Pete recently did an interview on BBC Radio 4 - You and Yours - which investigated a case history of failed cavity wall insulation.
Roofing an old house is not a very complicated job as a rule. What tends to muddy the waters is the problem of trying to work out what the house ORIGINALLY had on top, and whether the current roof structure is strong enough to carry what is actually now there. To give a real world example - I often see quite steep structures, with beautiful timber trusses beneath, which obviously had thatch on. Over the years, these have been messed around with, and now have a second generation of timber rafters over the old lightweight thatching poles, and on top of these has been at least two generations of tiles, often finishing with horrible great heavy concrete tiles. This causes problems with weight distribution, and often we see the walls bowing outwards at the top, and purlins (the long horizontal beams that carry the rafters) bent like a banana under the tremendous weight of the concrete tiles. Add to these problems the weight of insulation that's sometimes there, and you have a load of problems that need to be solved.
This doesnt mean that every old roof is a problem - most arent - and our biggest problem is usually getting the right form of breathable insulation into them to comply with modern building regulations. If more than 25% of the roof is being removed or replaced, you are supposed to put in an application for Building Regulations Approval. I find this a bit hard to take sometimes, especially when we are dealing with old buildings which are never going to comply with building regs in any other shape or form - the roof timbers themselves would never comply, yet we are supposed to put a new roof on which does comply... the whole thing becomes a bit of a joke. HOWEVER - there is an exit strategy which works well - buried within Building Regulations is a little known section dealing with 'Breathability'.