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Old 07-19-2011, 07:14 AM   #1
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Building out Rafters


Hi,

I have true 2x4 rafters in my existing cathedral ceiling. In order to get a better R-value, I have have been told that I could do two things.

The first being attaching sister rafters. That option I understand, but am not as keen on.

The second was to run 2x4's under my existing rafters. I like the sound of this, but am unsure as to the best way to attach them to the existing rafters in a way that will be stable.

Can I tie them together and add supplemental screws as needed? If so, what would be the best "tie" method. Some type of joist hanger? Or, would I have to run long screws, my guess would be 5-6 inch, through the new 2x4 and into the existing rafter?

I have not found much information on the latter option. It was recommended to me by an older relative, and while I consider his opinion of value, is this a method that was done in the past? or just an unpopular one?

Thank you,


Sean


Last edited by spcrooks; 07-19-2011 at 07:17 AM.
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Old 07-19-2011, 07:56 AM   #2
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Building out Rafters


Adding a 2x4 to the bottom of the rafters is a poor choice as the rafter and the scab are always moving as the weather changes.


The most successful method would be to add plywood gussets on both sides of the rafter and scab.
I'd make the rips out of 1/2" plywood--about 6 1/2" wide---

PL or liquid nails and nails will do.

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Old 07-19-2011, 08:07 AM   #3
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Building out Rafters


Thanks for the reply,

Just to make sure I read you correctly. Attaching the 2x4's underneath the existing rafters is viable, but in order to keep them in line together over time, gussets would greatly help...so far so good?

As for the gussets, I understand that they must span the width of the scab and existing rafter, but should they be a certain length? i.e. strips or blocks?

Do these gussets work in a way like collar ties do? That is, not look like they are doing much, but actually rather strong?

Sean
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Old 07-19-2011, 08:28 AM   #4
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Building out Rafters


You can run the new 2x4's perpendicular to the roof framing and use them like furring strips. This will accommodate the flexing mike spoke of and will give you an additional 1-1/2" of insulation space.
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Old 07-19-2011, 08:33 AM   #5
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Building out Rafters


An alternative is to put rigid insulation on top of the existing deck, with furring strips for ventilation, new deck, and new roof. This may only be cost effective if your existing roof is nearing the end of it's service life.

PS: since you are going to the trouble of adding depth to your rafters you should look into installing baffles for roof ventilation as well.

PPS: I think I prefer AGWhitehouse's suggestion, seems more stable.
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Old 07-19-2011, 08:41 AM   #6
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Quote:
Originally Posted by shazapple View Post
An alternative is to put rigid insulation on top of the existing deck, with furring strips for ventilation, new deck, and new roof.
If this is remotely an option, I would suggest a Hunter Cool-vent panel. Then entire process stated above is already done, just screw it on.

www.hpanels.com
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Old 07-19-2011, 08:41 AM   #7
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AGWhitehouse,

Would you mind expanding upon the perp. 2x4's you mentioned?

For instance, how many would I use? Also, as that the gussets would help secure the scab to the rafter, what would keep the perp. 2x4's from sagging? Would screws be sufficient?
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Old 07-19-2011, 08:43 AM   #8
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Building out Rafters


Oh, so you meant running the 2x4s perp. as an alternative to scabbing?
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Old 07-19-2011, 08:45 AM   #9
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http://www.stockphotopro.com/photo-thumbs-2/A2BXRD.jpg

Space the furring 16" o.c. for the length of surface you are going to apply sheetrock. Screws are sufficient. Sagging should not be an issue unless your rafters are further than 2'-0" apart.

I just had a thought...with 2x4 rafters, is you span such that this additional weight won't become an issue. If it is, then the original scab idea might better suit the structural requirements involved.
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Old 07-19-2011, 08:57 AM   #10
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Building out Rafters


I looked at the pic you posted, however, I am dealing with an open cathedral ceiling. The room is 11x20. I will be installing bead board as the ceiling.
Attached Thumbnails
Building out Rafters-cottage-roof.jpg  

Last edited by spcrooks; 07-19-2011 at 09:02 AM.
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Old 07-19-2011, 09:05 AM   #11
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Ah, a beam and purlin setup. What type of insulation were you looking to do? The span between them looks to be more than most batt type insulation is made for.

You could just use a rigid insulation product. Polyisocyanurate give approx. an R-7 per inch. With 3-1/2", that would make for an R-24.5. If that is your roof deck visible then I would only add 2-1/2" (R-17.5) to leave a 1" air space for ventilation.

Scabbing on 2x6 framing to the side of the beams would give you a 5-1/2" space. 1" airspace leaves 4-1/2" for insulation. At R-7 that makes for R-31.5.

Last edited by AGWhitehouse; 07-19-2011 at 09:09 AM.
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Old 07-19-2011, 09:15 AM   #12
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The distance between rafters is 2 ft. I was going to use batt, but am open to alternatives. I have never worked with the rigid...is it really more effective? If I did not have to alter my rafters in any way that would be the best alternative! I currently have vent chutes in, leaving me 1.5 in. to work with.
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Old 07-19-2011, 09:23 AM   #13
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Building out Rafters


Foam may be cost effective in conditions like this. Batt needs depth to give any kind of real benefit. R-30 in batt is 9-1/2" deep. R-30 in Polyisocyanurate is 4.5" deep.

if this were my house I would put 1-1/2" of rigid foam within the joist bays and then put 2" of foam over the entire structure. Then apply the bead board over that. The beadboard fasteners would need to be long to accomodate the foam, but you will end up with a continuous insulation barrier that has no thermal breaks, gives you an R-24.5 value, and shouldn't require any additional framing.
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Old 07-19-2011, 10:31 AM   #14
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Building out Rafters


I would think the beadboard would need furring strips between it and the rigid insulation?
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Old 07-19-2011, 10:38 AM   #15
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Quote:
Originally Posted by shazapple View Post
I would think the beadboard would need furring strips between it and the rigid insulation?
you could do that if you find the 3" fasteners too much for your liking in the beadboard. I wouldn't say it is mandatory though.

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