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Author Topic: Floating a second roof for shade above first roof....  (Read 3192 times)

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Offline metroplexchl

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Floating a second roof for shade above first roof....
« on: March 12, 2018, 12:18:22 AM »
So I've got this cool idea, but I can't find anything about it anywhere......surely I'm not the first to try this.  But I'm building a timber frame workshop in Texas where it gets hotter than hell.  I don't want to put in HVAC or window unit, so I thought it would be cool to put a basic roof on it and then put a second basic metal roof about a foot or two above that one that would allow air to flow between the top and bottom roof....while providing shade that would allow the structure to stay much cooler.

Does anyone have a name for this or know what its called? Or know where I can get some ideas on how to do it?

thanks

chris mccollum

Offline ljohnsaw

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Re: Floating a second roof for shade above first roof....
« Reply #1 on: March 12, 2018, 12:35:57 AM »
Well, here is what you can do that is faster, cheaper and WAY easier.  I did it in my attic.  Staple rolls of Refletix aluminized Mylar to the bottom side of your rafters.  If you can, provide ventilation exit at the peak.  Plus, you get great reflective properties - it will be nice and bright inside.  It reflects any heat that comes through the roof right back up.  I put it up in my attic on a fairly warm day and I could feel at least a 15 drop in temperature when I held it between me and the roof.  Really helps my house stay cool.  The rolls are 4 foot wide and I think 150 foot long.
John Sawicky

Just North-East of Sacramento...

SkyTrak 9038, Davis Little Monster backhoe, Case 16+4 Trencher, Home Built 38" cut Bandmill up to 54' - using it all to build a timber frame cabin.

Offline florida

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Re: Floating a second roof for shade above first roof....
« Reply #2 on: March 12, 2018, 07:42:24 AM »
It's a fairly common thing on mobile homes around and called a "roof-over." On mobile homes it makes some sense because it's the easiest way to reroof one. For a regular wood framed structure insulation is way cheaper and faster. The first inch of insulation will reduce heat flow but 50% and the second inch by another 50%. Nothing you can do for shade or insulation is going to do anything for humidity though. 
General contractor and carpenter for 50 years.

Offline Don P

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Re: Floating a second roof for shade above first roof....
« Reply #3 on: March 12, 2018, 07:53:55 AM »
I've seen it called a double roof, Texas cool roof or a smart roof;
http://www.houstoncoolmetalroofs.com/cool-roof-information/cool-roof-design-texas/

I've seen instances where it has fooled firefighters though, check with your local building or fire dept before doing it. It is considered a concealed space and can hide, feed, or harbor fire, smoke or gasses and provide a chimney to spread fire. Do your research before proceeding.

Offline metroplexchl

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Re: Floating a second roof for shade above first roof....
« Reply #4 on: March 12, 2018, 11:03:41 PM »
To give you an idea of what I'm thinking, see below.  The rest of the timber frame structure will be finished out, but the roof will be as it looks here for the most part. Obviously I'll have more support for the top roof instead of only a post at each corner, but I left it simple so you all could see what I mean with less blocking the openness in between the roofs.  It will be open on all sides with about 1' in between the roofs (or whatever I find is optimal through research).

 

 

 

 

Offline Crusarius

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Re: Floating a second roof for shade above first roof....
« Reply #5 on: March 13, 2018, 11:56:45 AM »
looks like the ultimate nesting grounds for everything you don't want. especially yellow jackets.
I knew what I thought I meant.

Offline mike_belben

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Re: Floating a second roof for shade above first roof....
« Reply #6 on: April 08, 2018, 12:05:27 AM »
Be certain you have inlet air enterring the soffit and outlet air exiting the peak or ridge and it will help.  The industrial aluminized flat rubber roof paint will reject quite a bit of heat as well.  And finally, if you are able to maintain a pond, pumping water up onto the ridge to drip down and vaporize will also help reduce temps on sunny days.  Just a drizzle from a perforated garden hose.


