I've seen the sloped roof on the "Winter Village Post Office" and similar using plates, and I want to build a roof with a similar angle:
What should I use to ensure they are well attached?
See the instruction for that set on pages 56-59, 65 and 68/69 for the 3 different roofs of it.
they're using a different technique and different pieces for all of them, so this perfectly shows up some alternatives (technic-like parts for the big roof to the right, blocks with studs on the sides for the small 90°-roofs and 1x1 with clips for the roof to the left).
To keep it simple I would use hinges placed on top of the wall, connect the roof plates and angle them to fit.
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Edit: You could even add a small plate hinge at the roof ridge, under the roof plates. This would keep them from separating.