warp, most very slight, some so bad you have to toss it out. When you are framing a floor system for example you need to look down the edge of the lumber and decide which side has what is called a crown. A crown will be the edge of the lumber that warps upward in the center when holding it on edge. In other words a 2×10 would be held up on one end with the 10″ dimension being vertical. Then looking down the top edge, if the center of the lumber rises or curves upward then that edge is the crown.
When installing the floor and ceiling joists or roof rafters it is very important to install them with the crown up. When framing the walls while they are laying down it is also very important to nail up the studs with the crown up, and if you are framing walls in place vertically you need to be sure the studs crown all face the same direction. This applies to the roof rafters as well. The reason this is so important is that the wood will usually warp in their natural direction and if you dont have the warping going in the same direction, when you install the drywall or interior/exterior wall finishes you will have some studs that have warped toward the outside and some toward the inside making it difficult to properly attach the finishes and produce a very unattractive finish as well as causing nail pops in the finish later on.
With the floor joists, ceiling joists, and roof rafters it is even more important because of the structural aspect. That is that over time gravity can cause the lumber to warp or sag in a downward direction. If all of the natural crown are installed up then that effect will take a very long time to notice. Just imagine if the floor, ceiling and roof joists were installed with the crown down,the floor, ceiling and roof would probably, if not immediately, very soon look like a bowl. Also if the crowns are not all the same then the floor, ceiling and roof will have noticeable rolls in them. One more potential problem that can arise if the floor joist warping that is going to happen happens in different directions it can cause cracking in the wood or tile flooring.
Make sure you always crown all the framing lumber installed in whatever you are building.