Is it the sill plate or the outside sheating that the 20' x 30' comes from ? ???
I don't have that exact plan available but the dimensions are usually the exterior 2x framed size, sill plate dimensions. Exterior sheathing goes over that increasing the actual OD a little. The sheathing can then lap over a little to cover the sill plate to foundation joint.
When planning the subfloor, the sheathing deserves some thought. With T&G sheet goods (OSB, Advantech, plywood) it is normal to lose about 1/2 inch of width as the adjacent T&G joints are fitted together. So if the floor framing is laid out to be exactly 20 feet on the outside of the edge of the framing your subfloor sheathing falls short by about 2 inches. It is not acceptable practice to use a narrow filler strip at the outside edge as the subfloor should be nailed to the rim joists to complete the floor assembly with the greatest strength. Two solutions: make the structure narrower by the required amount or rip panels for the edge start to be a few inches wider than 24". Then when the other side is reached you rip that final row to fit. There may be some waste but often the cutoffs get used someplace for something.
If there is a chance the framing and drying in construction phase may encounter rainy weather choosing Advantech (http://huberwood.com/advantech/home-advantech)for the subfloor is smart. It can handle puddles with no problem. It comes with a warranty.
I just got thinking about a sheet of plywood.
Very often I see great care given to plywood efficiency in floor and wall framing, and it all falls apart in the roof. If you are trying for efficient materials use keep drawing the framing plan on up through the roof.
One of the things I'm learning (slowly) is what I draw and how it comes out are not always the same. Its like "straight" wood .
Thanks,Guy's.
Also, where is the right place to start my 16" on centers from. Is #1 or #2 ?
(https://i.imgur.com/v69IUxP.jpg)
First, I never went to school to learn this stuff. With that in mind, I do...
#2. Start the OC measuring from the corner edge of the stud. Remember to adjust for the wall that meets the first wall at the corner; take the wall framing thickness into account. I measure on the exterior side and mostly don't worry about the interior side. My first sheet starts right at the stud edge. If we're using even sheets with no trimming I don't worry about the "corner gap" that results. That gets covered over with whatever comes next, foam insulation, stucco, whatever... When looking at corners also try to visualize how the insulation is to be inserted in the cavity if infill insulation is the method to be used.
When marking for stud positions I mark both sides of the stud and place a pencil X in between. That is my method. Pick something that works for you. If you have helpers you might want to mark them all yourself unless you have complete trust in the helper.
Thanks MDon.