I do like the effect you've achived.
Here's a couple of Ideas for your musing. I did a tutorial a couple of years back on a quick method for representing Roman roof tiles. (I can't find it but I'm sure someone else could
)
I could also added a couple of pictures of different techniques a spray can texture and Woodchip wallpaper which works better than the photo show's.
It's also worth noting once you get out of Rome andvinto the provinces you'll see more wood on display especially if the wood had been cut with a saw. As this was way more exspense to produce so it was used as a way of displaying wealth Rather than being hiden with a render. Render would be used around such timbers (even its just on the public view side of a building.)
The render on the Romano British church was done with a premade combination grout and tile adhesive.