Carpentry and Woodworking - Loft bed design - suspend from rafters

I'm unsure how to determine the best way to design a loft bed. Either I want to:

string two 2x6 boards from rafter to rafter to create two sides of the box frame, or I want to:

create a box frame for the bed out of 2x4's, screw one end into the wall, support the other end with two metal cables from the rafters.

The pitch is 45 deg.

I'm at a slight quandary - how do I determine the amount of weight the rafters can safely hold? I've seen pictures online where people have supported one corner of a loft bed with a cable, but never the entire bed hanging from the rafters.

Thanks!

The amount the structure above can hold is a tough question. There are some rules of thumb books you could find that may help - if you're comfortable with making the decision yourself, but the safest bet is to have a structural engineer or at least a well qualified contractor or framer look at it. A bed may be holding the weight of more than one person, and is not purely an up and down force.

Often roof structures are designed to hold much less weight (load) than floor systems. The amount they will hold varies depending on the pitch of the roof and your climate. If you don't want to bring in a professional, I would support the bed from the floor.