Roof replacement
Roofs are big and expensive, but you knew that already. The roof will almost certainly be the largest single expense in a building or renovation project. This is largely simply because of the size - roughly 20 - 50% more than the 'habitable space' area of one floor of the interior.
First consideration is - do you need a new roof? Barns and other old buildings usually have a roof of some description, so ideally you would like to retain it.
When we bought our barn we were told it had a 'new roof' and, being not very bright, we took that to be a good thing. That should make renovation cheap then, we thought. But a barn roof is not a house roof. It had been tiled with cheap, mechanical type tiles, with no structural work being done on the carpentry beneath.
Of course, it had no insulation. End result - all the mechanical tiles had to be removed and disposed of, the woodwork underneath repaired and strengthened, and the roof relaid with insulation and canal tiles. Total cost - more or less the same as if the roof had not been redone at some point.
Before adding a new roof or roof tiles you should always verify with your carpenter that the supporting structure can support any extra weight you are adding.
Traditional roofs usually consist of a solid, heavy wooden structure that supports thin wooden slats on which the tiles are fixed or placed. There may also be a continuous sheet of wood panelling (or even terracotta tiles in some areas) on which the tiles rest. Tiles and roofs vary a lot from region to region so I won't attempt to describe the different types.
Here in the south of France many houses have canal tiles that are placed rather than attached on a roof and manage to hold themselves in place, many areas have tiles hooked or nailed on to the battens.
These traditional systems all work reasonably well at preventing rain from entering a building. There is often a particular wind direction that can blow rain between the tiles but this is not usually a big problem. Keeping rain out is only part of the problem however. We want warm, dry, draught free and mouse free interiors rather than just dry interiors.
Making the roof (and property) comfortable and dry
The simplest and cheapest approach is to keep the existing roof, making repairs as they arise, and put insulation on the attic floor if there is one, and then forget about using the attic. Some properties already have an inaccessible roof so this may be a good solution.
Much the best way to insulate a roof is from above, at the time of replacing the roof. In this way the insulation can form one continuous sheet, and be easy to fit. Rckwool (laine de verre) is still the most popular soltion and is available in sem-rigid form which often makes t easier to install.
Sheets of extruded polystyrene are also popular in various forms, and the thin 'silver' insulation materials are also very practical (and seem to work well, in my experience, although they are not well rated by official organisations in France, and will not qualify for grants).
Frequently a new roof will have the appearance of the old roof but be constructed differently.
Canal Tile roofs now usually have rigid corrugated sheets called canalite placed on the roof first to provide the actual water-proofing, and then canal tiles are placed on top of these corrugations, essentially for aesthetic reasons. This enables old tiles, that may really not be very water-impermeable after years in the rain and frost, to be reused. The roof looks traditional and is also completely dry.
A further advantage in cost comes if the corrugated sheeting is the same colour as canal tiles, which is commonly available - you can put canal tiles on the 'ridges' of the corrugated sheet and ignore the 'hollows'. Doing this reduces by half the number of reclaimed tiles needed, saves a lot of money, and looks more or less identical unless you inspect it closely, and also weighs a lot less.
Other Tiles - most other tile types either hook onto the thin wooden battens or are nailed on (e.g. slate) As described in the insulation section, products are available that combine support for the tiles with superior insulating properties. Specific products vary with region and tile type, but a quick look around your local DIY store should help you identify the products for your region.
General
If you replace a light roof with a heavy roof you need to ensure the beams can support the additional weight.
The best time to clean and treat the carpentry in the roof is after the old roof has come off and before the new one goes on. Hydrosablage is a good way to clean the wood (pressure washng using high pressure water with sand in it) since it does a very good job at removing some of the rotten wood and ancient dirt without the potentially damaging effects that sandblasting alone can cause.
If any parts of the roof look weak, they can often be reinforced by fixing a length of good timber along the whole length of the existing beam, without removing the existing beam. this is much easier and cheaper than taking half the roof off to replace a beam.
Whether it is suitable depends on the condition of the existing wood, accessibility, and whether the ends of the new piece of wood can be adequately supported on the existing structure.
You will find that your local carpenter does not charge too much to do minor roof repairs and will have the necessary equipment to do it quickly and safely. This is often preferable to climbing around on a roof yourself.
Almost all new roofs, regardless of the method chosen, if well constructed and insulated, will cost upwards of 100 euros per square metre. Sometimes a bit more, sometimes a bit less, but I never found a quick, attractive cheap way of replacing a roof properly. Insulation and internal plasterboard finish will increase the cost.