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Old 06-02-2009, 02:27 PM   #1
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How to vent eaves if no overhang, metal roof, Texas?


I'd like to replace my shingle over 5/16" OSB (excellent
condition, but cheap materials) roof with a standing seam
metal roof.

The "attic" interior is inaccessible, just 18" high at
the ridge, down to nothing high at the eaves. There is
no eave overhang; the roof stops an inch past the
vertical plane of the exterior wall. This is a new
manufactured home, so I guess I don't need to have
overhang and gutters to keep water from the foundation,
because I don't have a foundation.

I'm willing to spend good money now to minimize costs,
both roofing and cooling (it gets warm in Texas) over the
next 30 years. I'm assuming costs of energy will go up
dramatically over that time span, as will inflation in
general. So best to do it now, for maximum benefit,
lowest cost, over time.

Currently the tiny "attic" is vented through the roof
with low profile cheap plastic vents -- about 12 of them
in all, six of them high up near the ridge, six of them
low down. The roof is a simple gable, 3:12 pitch, 76 ft
long over a 16 ft wide home, so should be about 8.5 ft
from ridge to eave on the slant. That should be about
1290 sq ft of roof.

If I stayed with asphalt shingles, then this would be a
natural for SmartVent on the eave inlet vents.

Main question: But how in tarnation can I get inlet
ventilation with a metal roof, when I have no eave
overhang or soffits?

Related questions: What sorts of underlayment, vapor
barriers, radiant barriers (above the deck, blown in
chips over the blown in fiberglass, ...?), air gaps
below the metal, and such would you recommend.

I supposed the outlet ventilation would be one of a
ridge vent or a bunch of dormers. I am concerned that
a ridge vent will impede the airflow too much.
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Old 06-02-2009, 05:15 PM   #2
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How to vent eaves if no overhang, metal roof, Texas?


Quote:
Originally Posted by pj99 View Post
If I stayed with asphalt shingles, then this would be a
natural for SmartVent on the eave inlet vents.

I am concerned that a ridge vent will impede the airflow too much.
Why would the Smart Vent Not work under a metal roof as you stated as your hesitancey?

Why would a Ridge Vent "Impede" the airflow?

Ed

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Old 06-02-2009, 05:48 PM   #3
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How to vent eaves if no overhang, metal roof, Texas?


Quote:
Originally Posted by Ed the Roofer View Post
Why would the Smart Vent Not work under a metal roof as you stated as your hesitancey?
Thanks for responding.

I'd have figured that the Smart Vent would get entirely covered up by the metal roof and associated flashing along the eaves. The best it could do would be to connect the "attic" airspace with the thin layer of air under the metal roof; nothing would connect to the great outdoors.

If I'm figuring wrong, it won't surprise me. I'm no roofer.

Quote:
Originally Posted by Ed the Roofer View Post
Why would a Ridge Vent "Impede" the airflow?
My doubts about ridge vents come from the website http://www.savenrgvi.com/attic_fans.htm.

However:
  1. that website seems to have an unusual way of thinking of these things, so could easily not be reliable advice, and
  2. whereas that website worries about hot air not naturally going downhill through the ridge vent maze, I'd worry more about the small holes and tight corners through which ridge vent expelled air has to navigate. Certainly on my clothes dryer vent, I strive to have a wide and straight path for the hot air to exit, not a winding and narrow path. Wouldn't the same apply to roof vents? Every ridge vent I've read about seems to put the exiting air through a bazillion small holes at some point, as part of keeping the weather and pests out.
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Old 06-02-2009, 06:33 PM   #4
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How to vent eaves if no overhang, metal roof, Texas?


Quote:
Originally Posted by pj99 View Post
Thanks for responding.

I'd have figured that the Smart Vent would get entirely covered up by the metal roof and associated flashing along the eaves. The best it could do would be to connect the "attic" airspace with the thin layer of air under the metal roof; nothing would connect to the great outdoors.

If I'm figuring wrong, it won't surprise me. I'm no roofer.



My doubts about ridge vents come from the website http://www.savenrgvi.com/attic_fans.htm.

However:
  1. that website seems to have an unusual way of thinking of these things, so could easily not be reliable advice, and
  2. whereas that website worries about hot air not naturally going downhill through the ridge vent maze, I'd worry more about the small holes and tight corners through which ridge vent expelled air has to navigate. Certainly on my clothes dryer vent, I strive to have a wide and straight path for the hot air to exit, not a winding and narrow path. Wouldn't the same apply to roof vents? Every ridge vent I've read about seems to put the exiting air through a bazillion small holes at some point, as part of keeping the weather and pests out.

