Your roof is one tile too "short" and one tile too "thin". Remember that the coordinates you enter should be the same as for the base FLOOR, and not the available space inside the walls. Compensate for the floor tiles which the wall occupies on the eastern and southern sides of your building.
Note though, that if you build both floor, walls and roofs through WorldBuilder, start with the floor (which will get nice edges around as well), then press the big arrow at the top of the Floors-menu, which will take you to the Walls-menu using the same coordinates. Press the arrow again, and it will take you to the Roofs-menu, again using the original coordinates you used for the floor.
Also, you will have to fill up the empty area between the arch of the roof and the wall below yourself, use the Walls-menu and wall-height "short".
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