Ahh right. Its a big piece! You've got a nice start. I think the peak of the roof could be off centre though, might need to be shifted up more. Approach the roof just like everything else, consider how the lightsource is shining on it and shade accordinly. It seems to be the light is coming almost from direct above-left (straight in terms of the X axis, if we regard the X axis as <----->).
Umm, it seems kind of tricky, doesnt it? I get the instinct to tell you to make the side of the roof tapering away from the viewer darker than the one facing the viewer. It just basically needs some shade to help define the form. I'm pretty sure that even flat planes have a slight gradient of shade to them. The "lowest" edges of the slopes should be slightly darker, im pretty sure, as should the rightmost edges. It depends on the nature of the material, which I'm assuming is that straw kind of roof...cant remember the actual name for it.
Oh which brings me to the other point; give it some texture. As for the wood...it looks quite plastic, due the kind of smooth edges, the uniformity and the slight shininess youve given it.
The left most and right most sections need texturing, also.
And what kind of game is it, btw?