Instead of making new resources all the time and occasionally blowing up the one's you've already spent the time to put in, why not just give players motivation to abandon a given safe-hold?
Something like zombies learning that the player is holed up there (attracted by the light/smell/what-have-you) over time could encourage players to use safe-places as temporary havens, moving on and opening the space for other players more frequently.
Or, you could make supplies dwindle over time in the area that is secured, such that the players have to travel further to get food/ammo and are thus at risk (outside of the safe zone) for longer periods of time because they have to travel further.
I'm not sure if these ideas are in line with your game-play mechanics, but I think, as a player, holing up in a building doesn't sound like much fun for long periods of time. If there were benefits to hopping between safe-zones (securing one, then abandoning it in favor of another), I think I'd wind up not only seeing more of the game-world because I'd be on the move, but because I'd probably log-in more frequently.
That's just my two-cents, though; again, I really don't know your game's design, and I just want to throw out ideas in the hopes of helping.