Help in sizing TJI for an attic please...

Hello. I need help in sizing a TJI for an attic I plan on making in my existing garage.
The garage design/layout is such. I've been told it is a cottage style roof system. There are no trusses. The roof is shaped like a pyramid. There are about 8 or so 2x4s that span the width of the garage. I assume they were for holding the walls true during/after construction. Other then that the attic space is completly void. The exterior walls sit on a 8" concrete footing. Overall garage is in excellent condition.
My plan was to build an interior wall and roof system on that 8" footing, inside the existing walls. The joists need to span 21 feet. The attic space is purely for storage, but needs to accommodate me walking around up there.
Anyone have experience in this area, offer some guidance? What size TJIs do I need?
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Freddy wrote:

Try here:
http://www.naffainc.com/x/cb2/tables/tjiflrjoiststable.htm#16TJI-PRO (20PSF)
For that type of span with the possiblity of engine blocks being stored up there, you should probably go with 16" Pro550s on 1' centers, but the span table says you can go with 19.5 " centers. The span tables assume 3/4" T&G plywood glued and screwed for flooring.
Place your studs on the same spacing as your TJI spacing, or double top plate with 16" OC. Check the rest of the website for stiffener and blocking requirements.
Or you could take your layout dimensions to the place that sells your TJIs and ask them to engineer it for you. They usually save you money. I always overengineer everything because I am not an engineer.
--
Robert Allison
Rimshot, Inc.
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Thanks for the link. Certainly no engine blocks, but i suppose over engineering is never a bad choice.
I would take my plan to the location where i'll get my TJIs, but they will only recommend if an engineer stamp is present. And that stamp costs more then the lumber :)
Thanks again.
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Freddy wrote:

Look for a different engineer. My engineer will take a plan drawn on a napkin (if it is simple), and give me a design drawing for about 250.00. Since I overengineer things on my own, he will take my parameters and design everything accurately, resulting in less materials to buy. When you figure in the time saved in doing my own engineering, installing the extra over engineered materials, the extraneous blocking, etc., I usually end up saving more than he cost me. If it is more complicated, it would cost more, but you can save more, too.
Not to mention the peace of mind and the engineers stamp on the plans when the inspector comes. (If there is one.)
I have worked with a lot of engineers over the years. There are a couple that have retired, but they do not lose their license and stamps. They are the ones that I go to with small projects like this. One of them just loves to do these small projects for me and he charges much less than one that is still making a living doing it. You might ask around and see if you can find someone like this. They are great sources of knowledge and help.
--
Robert Allison
Rimshot, Inc.
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If i was anywhere near Georgetown, TX i'd be calling you up for your engineers. I have tried a few but this city is so busy with construction you can't get the time of day from these guys.
Thanks again.
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Freddy wrote:

If you can actually talk to an engineer, ask them if they know any engineers that do small work or that are retired but still take on small projects. I will bet that they can recommend someone.
--
Robert Allison
Rimshot, Inc.
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Freddy wrote:

And it is worth more. Free advice is worth exactly what you pay for it ... at best, and is very costly at worse.
Matt
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Sorry, but i've had great advice from many people, and it never cost me a nickle. I'm not building a skyscrapper here, some common sense and knowledge of the products is all that is needed.
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Freddy wrote:

Is a skyscrapper anything like a skyscraper?
Yes, I've heard this before ... often right before the disaster...
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I have a garage that I built around 3 years ago, with 2 x 8 "floor joists", 24" OC for the attic. My local inspector ok'd it, but I noticed a significant amount of sag when I started storing stuff up there. There is around 2" +/- of sag at the center of the span. I talked to the building inspector about it a little more when he was out to look at another project, and his outlook on it was "it's just a garage". I would just leave it, but it is throwing one of my garage doors out of alignment enough that it "squeals" when in opens. I have around 600-800 lbs (total) stored up there, and you can feel the flex in the floor walking around. I have a 1/2" plywood floor up there.
Before you start getting the idea that I am a dingbat, I am planning on putting a double Microlam beam down the center to take the sag out and increase the weight capacity of the attic floor, That being said, I could "get away" with realigning the door track and not loading up the attic any more than it already is.
Span tables are great, if you are building something that you are going to live in, and need deflection measured as L/180 or better. Remember: 1) It's just a garage attic, 2) You will rarely be walking around in it, 3) TJI's are expensive, so getting ones that are bigger than you need or getting more of them than you need gets pricey real fast.
The exterior walls should be adequate for holding up the attic. You shouldn't need to build a new interior wall system to hold it up, but I can't quite see it from here. Also, it sounds like you have a "hip roof", although I can't see that too well either.
I have a structural engineer that does side work, with the "stamp" for $40 / hour. But I am guessing an engineer is going to tell you that it is just a garage attic, and Fonzie won't ever be living up there.
JK
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Thanks for the comments. You pain is my concern. You don't want to under engineer this.
What is you span of your garage?
And i have know idea what TJI run...what do they cost, any example?
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I can't help with the truss joists, but I framed our garage attic floor with 2x12's at 16" OC and 5/8" plywood flooring. The free span is just over 23 feet. I installed solid blocking every 8' along the span.
It's a little springy for a living space, but more than adequate for storage purposes as long as you're not storing engine blocks, massive libaries of books, or your gold bar collection. :) I have sheetrock on the underside of the joists, and after six years I have not had any cracking from the joists flexing. We store items like holiday decorations, pvc pipe and conduit, luggage, stuff for the next garage sale, etc. I do have extra roof shingles and several boxes of old books up there too, but nothing excessive.
If I had it to do over again, the only thing I would do different is to use 3/4" T&G OSB for the subfloor instead of the 5/8" ply (what the lumberyard recommended at the time). The flex of the subfloor is more noticeable than the joists flexing.
Anthony
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Thanks Anthony.
Your application is what i am looking for. 2x12's at 16" OC and 5/8" plywood flooring spanning 23ft...wow, i wouldn't of guessed that would hold well, good to hear it works for you. mind you i reinforeced our subfloor of the house with cross-blocking and it significantly improved the sturdiness of the floor.
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Freddy,

You may want to have a look at the Span Calculator at:
http://www.cwc.ca/Resources/E-Tools/SpanCalc?Language=EN
Using the US Span Charts for ceiling joists (my application) it shows
Doug Fir, 2x12, #2 grade, 16" OC spacing, 20 live/10 dead (drywall, limited attic storage), shows a span rating of of 23'-4". That's about 3" wider than my joists span. I honestly expected it to be springier than it is, considering the span.
You could always increase the spacing for more strength. For instance, just changing the spacing above to 12" OC gives a 27' span for ceiling joists!
In your case, sizing for floor joists (30 live/10 dead) 2x12's at 16" will free span 20'-3", and 2x12's at 12" OC will free span 23'-4". Either would probably work for your intended purpose.
Play around with the span calculator. You may find other options too.
Remember, the free span is the distance "between" the supports, not counting the wall or beam supporting each end. For example, my garage is 24' wide. With 2x6 walls at each end, my free span is about 23'-1".

I'm sure the solid blocking in my attic joists contributes a lot to the strength as well.
Again, I really recommend the 3/4" T&G OSB subfloor over the 5/8 plywood. We used the T&G subfloor in our house and it's a much stiffer floor.
Also, I recommend putting construction adhesive on the tops of the joists before you lay down each sheet. This will add to the strength, but it also helps eliminate nail popping.
Take care,
Anthony
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Thanks for the Span Calculator link and info Anthony.
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