I am making a loft bed for each of my kids. They each have full-sized beds, so the mattresses are 54"x75", and that's what I'm building the frame around. The verticals for the corners and ladder (part of one corner, so 5 vertical posts in total) will be 4x4s, and I'm planning on making this out of Southern Yellow Pine. The support structure underneath the mattress will be: a perimeter of horizontal cleats attached to the bottom/inside of the 1x6 rails, 1x4s across the width of the rectangle, and 3/4" plywood atop that.
So here are my questions:
Does the support for the mattress need a central piece down the middle halfway between the side rails, since it's a full-size mattress (dashed brown line in top view sketch below)? If yes, what size and best way to attach it to the middle short ends of the box?
1.jpg
What size should the supporting horizontal cleats be around the perimeter? Below I made a sketch with the cleats in green, as seen from above and then as a cross section.
2.jpg
3.jpg
Thanks for any suggestions. Apologies for my artistic skills. I hope my questions make sense.
Matt