I used to have snails. Now I have slugs. But the controls, I believe, are pretty much the same. If you're not willing to go on nightly snail raids or set up beer traps (google "slugs" and "beer" and you'll probably get lots of good hits, plus maybe a few about guys watching televised sports), you can go get a product like Sluggo. It's iron sulfite or something like that, and it's not toxic to much beyond slugs and snails--it eventually breaks down and might actually be good for the soil. I don't think slugs and snails munch all that much on roots (unless they're semi-exposed like through the drainage holes of a pot) so much as eat tender foliage, but I could be wrong on that.

For my slugs which should work for your snails I just save my egg shells . Just rinse good and crush and spread where needed they just seem to go off for softer pastures. Hope this helps

If you can get it, try cocoa mulch. Their little pseudopods don't like the sharp edges. I never had a problem with any plant that I used this on back home.

Or use Sluggo and be done with it.

I have heard that copper strips literally electrocute and kill slugs. I am sick of the slugs in my hostas and I am thinking of putting a copper strip around that bed.
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I had my best (least invasive) experience last year by not following the instructions on the slug bait I bought. Instead of spreading slug bait around each hosta as directed, I set out a few small 5" diameter split branches and sprinkled the bait around them and any other surfaces the slugs might want to take refuge under like stepping stones, logs, etc. It saved a lot on the cost of the buggetta because I used so little and was more effective. I also used pine straw as a mulch which on its own has never shown much of a deterrant effect for me. The idea is not to attract slugs to your plants by putting a ring of bait around them. Good luck!

I just reread your post. That's curious about the snail shells. Do you think something could be eating the snails and damaging the roots while it's hunting for them? Shrews maybe? I can't post another follow up so I had to start this over. Sorry