We had an identical experience. Went sale agreed at the asking. Surveyors report indicated structural issues in the roof, and elsewhere. We costed them and went back to the estate agent. Asked for a reduction in the price equal to half of the remediation costs, which we felt was fair. Vendor refused to budge so we pulled out. Vendor relisted the property and sold it again 6 months later to another buyer who either wasnt worried about the roof, or had more money that we had to fix it.
Nothing wrong in asking. But be prepared for your next step if they don't budge. Are you walking away or are you going to accept the property. We knew going back that we were walking way if they didnt move.
Similarly, when we sold a rental property last year, the buyer came back with a list of issues, wanting us to remedy them before signing. We didnt move, as it was a rental house that was tired and needed work and price reflected same. In that scenario, the vendor went ahead and completed the sale.
Hope this helps you.