The flat roof thing is a legitimate risk. Even if they are well maintained and regularly refelted (and most aren't) they are notoriously leaky. In a case where the insurance company does not cover the cost of repairing a damaged roof due to poor maintenance, the ensuing damage to your house and contents can end up being very costly and water damage is one of the basic covers on most policies. It's not just storm damage they are worried about. 100% flat roof is unusual too; generally flat roof extensions constitute less than 25% of the entire roof area and anything over 50% would set alarm bells ringing.
Even on Grand Designs lately, one of these really wealthy imaginitive program-participants got Germans over to isntall a better class of flat roof as part of well-engineered design and no sooner were they on their way back to Germany, than the thing was leaking left right and centre.
Rebecca