Purchasing a FannieMae HomePath Property

I am looking at a FannieMae HomePath property and I was wanting some advice on the amount to bid for it. This property needs a new roof AND has about 4 layers of roofiing that need to be removed. It also has some water damage as a result of the roof leaking. I realize that these properties are "as is" so I was wondering if I am better off offering the asking price minus the cost of the roof or offering the asking price and then,after the inspection demonstrates that a new roof is needed, negotiating the price downward?