Without Masonry plugs the screws would not grip intot he substrate and would not provide adequate fixing to allow for a secure fix for the works being carried out. Delta Plug masonry Plugs. Once you have chosen a drill size and made a hole in you substrate you are then able to push a plug inside the hole till flush, Once this is complete a screw is able to thread into the plug which expands the leafs of the plug at the back which in turn increases the friction of the plug on the inside of the hole.