My players are setting up to defend a village from a (small) invading army and they want to have some form of explosives, most likely would be barrels full of pitch or the like, placed under the bridge into town so they can blow it if things get too out of hand.
They've also got a cart, with barrels of pitch, just down the road too, so they can easily thin the herd.
From page 15 of the Scales of War: Rescue at Rivenroar (not what I'm running, it's just what I've found in relation to blowing things up) adventure I have found this;
If the casks on the back of the wagon take at least 5 fire damage, they start to explode. The casks in one of the 4 squares in the back of the wagon detonate each round. Each explosion is a +6 attack against Reflex defence in a burst 3 area, dealing 3d6 fire damage (half on miss). When the first casks explode, tell the players that it’s clear that not all the casks have exploded yet, and that the wagon is afire.
which I plan to use for the cart, however I'm unsure of how to translate that into blowing up a bridge.
I know the DMG has a section on page 65 about damaging objects but the way I see it a stone bridge could have up to 400HP so using that method seems like a bad idea.
I almost feel I could just say they blow up the bridge with something like a TNT style push level and fuse once they retreat back across it, since the first battle is initially minion spawning with a few standard monsters spawning every few rounds it would be an easy way to grant them a short rest, but I want it to feel like there's a bit of pressure on them and that there is a chance they won't be able to do it the first or even second time.
Think the classic movie trope where the lighter doesn't work until the very last second.
And so I have come here to see if anyone has other ideas or can direct me to an adventure that already has this covered.
Note: none of my players really have any ranged attacks (there's only 3 PCs) so they have village minions with shortbows to, at least, blow up the cart with flaming arrows.
Also if it helps the bridge is only about 20' wide and 35' long.