How do you know whether a house has to many issues before you buy?

I am a first time home buyer under contract on a home. The agreed sales price in $200,000. The inspection turned up problems with the rear wall foundation ("bending failure") and there are a number of leaks in the roof. The house was priced low because it is in general disrepair, however, these issues were unknown before the inspection. How do I approach the seller in the negotiation when these items were not in the seller's disclosure? How much do I deduct from my offer to account for repairs?