Rats are often found hiding out under garden decking, in fact this is one of the most common places I'm asked to investigate rat infestations.
While the decking provides a great place for rats to shelter its not the main reason that they are there. The main reason that you have rats in your garden is food. Most of the time this is seed from bird feeders but is can also be composting food waste and pets including chickens and rabbits. The decking is just a convenient place to hide out between trips to eat which could be in yours or one of your neighbours gardens.
Rather than removing your decking I would always recommend removing the food. If this is not an option and you are suffering repeat infestations then removing the deck might be the best course of action.