# garage needs an attic



## wopachop (Feb 12, 2008)

So my long time friend wants to build an attic/storage in his garage. Can anyone please help me find links that will teach me how strong of framing he needs? Right now its 2X6 at every 48". The joist span 23'....but there are 2x4's hanging from that top pitch of the roof down to the middle of the joists.

Can someone please show me a link that will tell me what size joists he needs? Should i sister the existing joists and use larger boards in between?


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## Cache (Sep 18, 2007)

wopachop said:


> So my long time friend wants to build an attic/storage in his garage. Can anyone please help me find links that will teach me how strong of framing he needs? Right now its 2X6 at every 48". The joist span 23'....but there are 2x4's hanging from that top pitch of the roof down to the middle of the joists.
> 
> Can someone please show me a link that will tell me what size joists he needs? Should i sister the existing joists and use larger boards in between?


I don't know of any sites that will tell you info like that. It is what engineers are paid to do. You are outside the realm of typical spans for dimensional lumber. That is a pretty gnarly span for dimensional lumber if there is no intermediate bearing. In order to figure it out, you need to know a little bit about structural engineering, such as modus of elasticity and Inertia, loads and durations.

If using western lumber you'll barely meet code with 2x10 @12"oc. Double 2x8 joists if fastened properly would also barely meet code @ 12"oc. 

If you use wood i-joists then even the lower series could do 9 1/2" @ 16"oc to meet code. 

Considering the 2x4 bracing from the ridge at the center of the span is very difficult. I have no idea how it is fastened and that would actually require an engineer's specs to be valid. You are essentially creating a king post truss, and the engineering becomes more complex.

If it were me, I would use the i-joists and I would build it to a 40lb live load and l/360 deflection at the least, so that when someone gets the bright idea to turn that attic into a play room or lounge area, it will be ready for it. In that case you would need 11 7/8" i-joists. If they are the lighter duty joists then 12"oc will be required. The heavy duty series could be spaced 16"oc. 

In either case, that king post is completely superfluous as the joists can make the span without it. Just keep in mind that the ceiling will fail if you remove the king posts before you've sistered the old joists to the new ones. I should also mention that the comments I'm making are assuming a common rafter roof structure. If they are trusses, DO NOT alter them in any way, including the removal of the king post. You'll know if these rafters are relying on the king post if there is no ridge beam where the rafters meet at the top of the roof.


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## ScipioAfricanus (Sep 13, 2008)

Oh. My. God.


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## greg24k (May 19, 2007)

I was going to post a link, but your level of stupidity left me speechless :whistling


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## boman47k (Oct 13, 2006)

I hope you guys are referring to the quote on proper deck spans, because if I read that right, I need to demo my deck and rebuild.


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## bwalley (Jan 7, 2009)

wopachop said:


> So my long time friend wants to build an attic/storage in his garage. Can anyone please help me find links that will teach me how strong of framing he needs? Right now its 2X6 at every 48". The joist span 23'....but there are 2x4's hanging from that top pitch of the roof down to the middle of the joists.
> 
> Can someone please show me a link that will tell me what size joists he needs? Should i sister the existing joists and use larger boards in between?


I recommend him hring a competent licensed contractor doing this job per plans drawn and sealed by an engineer.


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## tcleve4911 (Mar 26, 2006)

wopachop said:


> So my long time friend wants to build an attic/storage in his garage. Can anyone please help me find links that will teach me how strong of framing he needs? Right now its 2X6 at every 48". The joist span 23'....but there are 2x4's hanging from that top pitch of the roof down to the middle of the joists.
> 
> Can someone please show me a link that will tell me what size joists he needs? Should i sister the existing joists and use larger boards in between?


Too big a project for you and your long time friend.....:no:

Hire a GC...please....


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## Cache (Sep 18, 2007)

tcleve4911 said:


> Too big a project for you and your long time friend.....:no:
> 
> Hire a GC...please....



I don't know about it being too big a job, but as I re-read his post it did seem much more amateur than the first time I read it through. In any case, the project is not all that big IMO. There are just certain issues that you have to see coming. As I said before, i-joists can make that span, especially for attic storage. The bigger issue is bearing. Must be at least a 6/12 pitch if he wants to use the exterior walls for bearing, because a slope cut i-joist needs a heel of at least 4 3/8" if bearing on a 2x4 top plate. There will also need to be 3/4" ply web stiffener for two feet at each end and on both sides of every joist. Snug to the bottom flange; glued and nailed 6"oc. Additionally there will need to be 2x blocking between joists. Otherwise, he'll have to use hangers. 

Ok.... maybe he should hire a contractor.... :whistling


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## Cache (Sep 18, 2007)

boman47k said:


> I hope you guys are referring to the quote on proper deck spans, because if I read that right, I need to demo my deck and rebuild.


I don't know if the other posts were referring to my response or not, but what are your spans on your deck? We use Hem-Fir or DFL around here, and to make a 23' span, even with just a 20lb live load, you would need 2x10s if using #2&btr. My previous post was calculated at the absolute code minimum, where there is no possibility of any use other than limited attic storage. Span tables will actually tell you that you need 2x12s @ 12"oc spacing for that span. I was performing the actual calcs for the situation.

23' span
2x10s Hem-fir @ 12"oc
20lbs live load
*
=L/280 deflection*

That is a pretty bouncy floor, and you are deflecting a whole inch at the center under full load. Like I said, I wouldn't build the deck that way. I would build it to at least L/360 deflection @ 40lb live load.


