I stated with a basecoat of boltgun metal, and then gave this a wash of a 50/50 mix of black ink and water.
Having left this to completely dry I then stippled on a mix of bestial brown and fiery orange.
I then gave it another stippled coat with just fiery orange until the roof had a good coverage. At this stage the edges were also drybrushed with fiery orange.
After this was completely dry I gave it a wash of devlan mud.
When dry, another wash of 50/50 mix of black ink and water. I think this gives it a bit of depth and shade, and helps blends in the colours.
And here is the completed roof. I hope you agree but I think the finished effect is quite realistic and relatively easy to achieve....