Since this building is going to be quite a large model (122 scale feet long, 33 scale feet wide), it needs to have a solid base. I decided to build the core out of 1/4" plywood. However, plywood that thin still warps a bit, so I reinforced the interior with some scrap wood, to make sure the two walls are, and remain, at 90 degrees, and that the long wall remains straight. The walls were cut to the scale dimensions mentioned above.
Here I am test-fitting the core framework in the position where it will reside on the layout. It has to fit within the back and side edges of the layout, and the Chartiers creek. This photo was taken from the back of the layout.
The roof's angle is 18.4 degrees. I cut a strip of wood to that approximate angle on the tablesaw, and glued it to the end of the side wall. The first roof panel was also cut out of 1/4" plywood. I wanted its end to be flush with the side wall. So, in this photo you can see two metal weights held in position by two C-clamps. That way the roof could be forced against the metal weights, and I would know for sure that they would be flush.
I glued a small scrap piece of wood to the underside of the roof panel. When that was cured, I could use that to clamp the roof panel against the metal weights. With the panel in place, I carefully applied wood glue to where the roof panel touched the top of the long wall, as well as glued it to the top of the strip of white-painted board.
That assembly was, of course, very fragile, but it did guarantee me a good fit of the roof panel. Two additional reinforcement pieces were added, as shown in this photo. The compound angle of the roof panel where it will meet the backdrop (left side in this photo) was too hard to figure out. So, after the wood glue had cured on all the reinforcements, I took the building to the table saw, and cut the angle into the roof panel. Not easy to do, but I got it to fit correctly.
With the one roof panel in place, I then determined that the other half of the roof panel could still fit between the model and the backdrops of the layout. After some calculating, measuring, and cutting, I then had the other half of the roof panel for the building. It was just free-floating in mid-air, so I cut a piece of scrap wood to hold it up. At this point is all just held up with some wood glue, so that the roof panel would sit exactly where I wanted it to. The very back corner could, theoretically, model a portion of the roof of the second building, but after cutting a scrap piece and test-fitting it, it turned out to not be visible from the front of the layout, so I just left it off. That is why the vertical support board shown in the photo isn't flush with the edge of the roof of the first building.
I filled the gap between the two roof panels with a healthy bead of wood glue, and let that dry overnight.
Of course, reinforcement is absolutely necessary for such a delicate bond. I cut two pieces of plywood with the appropriate angles (a few trials were necessary), and then glued them to the underside of the roof. Due to the odd angles, I had to get creative with the clamping. Metal weights and a full bottle of glue provide plenty of downforce on the one side. I had put several strips of blue masking tape on the other side of the roof to hold the two panels together while I did this work. This photo only shows the one brace, but I installed another one in a similar manner on the other side of the roof.