The late 1920s picture above, from an unidentified building in Washington DC, was taken in an odd location. The gable roof above is a skylight. The plane that looks like the floor is actually a “laylight,” which is glass that looks like a skylight but is not. Typically, laylights are located below skylights, as seen here. The reasons for having two sets of glass is simple: the job of a skylight and the job of a laylight are different, not really compatible, and easily separated. A skylight has to be weather resistant and (in temperate or colder climates) has to be able to carry the weight of snow. A laylight carries no load other than its own weight and does not have to be air- or water-tight, but it is usually decorative.
The skylight is supported on gable trusses – hybrids of Fink and Howe configuration – made of steel angles. There are vertical hangers that support the framing for the laylight, dividing the span length for the heaviest pieces of that flimsy framing into thirds. So far, so good: there are clear and logical load paths for both the glass above and below. Here’s the problem: this support layout links the two. When it snows or when there’s a high wind and the skylight is heavily loaded, that load is transferred to the trusses, which carry it to the brick walls. In the process, the trusses deflect downwards slightly. Since the laylight hangers are supported by the same trusses, the laylight moves down as well. That’s not necessarily a problem, but laylight designs are typically (as seen here) not very beefy and are assumed to not receive any dynamic loading. I’ve seen laylights with glass and finish damage from decades of small up-and-down movement.
As a side note, those bundles of pipes on the brick walls are radiators, meant to prevent condensation on the inside face of the skylight and (possibly) to prevent snow build up.
Here’s a similar view of a different skylight, maybe a different unidentified building:
Similar framing except that the laylight girders are quite heavy, so they don’t need mid-span support. They simply span from one truss to the other. Because the trusses don’t follow the shape of the hip-roof skylight, we have those beams near the top to support little compression struts to hold up the skylight. Because of the way that the laylight is supported, its center (supported by the mid-span of the trusses) will move more than its edges (supported by the ends of the trusses).
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