There are a few reasons for choosing to re-roof rather than repair:
If your roof has quite a lot of missing or damaged tiles, then it may be a cheaper alternative to re-roof rather than keep repairing.
The roof is damaged, or in a bad condition due to the weather conditions taking its toll over time.
Most re-roofs are considered when the roof is starting to deteriorate due to the wear and tear and the weather.
Selling your home - if the roof isn't in a good condition buyers may have second thoughts about having to spend more money on a roof.
Roofs do protect your home and last for years so although it may seem an expensive purchase it will pay for itself in the long run.
The Process:
Remove the existing broken tiles.
Strip the roof - removed the slats and roof membrane.
Re-Lay new roof membrane.
Install new treated slats.
Laying down the new Roof Tiles (ie Rosemary tiles).
Fit new Fascia, Soffits and gutters if required.