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I have rat traps in my attic and I catch a rat every few months.

Answers on rat/mice problems - such this one There's a mouse in my house! - always include the suggestion to track down how they are getting in. Any suggestions for how to go about doing that? Finding a tiny hole in a large attic seems like a daunting problem to me.

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Look around the outside of your house for rub marks (dark greasy spots), rats tend to use the same path as they travel between nest->food->water and can leave behind markings of their travels. – Tester101 May 5 '11 at 12:19
Given that i catch a rat so rarely, how likely is it that the rats are infreqent visitors? Or is it more likely that I have a huge nest of rats that have learned to avoid the traps? – Kevin Lawrence May 6 '11 at 6:32
Rats and mice tend not to travel too far, they prefer the smallest area possible where they can get food, water, and shelter. If you are seeing them in the attic they either have a burrow in the attic, or somewhere close outside. Rats are very intelligent and it is possible they avoid your traps, or that you are putting the traps in the wrong place. As I said rats tend to follow the same paths, they also prefer to keep in contact with a vertical surface as they travel. You'll want to place traps along walls, and near possible food/water/nesting sources. – Tester101 May 6 '11 at 12:01
You could also use a black light to try to locate their entrance. Rat urine will fluoresce under a black light, and could possibly help you find where they are entering. – Tester101 May 6 '11 at 12:04

One solution I've seen people use is to get a fog machine, like are used for parties and start it up running in the area, then move around the inside and outside of the house looking for leaks. It won't work so well for low lying areas, but for an attic it might present a good option.

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