If there's no time pressure, I'd use a modified version of the 3e take 10 and take 20 rules. If they could open it on a roll of 10 or better and have just had bad luck on their initial roll, then they can take 10, spend 10 or so minutes beating on it with their axes and hammers and whatnot and get it open. If they could open it with a role of 20 then they can spend an hour hammering on it if they choose and eventually get it open. If there's no way for them to open it even if they had rolled a 20 then there's no way to open it. Depending on the context I'd probably allow for some kind of Int or Wis roll to determine if they could get it open in 10 minutes, in an hour, or if it was impossible so they don't waste an hour trying to get it open (fail the Int check spend the next hour beating on an impossible door - probability is a harsh mistress).
Of course the noise of them battering down a door for an hour will echo through wherever they are and attract attention - either alerting targets to flee the area or bringing sentries to figure out what the racket is all about. But from a purely practical perspective if you want die rolls to batter down a door I've found that approach works quite well.
