Most of the work will probably be needed to be done in Photoshop...or whatever image editor you are using. It also depends on whether you mean an 'English' thatched roof or a Polynesian/Caribbean one. If it's the former, you'll need to find a suitable thatch texture on the Net and make it seamless; then also find a suitable 'cut-end' texture for where it curves back underneath towards the wall.
If it's a tropical thatch roof, you'll need the same general thatch texture but also another one in transparent png format for the fringe of the roof. The actual geometry for both roofs can be left fairly simple. This Tiki Bar is little more than a fringed umbrella.
tiki2.jpg