I always heard sulfur around the perimeter of your house is supposed to deter snakes from getting in - don't know how true or reliable it is but they make the 'snake away' stuff thats primarily sulfur and people swear by it. I've had rat snakes get in the house plenty of times - older houses are more prone to getting snakes in as the foundation and all move and make openings that you don't anticipate for them and their prey to get in. At 12" your copperhead was young probably last season's hatch and it probably was born/hatched near your house so I would be careful to clean up any debris in the house, attic, basement, under the house and seal any crack, crevice, and hole in the house interior and exterior. Older model air/heat systems and vent pipes are an easy way for them to get into the house as well so if possible I would get some 1/4" hardware cloth and fit a piece in each ventilation cover and make sure your ventilation covers are secured/screwed into the floor to eliminate that as any possible entry point.
Where I live there are many many species of snakes including cotton mouths (my least favorite), copperheads (don't mind them actually even though I was bitten by one and still have nerve damage in the side of my foot similar to Mystic mom's daughter's finger) and timber rattlers have been found here as well. I have rarely seen a venomous snake and have not seen any snakes except a corn and 2 rat snakes this year.