The 'elements' means the weather - rain, wind, snow etc. If rain can get into a small hole in your roof, for example, it will do some damage and this will only get worse and do even more damage. So 'to get a foothold' means to start to do damage that will get even worse. Another example could be the wind. If there is a small tile loose on the roof, the wind can start to get under it (get a foothold) and make it even looser. Eventually it will fall off. If you don't regularly paint your door (if it is wooden) the weather will get a foothold and eventually take off old paint and rot the wood underneath.

HI,Ksenia. What a coincidence! I am also reading The Shining. Emma has given a perfect answer. It means wind or rain would enter the hotel through small cracks or holes.

Great! Foothold is a place where a person's foot can be lodged to support them securely, especially while climbing. Also, it is a firm or secure position that provides a base for further advancement. It is an initial accomplishment that opens the way for further developments too. "Can't get a foothold" Means " Can't infiltrate, Can't penetrate"