Rat traps are great for catching one rat, but try to catch 5 and you will fail because rats learn fast.
It's unlikely that you will have a single rat as they are a very social animal and tend to travel as a family or in family groups.
A solitary rat may exit a nearby drain or sewer and find its way to your home. He'll explore and if it looks good, he'll go back home and return with all his friends, leaving a sent trail to mark the way, much the same as dogs mark their territory.
The best rat traps involve catching the rat live when it enters a one way door that locks behind it. There are some large (washing machine size) and expensive ($1,500) commercial rat traps that have a drowning bucket and these can catch several rats per night since all the action occurs inside the machine where other rats cannot see.
If you have a rat problem I would suggest not bothering trying to trap them, but call us instead and get an effective job with a warranty and peace of mind knowing it's safe and done well.