So initially I thought I had a regular plasterboard. But I have a small measuring thing for wall depth, and after drilling a hole it seems it's deeper than 25mm. The plastboard thickness I see is 10 or 13mm online. So I am wondering if I even have plasterboard?
I am very new to this stuff so just sort of guessing. The apartment house is pretty old, I think it was built in 40s/50s. I am drilling the hole to fasten a standing shelf to it so that it doesn't fall over.
Do I just need a longer fastener? How can I determine how long do I need?