For the flashings, they can extend about an inch farther than the leading butt edge of the Smart Vent to still allow air flow to enter, if installed on top of the Smart Vent, otherwise it can be installed under the Smart Vent just like the Gutter Apron Drip Edge Flashing, just like it is shown on their website illustrations.

I have read that website link before.

"Sounds logical except for one basic fact: Hot air rises, it doesn't go down. A ridge vent design dictates hot air must fight gravity, travel down from the peak of the roof in order to escape. This of course is unrealistic and so is expecting this type of vent to be effective. The only "driving force" that makes hot air leave an attic is the differential density of the 140F attic compared to the more dense ambient outside air."

The air does not have to move downhill very much. It is almost a horizontal lateral movement. Fo to the Air Vent website instead and watch the videos they have reflecting the smoke canisters in comparison case studies by an independent laboratory showing the movement out of the attics under various wind conditions.

The hot, thermal buoyancy will push the warmer air upwards. The air already at the ridge vent slot area is not just going to slam on it's brakes and prevent the additional and continual force of more rising air from allowing it to be expelled by the interior forces at work.

Ed
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Old 06-02-2009, 07:40 PM   #5
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How to vent eaves if no overhang, metal roof, Texas?


Quote:
Originally Posted by Ed the Roofer View Post
For the flashings, they can extend about an inch farther than the leading butt edge of the Smart Vent to still allow air flow to enter, if installed on top of the Smart Vent, otherwise it can be installed under the Smart Vent just like the Gutter Apron Drip Edge Flashing, just like it is shown on their website illustrations.
Ah - anther reason that Smart Vents won't work here. Heck, they wouldn't even work with a normal asphalt shingle roof.

The reason is that the "attic" has about zero inches vertical height out at the eaves, which is totally stuffed with the blown in fiberglass insulation. Short of removing the roof deck so that I can put in those plastic chutes (what are they called?) to keep the insulation out of the way, I doubt that there is practical way to have any useful ventilation in or out of that roof within a foot of the eaves.

That roof will only ever have ventilation via penetrating vents, gable vents or ridge vents.

Which does remind me of an alternative -- remove all the roof penetrating vents and put in two powered fans, one on each gable, one blowing in and one out. Just blow a steady stream of air from one end of that long tunnel to the other.
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Old 06-03-2009, 02:03 AM   #6
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How to vent eaves if no overhang, metal roof, Texas?


Hello,

I posted a reply on Roofing.com


Keith
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Old 06-04-2009, 12:50 AM   #7
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How to vent eaves if no overhang, metal roof, Texas?


Build a new truss system SUPPENED OVER the current roof. Close in the new eves, add intake, sheetmetal, and exust your done. Now you have shaded your home, lower engery cost. LOOK GOOD TOO. I have done this mant times here in CENTRAL TEXAS.
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Old 06-04-2009, 02:58 AM   #8
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How to vent eaves if no overhang, metal roof, Texas?


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Originally Posted by texas115115 View Post
Build a new truss system SUPPENED OVER the current roof. Close in the new eves, add intake, sheetmetal, and exust your done. Now you have shaded your home, lower engery cost. LOOK GOOD TOO. I have done this mant times here in CENTRAL TEXAS.
Yeah - I suppose that would work. Well, with one risk. This is a dang mobile home, not a real house. So I should be a little more careful how much weight I add to it.

Anyhow, I think I figured out a different way to skin this cat, with what I hope will be just as long lasting (all I need is about 30 years, given my age ) and cool results. See further my reply http://www.roofing.com/forum/sutra55866.html#55866 over on a thread at roofing.com, and my post over there "THERMAL CONTROL MEMBRANE (TCM) -- any good?" http://www.roofing.com/forum/about7944.html

In short, I am considering venting it from gable to gable, not eave to ridge (this trailer is real long and skinny.) For the actual roofing construction, I'm considering using THERMAL CONTROL MEMBRANE (TCM). I'd put the following layers, top down, on my roof: 1) standing seam metal in some "cool" color, 2) this TCM stuff, 3) the existing cheap asphalt shingles, almost new, 4) existing unknown cheap underlayment, 5) existing 5/16" OSB decking. That's it.

The primary risk I see is that I'm expecting too much of this TCM stuff. Well, that, and the fact that a month ago I couldn't even spell the word "roof".

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