I've just moved into a new-build and got made-to-measure recess blinds. I need to screw the brackets on each side. Question is do I screw it to the top horizontal recess wall or the vertical recess wall? The vertical recess wall would probably just need plugs and screws but if screwing into the lintel how do I determine if the lintel is metal or concrete? If it's metal do I just drill a small hole with drill bit for metal and use self tapping screws? If its metal, how deep can the screw go before it hits whatever material is sitting on top?