If you havent built this structure yet, consider putting a few coils of 4" drainage pipe down a few feet in the dirt under it, to a plenum somewhere in the home.  A fan in the roof of the house blows hot air out, while makeup air is sucked in from a shady spot outside, through the cool earth and then into the house. Obviously youll want some screen for the critters.


I think it was university of colorado that documented a greenhouse in a very hot and humid climate where they had socked corrogated drainage pipe going through raised beds and a big fan in a 55gal drum plenum just circulated the entire room through the bedded pipes.  The plant roots consumed the humidity and dropped the temps.. Thus it "felt" like AC.  They had formulas for tubing quantity and cfm requirements in the document i read.
Revelation 3:20

Offline Don P

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Re: Floating a second roof for shade above first roof....
« Reply #7 on: April 08, 2018, 08:44:01 AM »
Google earth tubes for that. If the tubes actually connect to soil rather than just going through it... not good for the house and occupants, moisture and radon. Earth tubes were done in the '70's but have been largely abandoned, not enough heat transfer per length. Ground source heat pumps kind of took their place.

Offline mike_belben

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Re: Floating a second roof for shade above first roof....
« Reply #8 on: April 10, 2018, 07:42:38 AM »
Yeah the tubes pass through the dirt, they dont terminate there. 

The slower the air speed through the tubes, and deeper they are buried (to a point) the more effective theyll be.  So it kinda fits the solar off grid model, the whole thing works on just a small exhaust fan in the attic, naturally its most effective when run overnight to pull in the evening air and then shut down during peak day.

I spent most of my life with a night time attic fan schedule to endure summer.  The more mass in the home's construction the better it works.
Revelation 3:20

Offline 711ac

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Re: Floating a second roof for shade above first roof....
« Reply #9 on: April 11, 2018, 06:06:17 PM »
The term is a "cold roof" used often in cold climates where the heated space (under the roof) is "leaking'' heat through to the cold and doing a little melting of the snow, creating ice and roof leaks. It will work for your purpose as well (along with a white roof). Here's a vid but use that term for searching.
 https://www.youtube.com/watch?v=nLTola8SxiA

Offline 47sawdust

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Re: Floating a second roof for shade above first roof....
« Reply #10 on: April 21, 2018, 08:23:35 PM »
You might check out this video on youtube.

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Offline D L Bahler

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Re: Floating a second roof for shade above first roof....
« Reply #11 on: May 27, 2018, 12:09:01 AM »
In Switzerland you will see this done quite often. 

Only in this case, it is done for insulation against the cold rather than heat. But it works both ways. 
What you will do in this case is to place a thick layer of insulation on top of the primary roof structure. Then another set of rafters are laid out on top of those and fastened through the foam to the roof structure below. Strapping is then laid across these upper rafters and the roof tiles laid on top. This creates a thermal barrier that performs very well. 

On my own cabin I did something like what you are describing. I made my roof with sheeting nailed across the rafters. I then fastened 2x4's on edge with long timber screws through to the main rafters, and nailed another set of sheeting across these. They are open below, with a ridge vent above. I can't say what this does for heat, as the cabin is in the woods, but the upper level is always quite comfortable  

Offline shinnlinger

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Re: Floating a second roof for shade above first roof....
« Reply #12 on: June 29, 2018, 09:56:56 PM »
I suppose I too have a cold roof.  I have rafters with purlins underneath.  Sheathed and then stacked with insulation from old lumber kiln.  I sandwiched that foam with 2x running vertically attached to the purlins underneath with panel screws and than strapped the 2x horizontal as nailers for ribbed roofing.  It created an air space which breathes  at the top and bottom.






Shinnlinger
Woodshop teacher, pasture raised chicken farmer
34 horse kubota L-2850, Turner Band Mill, '84 F-600,
living in self-built/milled timberframe home


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