I’m planning to make some floating shelves 150 cm wide and 30 cm deep.
I want to mount them on a wall that has a plasterboard of thickness 13mm behind which is a plywood backing of 15mm according to the details provided by the contractor (see the attached image). This construction was apparently intended for hanging TVs and they recommend using wood screws that are not longer than 30mm.
One of the shelves will have my record collection which could weigh between 25-50kg. What are some options for hardware I can use to mount the floating shelves?
Yea, the pocket door is a challenge, but there should be plenty of room - there should be a lot of space from the door to the ply panel. Once installed, a Molly should only extend about 1cm. Just make sure to cut the screw down once installed (the screw is longer so it can compress the Molly, easy to cut with a pair of dikes).
Optionally, you can use a wood screw for that end, one that's long enough to just go through the ply when set at the right depth. This will ensure you have maximum thread engagement.
Wood screws will probably be OK, it just makes me a little nervous with that weight. Of course, more screws=more distributed load, but then you have to get them all to line up perfectly, which isn't easy.
I think you’re right! I just checked and the pocket door is definitely much further behind.
Now the question is - how exactly do I use these to mount the shelf? Is there some sort of hidden bracket I can buy?
Lots of ways to do it, the most common I've seen are like this:
https://www.ultrashelf.com/collections/floating-shelf-brackets
Where holes are drilled into the shelves and then they're slid onto the rods screwed to the wall. Hard to level but looks great if done right
Wow, 50 lbs (~20 kg) per stud, with a bunch of screw holes in the base plate. Problem solved, and you could just use a number of wood screws into the ply panel, just spaced well apart (like 2.5-3 cm), and anchor into a stud where you can with a long lag (3"/~8cm).
Or if you can only hit a stud on one side, use a single Molly on the other and a couple wood screws in between.