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Old 10-28-2011, 10:21 AM   #1
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Lightweight Vaulted Ceiling Roof


I want to build a lightweight vaulted ceiling/roof with good insulation. The vertical spans for the roof joists are relatively short (<9') but the horizontal spans for the support beams below are long (~16'). So, I want to minimize dead load on those beams. My plan is to use 2x6 rafters and stack some 2x4 spacers on top of them to make room for R30 insulation. 2x4 strapping (purlins) and corrugated metal roof will then on on top of the spacers (see drawing). A metal roof without sheathing is a bit unusual, but from the research I've done this seems to be widely accepted. However, the use of the 2x4 spacers is pretty novel. I was wondering if anyone had any comments about this design before I run it by the inspectors office. The drawing shows horizontal and vertical cross sections. I know I could just use 2x10 rafters, but I prefer to work with 2x6s and I like allowing airflow among the vertical columns. Any comments positive or negative would be appreciated.
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Old 11-01-2011, 12:21 PM   #2
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Lightweight Vaulted Ceiling Roof


No comments at all? Any concern about wind lift?

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Old 11-01-2011, 01:07 PM   #3
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Lightweight Vaulted Ceiling Roof


I think 2x10's would be less labour intensive, considering all the work you'll have to do to detail around the eaves, ridge, projections etc. Not to mention the amount of spacers you would have to cut for each piece of strapping. Construction would have to be quick since you don't have a felt underlay to act as a temporary roof, nor is there anything to attach ice and water shield to.

3" of airspace is quite a bit. Why? What does your local code specify?

I would put strapping directly on 2x10. With a corrugated roof I think there would be more than enough ventilation. If you still want to use 2x6 then you could put metal roof, strapping, 2x6 with R20 batt, 2" foam, and gypsum.
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Old 11-01-2011, 03:58 PM   #4
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Lightweight Vaulted Ceiling Roof


+1 on running with 2x10s and simplifying the process. It will not add that much weight when compared to all the strapping and purlins.

3" of air space is more than really required if you have continuous venting from top to bottom.

I would also apply some sort of air barrier/tyvek to prevent the wind wash on the insulation. You could go with rigid foam and get a higher R-value per inch as well and keep the 2x6 framing option. You can just as easily use rigid on the 2x10 option too.
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Old 11-01-2011, 04:30 PM   #5
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Thanks Windows.

If I was to use 2x10s, with the tyvek barrier you recommend, where would the tyvek go? Just under the metal? Between the joists and the strapping? Laid loosely over the insulation (perhaps with some staples to hold it)? I agree that it would be good to protect the insulation, but I'm not sure how given that there is no decking and that I need to maintain ventilation.

The rigid insulation is out--just too expensive. Unless I'm missing something.
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Old 11-01-2011, 06:46 PM   #6
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Lightweight Vaulted Ceiling Roof


Assuming that you are in Oklahoma, your 2x6 rafters will not meet code. Now that depends on what your local code for rafter spans are including what allowable dead loads and ground snow loading are.
Assuming 10 and 30 which maybe completely wrong you can only span about 14' with Dough Fir #2 on 16" canters.
I kind of like your idea of spacers to get the depth you need for the insulation but without knowing the whole scope of the project I can't be of much help.

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Old 11-01-2011, 06:49 PM   #7
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A second thought is that you may need a butt load of spacers to keep the attached metal roof from ripping off during a high wind time.

Also, would you have leaking issues from roof to spacers during rains?

Andy.
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Old 11-02-2011, 01:14 AM   #8
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Thanks Windows.

If I was to use 2x10s, with the tyvek barrier you recommend, where would the tyvek go? Just under the metal? Between the joists and the strapping? Laid loosely over the insulation (perhaps with some staples to hold it)? I agree that it would be good to protect the insulation, but I'm not sure how given that there is no decking and that I need to maintain ventilation.

The rigid insulation is out--just too expensive. Unless I'm missing something.
Rigid as in Poly-iso.

If that is too expensive, a good high density fiberglass batt will work.

You can either run baffle to prevent the wind wash, run 2x8s with tyvek over the rafters and 1.5 inch spacer (2x4) attached width wise to the 2x8. This would give you about an R-25 out of the 7.25" of fiberglass and a completely air tight and protected insulation.
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Old 11-02-2011, 08:48 AM   #9
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Thanks for the continued support.

Andy, The max span is a little less than 9'. The longer spans apply to the beams below the rafters, which will be 14" LVL. I'm assuming loads are 20 and 20, although the dead load is actually quite a bit less I imagine.

However, I think the 2x6s are out because I really like Window's idea of putting down 2x8s, then a layer of tyvek on top, then 1.5" strips longways on the 2x8s for spacing, then the strapping and the roof. Sounds like a good balance of simplicity and quality. Also, a 2x8 will contain the R30 rock wool insulation I want to use. Another plus is that I will have some rain blockage once the tyvek is up, so I can take my time and do a good job on the roof.

As for rigid insulation, I was pricing 2" poly-iso at about $30+ per sheet (4x8). and I would need to triple the depth to get R30. A bit much for me.
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Old 11-02-2011, 09:47 AM   #10
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Lightweight Vaulted Ceiling Roof


Yeah, rigid insulation is $texas.

Here is the tyvek product Windows mentioned. An alternative to the strapping method is C channels to provide your vent space. It would be interesting to see the price difference.
http://www2.dupont.com/Tyvek_Weather...atticwrap.html

PS: My local code specifies a 1:300 vent:ceiling area. I think you could easily get that with your new setup.

Last edited by shazapple; 11-02-2011 at 09:50 AM.
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Old 11-02-2011, 09:47 AM   #11
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Lightweight Vaulted Ceiling Roof


If you are putting up Purlins (i.e. strapping) across the roof, you don't need the filler strip.

At that point, I would put down the tyvek/underlayment across the 2x8s and either put down your strapping and have your 1.5" of venting or better yet, put down a layer of 1" foam over the rafters and apply your strapping to that.

That will give you every bit of the R-30 assembly and create a complete thermal break across the rafters which is ideal.
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Old 11-02-2011, 10:28 AM   #12
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Thanks again guys. To me it seems like the spacers are necessary. If I only have horizontal strapping with no space below, then there will be very little vertical airflow (allowed only by the channels in the metal roof). Seems like the very worst situation.

The atticwrap product is interesting. I can't find official pricing for it, but another forum says it is about $200 for a 4x100 roll. No info anywhere about those pvc rafter caps. I like the idea of just stacking 2x material on top of the rafters. Then you can just stretch the tyvek and hammer away.

That about does it for me. I think I have a good idea of what I want to do now. I'll try to remember to post some results.
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Old 11-02-2011, 10:38 AM   #13
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Actually the purported reasoning for venting the roof area above the insulation is to keep the roof material cool.
No need for this with a metal roof.
Andy.
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Old 11-02-2011, 11:10 AM   #14
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Originally Posted by AndyGump View Post
Actually the purported reasoning for venting the roof area above the insulation is to keep the roof material cool.
No need for this with a metal roof.
Andy.
+1

If you have the moisture diffusion under control, you really don't need to vent a metal roof.
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Old 11-02-2011, 11:27 AM   #15
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The venting was for moisture. I don't know the degree to which I will hae moisture diffusion under control. There will be a bathroom (with a fan vent of course) under this roof, but otherwise no steam emitting objects (except people). I figure venting is not a bad idea, and I imagine the inspectors will want to see it.

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