Roofs are the main cause of price re-negotiations in my experience. Pitched roofs, supported by wooden structures, should survive for a very long time in the right conditions. Flat roofs, on the other hand, tend to need replacing more regularly, because water does not run off so readily and this may eventually lead to problems.
Some problems are just a result of time and the elements. The cement around chimneys can be affected by water penetration, and this causes a chemical reaction in the cement, which makes it expand, leading to a tilt in the chimney. This may require the chimney to be taken down or rebuilt. But it’s not an enormous building job.
Many older properties have tiled roofs. Slate tiles were usually fitted by nails and these may rust in time, causing the tiles to slip. They usually just need re-nailing or replacing. If the slate tiles have been replaced with heavier clay tiles, this may cause bowing in the roof structure, because it was not designed to carry the heavier load. This can be a serious problem.
Some problems are the result of ‘improvements’. If the roof is not ventilated properly, rot or woodworm can attack the timbers. Most houses were built with proper ventilation, because these risks have always been well known. Such problems are often the result of over-zealous insulation round the rafters by later homeowners which prevents moisture from the house escaping through the roof.
Damage to a roof may be evident from outside. If you look at the roof through binoculars you should be able to see if the chimney is leaning over, or if the roof line isn’t straight, or if there are slipped tiles. Slipped tiles are easily put right. But if the problem is damage to the roof structure itself, then that is likely to be quite expensive to put right, and you may well have to reduce the price for that.
However, there is one situation where you should definitely resist any price reduction. That is when the surveyor announces that there is nothing wrong with the roof at the moment, but it may need replacing in x years’ time. That is just the surveyor covering himself. You may well find your own surveyor said something similar in his report when you bought the property, and the roof is still going strong. So, don’t readily agree to reductions on the basis of this kind of nebulous threat.