The roof substructure is easy to build. In the photo I put some metal weights on the parts while the white glue is drying. This also made sure the whole assembly didn't warp due to the wet glue.
I then had to do some very minor filing to get the part to fit inside the building. Because of that filing, I marked where the second floor door is on the roof part. That way it is always inserted in the same orientation.
Because of the large windows, I had already decided that I wanted to detail the interior. To emphasize that so that members of the audience can see that detail at train shows, I decided to install LED lighting. Here's a photo of the LEDs hooked up to some power source. You can see the light in the second floor, despite my layout room's main lighting being on.
The way I did it was to use a small strip of three LEDs. These are a section of a larger 16-foot LED strip that I still had left over. These have built in resistors, so all you have to do is hook up a 9- to 12-volt power supply. I painted the ceiling white. I later painted the upper part of the roof structure as well (the hidden area), because I could tell the wood started warp a bit with just the one layer of paint.
Here's a view looking up into the second floor with the lights on. From a normal viewing angle you can't see the LED lights directly, though.
With the electrical wiring done, I could then get back to finishing the roof structure. Again, white glue was used.
To complete the roof construction, peel-n-stick cardboard (simulated tarpaper) strips are applied. The key here is to center the first two pieces on the roof, because their ends will overlap the other sides.
After applying the other pieces, the overlaps are pressed down. I found my fingernail to be a handy tool for that, avoiding a sharp edge from damaging the paper.
The other layers don't have an overlap to them, because there will be separate pieces applied later. It is important to pay attention to where the smoke jack's hole is.
The corner pieces are applied next. The first one I folded along the fold line, but I found out that it is easier to just apply it flat, and then fold it across the roof once it is in place. It is easier and more accurate. Again, use your fingernail to really rub the parts in place.