Why is this the universal representation of a factory?

March 17, 2005 6:59 AM

Why does the universal representation of a factory include a series of right triangles on the roof? (warning, .gifs inside)

For example:


I thought perhaps they were designed to increase the surface area of the roof and let in more light. But wouldn't light only get in when the sun was at a certain angle?

posted by schoolgirl report to Grab Bag (18 answers total)

Er, I hope those images didn't mess up the formatting. I couldn't find anything smaller.
posted by schoolgirl report at 7:04 AM on March 17, 2005

It is called a sawtooth roof, and it was in fact designed to let in more light, particularly to the interior of large buildings. As long as the windows in the vertical sections of roof face south, you are getting enough light to illuminate the previously dark core of a huge factory building. IIRC it was developed by Ford.
posted by Rock Steady at 7:06 AM on March 17, 2005

The vertical part faces south. If you can find a nice diagram of the tilt of the axis of the the earth in relationship to the sun, this will make sense. It's also why if you can find moss growing on only one side of a bunch of trees in the forest, that's probably south, and why the north face of a mountain is generally nastier, vis a vis coldness and attempts to climb it.
posted by mzurer at 7:10 AM on March 17, 2005

I'd love to see a diagram of how this works. I'll have to hunt around. Great answers in the meantime, thanks!
posted by schoolgirl report at 7:15 AM on March 17, 2005

Actually, the moss grows on the north (cool) side of a tree. But the rest is all good. Remember that early in the industrial revolution, electricity was not the universal power source. If you go to Henry Ford Museum and Greenfield Village in Dearborn, you can see in one of the old factories (a hobby shop by today's standards) that power was transmitted within the factory by means of belts riding on pulleys which were driven by shafts by a central power source. This source was either a large, stationary steam engine or sometimes a paddle wheel in a swift-moving river (which is why so many manufacturing cities grew up near rivers- it was their power source). Anyway, back in those days, you couldn't really light an area with steam or hydro power, so they needed the funky roofline with lots of glass to light the interior of the building.
posted by Doohickie at 7:27 AM on March 17, 2005

See this summary of indoor sunlight from A Pattern Language for some more thoughts on designing buildings so their interiors are illuminated by natural light.

Also, moss growing on a certain side of a tree is a myth. Use the sun or a compass instead.
posted by driveler at 7:30 AM on March 17, 2005

This page has a helpful diagram...
posted by fake at 7:36 AM on March 17, 2005

As long as the windows in the vertical sections of roof face south, you are getting enough light ...

That is, of course, assuming you are in the Northern Hemisphere. In the Southern Hemisphere, the windows shoudl face North.
Unless you're in the Tropics, then all bets are off.
posted by signal at 7:40 AM on March 17, 2005

Here's an axis/sunlight diagram, which confirms what signal said as well...

grumble... grumble... moss thingy, but if it's a myth, maybe I'm not as wrong as if it exists...
posted by mzurer at 8:01 AM on March 17, 2005

This is most interesting; I've just learned why my bedroom gets so hot in the summer - it's a south facing room. I never understood why this was an issue before, but now I understand it. Thanks!
posted by salmacis at 8:18 AM on March 17, 2005

No no no NO!!!! You want the vertical parts to face north if you're in the northern hemisphere so that you don't get direct sunlight! North sky light (i.e. the sunlight refracting through the atmosphere) is even, producing no glare and casting no shadows, similar to an overcast day. You get less overall light, but that's why there are so many on the roof, not just one or two. If you had those giant sawtooths facing south, your workers would be hating life. Plus the heat gain mentioned by salmacis.
posted by LionIndex at 9:11 AM on March 17, 2005

It's not just for light. Those windows would open by turning a crank mounted down near floor level. Opening the windows let the hottest air out of the building in the summer. Opening other windows lower down created a chimney effect, ventilating the whole place.
posted by Kirth Gerson at 9:14 AM on March 17, 2005

No no no NO!!!! You want the vertical parts to face north if you're in the northern hemisphere

Wrong. Take a look at mzurer's link. The angle of the windows means that you'll get direct sunlight in winter but not in summer, which is a good thing.
Northern orientation in the N.H. is good for artists studios and galleries, places where you need indirect lighting and can afford the heating bill.
posted by signal at 9:44 AM on March 17, 2005

"It's also why if you can find moss growing on only one side of a bunch of trees in the forest"

actually moss grows wherever... it's lichen that grows on one side of the tree (unless i'm mistaken) IANAB

*shrug*
posted by rampy at 10:06 AM on March 17, 2005

Wrong. Take a look at mzurer's link. The angle of the windows means that you'll get direct sunlight in winter but not in summer, which is a good thing.

That's for houses, not factories or work places, and yes I'm quite aware of the whole solar angle thing, thank you. You're totally misinterpreting the relationship between the information in that link, dealing with apartments, and how the light monitors on top of factories work. The sawtooths are there to provide even, ambient illumination, not direct task lighting. For factory work, you would not want an infusion of bright, direct sunlight nor would you want the glare it causes, whether in summer or winter. Skylights, in general, should always have a northern orientation, no matter what their application. For windows on vertical faces, it's okay to put them on a southern face because you can control the sun entry via overhangs, whereby you could admit light in winter but not summer. Look at the drawings at the top of the thread, look at any actual photographs of factories with sawtooth roofs, and you will see that they have no such overhangs.
posted by LionIndex at 11:35 AM on March 17, 2005

So LionIndex, you're saying the light does indeed come in via the vertical faces, but that it's softer northern light rather than more direct southern light. Correct?

As a sort of aside, is it at all common to have solar panels on the angled faces? This would seem to be a good way to make use of that surface area, as it's not being taken up by windows.
posted by schoolgirl report at 11:55 AM on March 17, 2005

So LionIndex, you're saying the light does indeed come in via the vertical faces, but that it's softer northern light rather than more direct southern light. Correct?

Exactly. Otherwise, you'll have a swath of direct sunlight sweeping across the factory floor every day, causing inconsistent lighting patterns and harsh contrast (glare), making it difficult to concentrate on tasks. Northern light is steady, making for consistent lighting scenarios for makeup interior and task lighting, thus more economical for building managers.
posted by LionIndex at 1:03 PM on March 17, 2005

(Otherwise the factory operator has to provide for a number of daylighting conditions--overcast, shade, direct sunlight--requiring multiple configurations of artificial lighting, adding cost to construction)
posted by LionIndex at 1:04 PM on March 17, 2005