I agree with the post above, that using hardware cloth is best. I used coated welded wire, with I think 2" x 4" spacing, buried just below the ground to about 18" out, stapled to the bottom horizontal beam of my pen. My yard is also fenced so I'm less worried about bigger predators like dogs or foxes. But I have rats, rats and more rats that have no problem digging under and through the spacing. If I had used hardware cloth the problem wouldn't be so bad, though I know that rats will tunnel in somewhere no matter what. I put 12" square cement pavers all along the edge of the front of the pen to make it look neater but now they're lumpy from where the rats dig and I even twisted my ankle today on one. Stupid rats!! Better than the other dangerous furries though!