The upper deck should be a minimum of 5/8 inch thick material. Either OSB or plywood. The best material to use for the desktop is OSB that has a finish veneer. A finish veneer such as Formica works well. Most big box retailers sell pieces of Formica veneer OSB in 16 inch widths with finished edges.
Figure 9 - Front view of installation of upper loft bed deck and lower desktop
Figure 10 - Side view of installation of upper loft bed deck and lower desktop
The upper loft bed deck should be screwed to the 2 x 4 frame ever 6 inches using 1 1/2 to 2 inch, #6 or #8 flooring screws, the use of carpenter's glue, while not mandatory, adds additional strength.
The desktop should be held in position using angle brackets (Figure 11) mounted from underneath the frame. One angel bracket at each end is sufficient to hold the desktop in position.
A 2 x 4 desktop support, as shown in Figure 9, should be added to the center of the desktop. Without the support the front of the desktop may sag, depending on how much weight is on the desktop and whether or not the user of the desktop rests their weight on it while working. Use two angle brackets (Figure 11), one on the bottom and one on the top to hold the desktop support post in position.
Figure 12 - 1 1/2 x 1 1/2 inch slotted angle steel
As an alternative to the desk support post a piece of 1 1/2 inch slotted angle steel (Figure 12) can be run from one end of the desktop to the other, on the bottom side of the desktop, approximately 4 inches in, from the front edge, as shown in Figure 13. Use short woodscrews and washers if necessary, to fasten the slotted angle steel to the bottom of the desktop.
A standard twin size mattress could now be placed on the upper loft bed deck and for all intent purposes, other than a ladder to get up to the bed (construction shown further on), you could call the project complete.
However, in most cases a small railing is installed around the mattress. There is no need to install a railing against the two solid walls. You may wish to have a railing on the open front and the open side.