I saw the threshold I need at a hardware store today. It's for an external
door and comprises of a piece of wood the width of the door and a piece of
metal to stick out as a kind of step. There are screws in the wood to raise
or lower the threshold to meet the bottom of the door. If I had know that
they were for that, I would have raised the threshold on our own door years
ago to prevent the damp getting in and causing the wood to rot.

Anyway, if those screws are for raising and lowering the threshold (and not
screwed into the slab like I thought), how is the threshold held in place.
There's plenty of caulk around where it meets the door jambs but how is it
actually held to the slab?

Thanks.

Paul