I understand working with layers on lines only to the extent of a road over a railway, say, and applying 1 or -1 to either as applicable. This does not appear to work with a line and an area. I have two buildings connected by a bridge and a road running underneath the bridge and between the buildings. The principle of another mapping application that I used to work with was that points appear on top of lines and areas and lines appear on top of areas. Is it possible to place an area on top of a line? If so, what key(s) and value(s) must I apply to achieve the correct appearance? Is there a tutorial for working with layers? All my editing is in JOSM.
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In general: Drawing objects (above or underneath other objects) is bussines of the rendering program. The layer-tag is only a clue for the rendering engine to know which object is “above” and which “below”. If this is not working correct the render program needs to be fixed, not the data.
In you case I’d tag the buildings and the raod without layer tag (equal to layer=0). The stucture connecting the 2 buildings (either way or area with building=yes) should have bridge=yes and layer=1 (or hogher if the road already has a layer tag for other reasons).