If you’re not attaching correctly parts of the models, you’re going to have some Z fighting visual artifacts where faces are overlapping with each other (and you forgot to CTRL+N to recalculate the normals, there are several inverted faces in your model)
Additionally, if you’re not making the model solidbecause you want to save polycount, you can save much more by actually not having some so many quads faces that do not serve any obvious purpose :
to the left your model with 200 faces and to the right only 123 faces for the same exact shape (2 more faces could be saved too, but it’s negligible at that point).
Allowing you then to use the faces you saved to attach correctly roof and floor to get a solid model, avoiding then some Z fighting artifacts where faces are overlapping.
And less faces mean less vertices and edges and so less time putting seams for unwrapping.
For the seams, i uploaded an example .blend on how i would do this :