Currently preparing an adventure and need some help. The PCs will have to destroy a sealed hatch (presumably using explosives), but I can't find any rules for attacking structures, other than the cover rules (which aren't really appropriate). Am I just misisng them somewhere? For now, I'm just going to rule that the hatch has an AP of 15 and can take 20 damage before they can breach it, but are there any stats in one of the books?

I haven't seen any rules about attacking structures, but maybe someone else has.

I think your stats are a pretty good baseline, but if your players are not in a particular hurry (i.e. they're not being chased by the Hounds of Khorne) I see no reason not to let them strategically set one Krak grenade and blow through the hatch that way. If every Turn is precious however, be prepared to throw them a bone in case they run out of explosives before the hatch opens. Maybe they've managed to weaken it and a good Strength check will force it open, maybe there's a hole they can squeeze through with the Contortionist skill, maybe there's a Lascutter lying on a bench that they missed earlier, but they'll have to brave enemy fire to retrieve it.

Just be prepared for when your players fail to get through the hatch the way you expect them to. This happens all too frequently in my experience (not that it's a bad thing).

(Edited for clarity)

The funny thing about RPGs and being a GM is that whenever you come across something that isn't spelled out in the rules you get to make it up. Personally, knowing that a krak missile is used against fortified targets, and having come across instances in the 40k universe where a single one can easily demolish a bunker or main battle tank (with a little bit of luck) I'd say it would all depend on the levels of success that the Acolytes reached when attacking the building with their missile launcher, or their demolition test. The higher the success, the more impressive and complete the destruction of the target. You can still use narrative styles even during combat, and remember that this is your very own living world, and not everything requires statistics to overcome.

there is also melta gel page 165 of the inquisiters hand book. this is designed for cuting through structural materials and could also be used. Also page 178 same book have many other breaching systems to use. i think the best thing you can do is just use the cover level of the stucture and the damage of the breaching agent.