Ah thats pretty straight forward.

I would model the roof as its shaped here, its a fairly easy shape to block out when you break it down. the surface itself is fairly flat so by adding a little bit of geo to it, it will help uneven the surface. A good texture will help you for sure, as Dom mentioned you could use PFX, but paint the texture dont use the actual strokes as that will kill the scene and weigh it right down.For the edges/sides of the roof, use a different texture - end of straw, then aplly a good bump map for effect. For actual strands of straws sticking out, just place some random straw shaped geo, you dont have to go mad with it but use enough to sell it.

I hope that helps - keep it simple, most of the time it works best that way

cheers
Jay