The Family Handyman is a great magazine for projects like this, and they have a nice tutorial on building floating shelves here:
Thank you so much Steve! Great article. :)
Those shelves use what is called a "torsion box" design. A hollow core door is not really designed for these type of loads. Most the floating shelf designs I have seen in my 30 years of wood working, are a bit more robust. The skin of a hollow core door is only 1/8 to 3/16" thick luan. For me I would built from scratch using 1/4" ply with fill blocking.
This could be done in "paint grade" or "stain grade" depending on you style needs.
Thank you KMS Woodworks - I actually have 3/4 inch plywood scraps I was going to try to use, how wide (out from the wall) do you think I can build them using a 1 inch 'cleat' screwed into the wall and still be secure? I was thinking 10 inches ... the shelf will be 5 feet long.
With a torsion box design, ply on top and bottom and fill block sides, glued and screwed shelf...set with screws to the cleat. I have seen these out to 16 or so...the deeper the cleat is inside the torsion box the more strength.
Without a torsion box 10" is about the limit. Keep in mind that the cleat needs to be set firmly to the studs.