Hello. I need help in sizing a TJI for an attic I plan on making in my
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
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?
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.
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 :)
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.
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.
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
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.
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
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.
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.
You may want to have a look at the Span Calculator at:
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
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.
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