We use 2x6 tongue & groove with a V joint for our beams to simulate the log look . The logs are made from # 1 & 2 Spruce or Pine. The actual species is Eastern White Spruce, Kiln Dried. Eastern Spruce does not have the problems other softwood has of gum (resin) bleeding from around the knots and causing unsightly dripping down the sides of the walls.
The floor is made from 3/4" tongue and groove either White or Red Pine. The roof boards are made from SPF - solid 3/4" thickness. This is superior in all aspects to either particle board or plywood, as it will not delaminate in wet weather if it happens to be erected and then sits for any period of time without shingles being applied.