When Roger Ebert tried to explain the reason for the success of the original “Willard” years ago, he opined that it might have been that people had been waiting for years to see Ernest Borgnine get eaten by rats. Well, Borgnine isn’t one of the actors who provide nourishment for the nasty vampire bats that populate “The Roost,” but anybody looking for an old-fashioned, poverty-row level horror flick might check out Ti West’s no-budget no-brainer, about four twentysomething pals attacked at a remote farmhouse one night by a bunch of flying, blood-sucking rodents, anyway. (Not only that, but their bites turn the victims into flesh-eating zombies–for reasons that remain conveniently unexplained.) Normally, ever since “Attack of the Killer Tomatoes,” or at least since “Scream,” this sort of thing would be played with a knowingly satirical wink to the audience. But here it’s done pretty much straight–with the elderly couple who own the place (Richard Little and Barbara Wilhide) quietly offed before the quartet (played by brother-sister duo Wil and Vanessa Horneff, along with Karl Jacob and Sean Reid) arrive after their car’s run off an isolated road into a ditch. Then the four are quickly threatened by the bats themselves. Naturally they’re picked off one by one until a properly shocking finale. The only other characters who show up during the carnage are a cop (John Speredakos), who according to the conventions of the genre, proves of very little help, and a tow-truck driver (Larry Fessenden, who made the creepy “Wendigo” and executive produced here), who’s even less.

On its own, “The Roost” has some of the virtues–and plenty of the defects–of this sort of bare-bones shlock fare. On the one hand, it manages some genuine fright moments; West exhibits a skillful hand in staging some of the sequences (like one in which the old woman suddenly appears in the distance from outside the frame while one of the youngsters cowers in terror), and he and cameraman Eric Robbins fashion some really striking compositions using light and shadow–a necessity here, since most of the time the action is shot in such darkness (in a huge barn) that one suspects the producer couldn’t even afford the bulbs to illuminate it. (At times the screen is entirely black except for a few shards of light, and sound effects must carry things forward.) By the standards of such stuff, the acting isn’t too bad either, with Horneff–whom you might remember from kid roles in movies like “Ghost in the Machine”–cutting a stalwart but hardly flawless character as the most heroic member of the group.

But the tension generated by the little flick is severely undercut by a wraparound device featuring Tom Noonan as a ghoulish TV host introducing bad movies on some cheesy local channel frightfest series. By itself these segments are amusing (though why they’re in black and white while the movie is in color is something to ponder), though they’re played awfully slowly and go on rather too long. But they have the effect of treating the movie as though it belonged on “Mystery Science Theater”–a feeling that might all too easily infect the audience. One Noonan bit in particular, which interrupts the picture in mid-stream, saps the energy just when it should be accelerating.

Of course “The Roost” can’t be generally recommended; the production is so threadbare that it often looks like something made by students in their own back yard. But West shows signs of real talent, even if he doesn’t have the resources needed to put it to best use. Still, for those who appreciate this sort of thing, it’s worth searching out.