A coop secure against raccoons will also be secure against snakes - the reason that I say that is any opening a shake that is large enough to pose a hazard to your flock will require an entrance that will also be sufficient to be taken advantage of by mammalian predators as well. Snakes are not always bad, in fact most are very beneficial and few actually do harm to birds - that said, you don't want them in the coop (outside they are great at controlling rodents, etc drawn in by the grain based feeds) . Half inch hardware cloth on any opening larger than an inch will secure your coop against predators.