[MikeSid]

Growing up in an 1920's brick colonial this was a constant problem since the squirrels can easily scale brick. They usually got in to the attic at a corner of house where the roof eaves/gutter meet the brick in a not so perfect fit.

Step 1. Seal the hole. Seems odd to do this first, but trying to time it so they are out of the attic when you seal is a fool's errand. Just seal your perimeter in broad daylight hours and hope you get lucky. Use metal carpenter paper or tin flashing - something their teeth can't chew. Trim any trees or branches that are giving them easy access to your roof. Wires are easy access points too. Slice a plastic funnel to enable you to place it on any wire going to your roof. Face the large end of the funnel away from the house to prevent squirrels from traversing the wire. They'll start to push the funnel toward the house, so set it far enough away to buy some time, but close enough to capture any access point they have to the wire itself. If you're fortunate enough to keep them out (as opposed to trapping them) you may have to move the funnel back several times as they will keep trying their usual access point.


Step 2. If squirrels remain in your attic, set up a one way live trap in the attic and buy yourself some heavy gloves. They will fight like hell in those metal traps but they work. Conventional rat snap traps don't seem to work too well with squirrels - at least in my experience. The squirrels will bloody their faces trying to chew their way out, so the gloves are very important.

Step 3. If you unfortunately have to transport a trap with a live squirrel, take it across a body of water if you can, or at least a sufficient distance. Don't unload him in the park down the street.
Face the trap opening toward the base of a tree and open the gate with a long stick.

We had a neighbor that would accomplish step 3 in a much more simple fashion using a large garbage can full of water. That sort of thing might not be to your taste, but it solves the problem conclusively.

Good luck.

EDIT: That Amazon link you posted will work, with one very critical condition. You have to be able to adequately mount the trap at the access point such that they can't get around it. If they're coming in at a spot that is in the middle of a wall where you can reliably attach all 4 of those flanges to the siding, great you're in business. But in my experience, those traps are too clumsy to mount to the exterior of the access point and won't completely seal the area. They are likely getting in at a seam or corner and that trap probably won't just fit perfectly in the area you want it to.

Nevertheless, if you try and it doesn't work, you can use it as an interior trap. Otherwise you can typically rent interior traps at tool rental places.