Roofing work is never 'easy', but some jobs are less difficult than others.
You would first need to cut into the flat roof in order to expose the internal wall and subsequent wall plate. This will allow the roofer to determine if he can spring off the plate and use the existing joists as part of the roof build.
If he can use what is there - fine.
If not, it may, mean a total strip and re-build though this is unlikely.
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local