There are many different types of thatch, but two basic methods of thatching.
One method is to remove all the remaining old thatch and fix a new coat of thatch directly to the roof structure, a method which is used throughout the world.
The other method which exists only in the British Isles, is to secure the existing thatch to the roof structure and fix a new coat of thatch to the existing old thatch underneath.
Both methods have been around for hundreds if not thousands of years and are used throughout the British Isles today.