You can use 2 X 4's for the walls and 2 X 6's for the ceiling/roof joists but it depends somewhat on local weather conditions. If the cabin is in a cold area in winter, you may want to opt for thicker 2 X 6 walls and add extra insulation. For other walls, you can tear out the interior paneling and add 2 X 2's to existing 2 X 4 studs and add extra insulation as well. Or build a 2 X 4 wall and add 1/2" of plywood or chipboard exterior sheathing and 1 1/2" of foam board or more to the outside under any siding to make up the difference between 2 X 4 and 2 X 6 dimensions as added insulation. The problem with that method is using very long nails to attach any siding...better to just build with 2 X 6 and insulate between studs.

And if there may be some considerable snow load at times, 2 X 8's would be better for the roof...especially if they are installed on 2' centers rather than common 16" center to center.. Add or sister in 2 X 8's alongside any old 2 X 6's as better support for joists that will not be replaced due to mold or rot. But for areas with little to no snow load, 2 X 6 should be fine for roof joists of that span of 15'.

My dad has just recently given me my family's cabin. It was originally built with one room about 30ft long and 15ft wide. The front and sides were made out of old wooden military boxes. The back of the cabin was connected to an old school bus that my family used as the sleeping area. As time went on, a kitchen was added to the front of the cabin, a covered porch, a bathroom, and siding all the way around. The kitchen and bathroom were made out of 2x4 walls and 2x6 ceilings. About 3 years ago, the roof began to leak in a few spots in the original room which is now the living room and in 2 spots in the kitchen. Since no one has taken care of it for a few years, black mold spots have formed in the leaking areas of the ceiling. So now that it is mine, I decided I wanted to fix up the place and turn it more into a home/cabin. After taking off some paneling on the inside wall where it has been leaking from the ceiling down into the wall, the ammo boxes also developed mold. This is the front wall of the cabin. So after removing a few boxes, I discovered that the walls were built basically 6" thick because of the width of the ammo boxes. So question is, can I put up a 2x4 wall in place of the ammo boxes underneath the original 2x6 header, or should I go ahead and keep it as a 2x6 thick wall? The roof pitch is 2/12 or 3/12 with no attic. Can I replace the 2x6s in the roof with new ones or do I have to go with 2x8s? No building permit is required. I checked already. Thanks in advance.