Factory roofs

I have seen an number of (older) factory buildings whose roofs look “serrated”–that is, the top of the building zigzags, presumably to let in light through windows on the roof. Am I right?

Yes. The saw-tooth roof design is to admit more natural light, thereby reducing your electric bill. (The vertical sections have windows.)

When electricity got cheaper and labor (to replace broken windows) got more expensive, factories started getting built with no windows at all, depending entirely on artificial lighting.

I’ve always wondered about this myself. For a while I just assumed it was gantry cranes to move back and forth, but then you would still have a flat roof. My dad mentioned that back in the day they had windows to let light in and if the building was a foundry they would open to let the heat out. I assume that after a while they got tired of replacing windows and just covered them up.

Mostly for daylight AND cooling in the summertime. Glass replacement problems are on street side windows, even high up. Some places use translucent fiberglass panels.

*"Factory windows are always broken.
Somebody’s always throwing bricks,
Somebody’s always heaving cinders,
Playing ugly Yahoo tricks.

Factory windows are always broken.
Other windows are left alone.
No one throws through the chapel window
The bitter, snarling derisive stone.

Factory windows are always broken.
Something or other is going wrong.
Something is rotten – I think, in Denmark.
End of the factory-window song."*

Factory Windows Are Always Broken, Vachel Lindsay (Public Domain)