Stacking two full-size beds one over the other, better known as "full-over-full" bunk beds, is becoming more and more popular. Full-size mattresses typically measure 54 x 75 inches (137 x 190 cm), although some manufacturers size their mattresses a little differently. There are plans available for full-over-full bunk beds, but you probably will not find any free plans on the web. Fortunately, it is not hard to make your own plans.
Because full-size beds are not much larger than twin beds, it is a fairly simple matter to use plans for twin bunks and resize them using plans for a full-size or double bed.
Do you want a single unit with two beds, or do you want beds that can be separated? The former is a straightforward construction project, but the latter requires some extra thought. The key is to build two headboards and two footboards that can be used in any combination. The easiest way to accomplish this is to use commercial hardware for connecting the rails to the end pieces. You will also need to center and drill eight 1/2-inch holes--one in the top of the corner posts on each headboard, and one on the bottom of the footboard corner posts. Then you assemble the bottom bunk using both headboards and the top bunk using both footboards; inserting 1/2-inch diameter dowels into these holes will hold the beds in place.
Head space is also an important consideration. You should try to leave at least two feet of head space; platform beds make it easier to create this space than beds using box springs.
Strong corner posts are important on all beds, but especially so as the beds get bigger. Though not much bigger than twins, fulls should never use anything less than 4 x 4s or 2 x 6s.
Mattress supports should be thicker for full-size beds as well. You can use 2 x 4s for the slats beneath the bed. Side rails should be at least 2 x 6s, with 2 x 2s glued and screwed to the inside of the rails to carry the rails. You can also bypass the 2 x 2s and screw the 2 x 4s directly into the rails, but rotate them so a 2-inch side faces upward for greater strength. In either case, you will need only five or six slats to support box springs; if you use a platform made of slats to support the mattress, you will need enough to make an almost solid platform.
The box formed by the rails should be an inch or two larger than the length and width of the mattress; that way, you will have enough room for bedding.
Do not forget to include guard rails. They should rise five or six inches above the mattress to be effective.
Strong ladders are useless if they are not attached securely to the bed. You will probably want to use metal mounting hardware for this purpose.