# Roof Ventilation Question



## Dwight Ennis (Jan 2, 2008)

How was hot air vented through this kind of roof ventilator?










Were periodic slots cut into the roof? Was the entire roof cut away (other than the rafters and supports) under it? Were there a series of smaller cutouts?

If the building was lit on the inside, would light shine from the ventilator?


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## chuck n (Jan 2, 2008)

Dwight

Hot air is less dense and rises. With a large opening in the roof, with vents to keep out the rain, air heated in the building will rise and exit through the roof. It will be replaced by cooler air coming in the from the lower, ground level. This creates a draft that helps cool the building, before air conditioning.

It is really just a big chimney.

Chuck

PS. hot air rises, that is why we have politicians, not statesmen.


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## Semper Vaporo (Jan 2, 2008)

What I vaguely remember of seeing a building that had ventilators similar to that (three smaller cupola's, instead of one long one) there was light barely visible in one of them... not enough to shine out onto the roof (so the lights were obviously not 'in' the ventilator) but definitely visible at night reflecting up into the cavity behind the vents. But I don't know what was in the building. I just remember driving past it sometimes late at night.


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## Totalwrecker (Feb 26, 2009)

I think; that's an Attic vent. Air holes w/ screen between horizontal slats in cupola.
John


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## Dwight Ennis (Jan 2, 2008)

Hmmmm... maybe I'm not wording my question clearly. I want to know what the roof looked like under/inside the ventilator. I would think there were no shingles certainly, but did the roofing boards continue? Were there sections of them missing, or was the whole roof open? In other words, if I were inside this building and I looked up, would I see all the way up to the ventilator roof and slats, or were there just a few holes or notches in the roof proper which allowed air to travel up and out to the ventilator?


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## chuck n (Jan 2, 2008)

Dwight

My guess is that it was open, only the joists/rafters holding up the roof. The less blocking the flow of air the better.

Chuck


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## Mike Reilley (Jan 2, 2008)

It was a hole in the ceiling... My son's shop in Oakland is an old foundry...and there's a square hole up there with rafters crossing it.


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## R.W. Marty (Jan 2, 2008)

Dwight,
The roof below the ventilator was/is generally an open hole except for structural framing.
As has been stated convection creates a draft to cool/ventilate the structure.
Lights in this type structure were/are usually directional, pointed down with reflectors,
so any light glow at night seen through the cupola ventilator will be very dim.

Take care bud.


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## Dwight Ennis (Jan 2, 2008)

Thank you gentlemen - that's the answer I was looking for.


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## Wesley Furman (Jan 2, 2008)

Yes the structure would be in place, and needed. Being in a factory there probably wasn't much concern about the visual view of the ceiling.








I'm sure some light would leak out. But it would be a low level since it is reflected light off the walls and floor not direct light from any fixtures.


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## CliffyJ (Apr 29, 2009)

Dwight, here's one example that may shed some light on your structural question.

This was a huge mill in Virginia City, and like most large mill & mine structures there, at this time, it had a "clerestory" roof for both ventilation and lighting. 










Here are a couple of interior shots of the same building.



















There are many other ways to frame this I'm sure, but in this case there were main trusses that went all the way to the peak, and which were spaced maybe 10-12 ft apart. A number of stringers tied these together, and rafters rested on those. The rafters and sheathing planks were cut at the opening, not the main trusses. 

In the case of your model pic, I'd guess that the trusses also went all the way up, with stringers at the base of your vent walls, and/or a ridge beam buried inside the vent structure. 

Since the model is in an electric lighting era, the "skylight" effect wouldn't be as important. But I'd still think that light would come through, because I can't picture what might otherwise baffle it then the slats. Unless you have an unlit attic, that would baffle the light for sure. 

I'm no expert though, just putting in my 2 cents.

Cliff


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