Why are some English Thatch roofs 30cm or less and some 120cm or more?
Generally straw roofs have multiple layers of thatch and many thatchers will secure the new coat to the one below using twisted hazel fixings. Over a period of time the roofs will become very thick because the Thatcher will not strip much of the old thatch away. The problem with many of these re-coating jobs is that in order to maintain a steep pitch at the apex of the roof the Thatcher will have to build up the height of the ridge to such an extent that in some cases the new thatch will get very close to the top of the chimney.
Water Reed thatch is generally one layer thick and when the roof is re-thatched all the old thatch is removed and the Thatcher will start again from the roof timbers. This is because water reed is longer than straw and lies better on a flat firm surface. It is also harder to bend than straw and so where as straw will mould itself to the hollows and bumps of a base coat, reed will not.
As in all things to do with thatching the above is just a general rule of thumb and many thatchers will fit straw on top of reed and reed on top of straw especially if it avoids exposing the roof timbers and carrying out costly repairs to these.