Epithelia are tissues that mark the separation between the outside (apical surface) and the inside (basal surface) of an organism. The tight packing of cells within them guarantees their cohesiveness and impermeability, essential to their functions. Yet, how cells spatially arrange in three dimensions within an epithelium is not fully understood. Until recently, most theoretical or experimental studies focused on the organisation of the apical surface, and not on the shape of lateral cell junctions. Lately, however, the idea that cells can be tilted and skewed has gained traction and is more and more recognised as relevant and integral to the understanding of epithelial packing. Here, we present an analytical description of epithelial tissues resting on a flat substrate. From discrete cell outlines on the basal side, 3D cell shapes are assumed to emerge from a continuous vector field mapping the basal side onto the apical surface. The field is split into its vertical and horizontal components, allowing to define height and tilt fields at the tissue level. We additionally make explicit the intrinsic geometric relation between cell tilt and cell skewing, the skewing being the divergence of the tilt. We then consider a simple energy function including apical, basal and lateral surface tensions, and show that, by coarse-graining above the level of individual cells, one can obtain a fully analytical description of epithelial mechanics in terms of continuous fields of cell density, height and tilt. This bridges the gap between the description of individual cell geometries and the tissue scale, and allows to perform tractable analytical computations to study the general principles that govern 3D epithelial mechanics. In particular, we show how spatial fluctuations of tensions can lead to correlated spatial patterns of height and skewing.