Are skill modifiers cumulative (even beyond plus/minus 30%)?

I haven’t GMed Warhammer FRP 2nd edition for quite a while and my knowledge’s gone a bit rusty, I’m afraid.

Are skill modifiers cumulative (even beyond plus/minus 30%) in WFRP 2nd edition?

I tried to find the answer in the core book, but all I found was that skill difficulty modifiers are mostly up to the GM, and that they very rarely exceed +/- 30%. Is that correct, or have I missed something?

For example, Hans is about to parry with his off-hand weapon, which gives him a -20% (as it’s not defensive and he’s not ambidexterous), and it’s pouring at night, in a back alley, which I’d say is also a -20% (because it’s slippery and dark and Hans can’t see much.) Does he roll with -40%, or does it stop at -30% (as all these things make it a “Very Hard” roll which means -30%)?