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## tcleve4911 (Mar 26, 2006)

wopachop said:


> Should i sister the existing joists and use larger boards in between?


What I was addressing by "too large" was the learning curve of talking the talk. 
Like in any trade..if I go to the auto supply and I ask for the flippy thing that makes the engine run...that could be anything from the key to the solenoid in the starter.

If he goes to an information source and asks " should I use larger boards between them ?" :blink:they're going to have to educate him so he's talking the language so he can get real answers.

"but there are 2x4's hanging from that top pitch of the roof down"

just another example.................at least get on board with someone in the trades. They can at least be your translator........good luck


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## boman47k (Oct 13, 2006)

boman47k said:


> I hope you guys are referring to the quote on proper deck spans, because if I read that right, I need to demo my deck and rebuild.


 
Of course I meant to say "are* not* referring..."


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## boman47k (Oct 13, 2006)

Cache said:


> I don't know if the other posts were referring to my response or not, but what are your spans on your deck? We use Douglas Fir around here, and to make a 23' span, even with just a 20lb live load, you would need 2x12s if using #2&btr.


 
Deck /floor joists *2x8 pressure treated pine 8' long on 16" centers.*

That being said, maybe I should not have commented on this as I am no carpenter per se, and have no real knowlege of the limits of the mat you used in this situation.

Made me go look at it. 12X16' deck 3 doubled 2x*16' beams. 2X8X12 joists fastened to the outside beams and going across the top of the middle beam.


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## Cache (Sep 18, 2007)

boman47k said:


> Deck /floor joists 2x8 pressure treated pine 8' long on 16" centers.
> 
> That being said, maybe I should not have commented on this as I am no carpenter per se, and have no real knowlege of the limits of the mat you used in this situation.


Yeah, your deck is good. Your post made it sound like you'd used 2x8s for a 23' span. You had me concerned there for a second. Thought I might have to ruin your day. Your deck is L/980 by a quick calc. Under full load, you'd only deflect 0.1"

Most dimensional lumber is at least in the same ballpark. The main difference is modulus of elasticity (ability of the wood to bend) for different species. Eg. if you used Hem-fir for your deck instead of PT pine, you'd only have 0.08" deflection instead of 0.1".


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## boman47k (Oct 13, 2006)

> Yeah, your deck is good. Your post made it sound like you'd used 2x8s for a 23' span


:thumbsup: I left a small detail ( length) out. :laughing:


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## Cache (Sep 18, 2007)

boman47k said:


> Made me go look at it. 12X16' deck 3 doubled 2x*16' beams. 2X8X12 joists fastened to the outside beams and going across the top of the middle beam.


WOW!! If I'm reading this right, (2x8x12' joists with bearing beam midspan) you actually have L/2100 deflection rating. You could use that thing as a garage floor. :laughing:

For future reference in your deck building endeavors, most decks are small enough as to only need 24"oc joist spacing. It is the supporting beams that are usually the bigger issue, because people are always wanting as few posts as possible. In your case for instance, you should have a post supporting the middle of those 16' beams.


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## boman47k (Oct 13, 2006)

> You could use that thing as a garage floor


. :laughing:

Wife might disagree with you on that! :laughing:

She loves the deck! Glad I made it strong. Now if could do something about that dang street light at the corner. :whistling


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## Snobnd (Jul 1, 2008)

I had Trusses made to complete my attic space above the garage. Span 24' x 32' deep.
I end up with a 12' x 32 attic space -
2x8 bottom cord 
2x6 top cord 16" oc - 3/4 ply for the flooring.:thumbsup:


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## boman47k (Oct 13, 2006)

Are the front top cords jsu butted togetrher then fastened?

I have an old house I bought and need to brace the roof. It is about the same size as your shop. I don't have to worry about the rafters , but I need to put some bracing up there as there is none now. Previuse owner had gutted it and replaced some lumber. I don't know if he removed the bracing or it just never had any. I have not got up and really checked it out, but the ridge is showing some bowing. I might brace it the way those trusses are. The again, being trusses, the bracing specs may be different. I guess I will find out when or if I do anything with this old house.


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## Cache (Sep 18, 2007)

boman47k said:


> Are the front top cords jsu butted togetrher then fastened?
> 
> I have an old house I bought and need to brace the roof. It is about the same size as your shop. I don't have to worry about the rafters , but I need to put some bracing up there as there is none now. Previuse owner had gutted it and replaced some lumber. I don't know if he removed the bracing or it just never had any. I have not got up and really checked it out, but the ridge is showing some bowing. I might brace it the way those trusses are. The again, being trusses, the bracing specs may be different. I guess I will find out when or if I do anything with this old house.


In trusses, long top chords are always just butted and gang nailed, but you'll notice from the picture that there are web members within a certain distance of the splice on either side. Those are engineered to compensate for the weak spot at the splice. The web members also reduce the span of the bottom chord so that 2x8s are sufficient for the span. Instead of a 24' span, he effectively has only a 12' span as long as the top chords can take the stress.

When bracing rafters like you're talking about, a collar tie like in the picture would do a lot to take some of the stress off the ridge beam.


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## Cache (Sep 18, 2007)

Snobnd said:


> I had Trusses made to complete my attic space above the garage. Span 24' x 32' deep.
> I end up with a 12' x 32 attic space -
> 2x8 bottom cord
> 2x6 top cord 16" oc - 3/4 ply for the flooring.:thumbsup:


Looks good. :thumbup: Every home I design now has room trusses above the garage and, when applicable, in the attic. The trusses don't cost much more and the room to grow or storage is nice.


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