Rob,
I've had "reasonable" success with making thatched roofs using Terrain Tools.
You'll need to work outside the box.
Create the house using a conventional roof
Create the thatched roof in a separate plan as Terrain
Make the terrain perimeter size and shape you want it to be in the final model
Use terrain elevation lines to achieve the shape of the roof
When you are done, apply the thatch material to the terrain and export the terrain as a 3DS file
Import the 3DS file into the working model (with a conventional roof)
Place the 3ds Model on the roof top - kind of like a hat.
I hope this helps.
Good Luck. Have fun with it.
Zoome
Tiki-Hut.plan