I'm working on a railroad car from 1898 which was built in Detroit, Michigan by the Michigan Peninsular Car Company.  It has a Drake & Wiers asphalt roofing material which I'm at the point of testing out to see what might be the best approach.  See Drake & Wiers -1 for the 1898 product info and the other image  is a model view.

My first though was to make a some surfaces and knit them together,  and they apply a texture or color to the surface.   Actual thickness/elevation is not a concern, just for visual information.   

Then I was thinking maybe a decal could be used to fit over the irregular surface.  But the first thing the documentation says is probably not a good option for assemblies. 

So I think I'm back to making surface segments and going that route.   Unless someone has a idea on another option. 

It would be nice if there was a 'Blanket Surface Area' where I can automagically create the surface area with just specifying some of the boundary areas.  

In the Drake & Wiers -1 image for their product,  the specific part is labeled as "Felt" -  it's shown as being unrolled and lapped over the sub rafters which in turn get covered with some additional lumber and the exterior course of roof boards.  These inside roof designs were common and then later replaced with exterior metal roof designs.

Any ideas are greatly welcome.