So you’ve secured the doors and windows. You’re hunkering down and most of the undead are staggering by your house, paying you no attention.
You’re sitting there, feeling all safe, but your getting a little thirsty and the only water left in the house is sitting in the toilet bowel. Which wouldn’t be too gross, except your last meal before the utilities cut out was burritos, and there is a long brown stain smeared across the curvature of the porcelain beneath the cool, beautiful water.
Of course, you could have planned ahead and you wouldn’t be in this situation. Go buy yourself two fifty five gallon drums used to haul fruit juice concentrate. (Make sure whatever you use is safe and won’t leach anything dangerous into your water.)
Place these someplace on the second floor, someplace safe and out of view. Now comes the tricky part. You need to cut a hole through the wall and tie the downspout from the roof to a supply line feeding the buckets. Take the overflow line and drain that back into the downspout.
If you do a good job, no one will be able to tell from the outside, and the overflow will keep the water from getting too nasty each time it rains. When the undead start rising, you can drain them into your tub to collect as much as possible the next time it rains.