I have a set of architect drawn plans for a house. There's an A-Frame
section in the middle with vaulted ceiling (i.e. no 2nd floor) in the front
and a 2nd floor loft in the back. The plans specify the use of 3x10s on 12"
centers. These would be 23ft long with no support other than at the ends.
Seems to me that's pushing it a bit too far and the floor for the loft will
be very spongy. Any thoughts?
Also, there is a bathroom in that loft and I really prefer to put down
ceramic tile. I'm concerned that a flor that has much deflection will cause
grout joints to constantly crack.
These are a set of ready made plans. I did not work with the architect, nor
do I know how to contact him.
Even if I was able to, I'm not sure I would. My past experience tells me
that jst because an architect has a degree means nothing about his
competence. Just like any other profession there are lots of degreed
I'd rather not start a long discussion debating the value of the
architectural profession. I'd rather get first hand feedback from others
with a similar situation.
Thats why the intelligent person will require verifiable references.
In your opinion, it seems, that just because someone has a degree he is
This is called immature jealousy.
Lemme get this straight.
You won't do business with a local architect because he might not know what
he's doing, yet you purchase a set of stockplans that probably won't work in
your area and then you ask for solutions to errors on said plans from
anonymous persons in usenet?
Its apparent who's incompetent here. LOL
That span, even with 12" spacing, is long enough so that choosing
merely typical lumber for the joists will make it fail to meet a
30PSF live load specification. You're right, it's too bouncy.
Boise-Cascade's AJS-25, 9 3/4" I-joist is good for 22'11",
assuming L/360 and 12" spacing, and the solid lumber tables
vary from 22'9" to 15'9", depending on what you're using.
For your purposes, you want spans of no more than 18' for 2x10s,
or 22' for 2x12s. Are the floor-to-ceiling distances small enough
so that you're clawing for vertical space? If not, you should
probably switch to 12" I-joists, like
Boise-Cascades (AJS-25, 11 7/8")
In this situation, the maximum deflection of a beam (joist) will grow
with the cube of the span, increase linearly with increased spacing,
decrease linearly with increased beam width, and decrease as the cube
of the beam depth. Knowing this, you can extrapolate from standard
span tables to your non-standard situation.
For example, according to my Code Check Building book, the 1997 UBC
allows 2x10s 12" o.c. carrying a 40 lbs/ft^2 live load to span 18'0".
The 3x10s are (2.5"/1.5" = 5/3) times as wide, so they can span (cube
root of 5/3 = 1.186) times as far, or 21'4".
By contrast, the 1997 UBC allows 2x12s 12" o.c. to span 21'11". To
get up to a 23' span, you need something ((23'/21'11")^3 = 1.156)
times as stiff, e.g. 2x12s at (12"/1.156 = 10.38") o.c. Or you could
use 3x12s and increase the spacing by a factor of 2.5/1.5 = 5/3 to
At least, that's the theory as I understand it.
3x10"s?????? Is this engineered lumber or a typo?
Just because you have a set of plans does not mean they will meet the local
building code if there is one, everywhere in the world. I had a friend that
bought a set of engineered drawings "guaranteed to pass permit inspection
any where". Took 7 months of changes before he got his permit. The plans
company FINALLY made all the necessary changes. Only cost my buddy an extra
$7k for the foundation and extra review fees. The foundation drawn did not
allow for frost in the ground.
2x12 12" o/c is fine if lumber is dougfir#2 or better. Grade and type of
lumber is important, the 'engineered' woods tend to be less stiff than DF.
Double up on decking with stagg joints if u wanna be 110% certain. Glue and
screw, don't cut corners!
Some others had good advice. I'm spanning 28' with 14" I-joists on 2'
centers in my garage. Of course this is a roof, not in snow country,
not a floor so deflection isn't as big an issue.
If I were you, I'd use I-joists, with glued and screwed decking and
perhaps some blocking at midspan. Also, the underside treatment
(ceiling?), depending on what it is, can add considerable stiffness.
Where are you going to find 3x10's? If anything you'd end up using
engineered wood I-beams. Even then that's an awfully wide span on which to
put a second floor.
You could potentially beef up the subfloor underneath the bathroom to
increase it's rigidity. Or you could add support columns underneath. But
otherwise, yeah, it's likely to end up with cracked grout.
I got these plans about a year ago (had to postpone our construction plans)
and I was immediately concerned about that span. I remember I looked it up
in the span tables and there is a grade of DF (not just No2 or better) that
will just support the 23ft span. But I'd rather not push this to its limit.
So I think I'm jgoing to do some redesign work, and break up the 23 foot
span with a beam and a post.
Just wanted to thank you guys for confirming this is a legitimate concern.
That seems vastly undersized to me, especially if you plan on tiling up
there (which your post seemed to indicate).
I built a garage a few years back that is 24' wide, giving a 23' free
span. I used 2x12's at 16" OC for the attic floor. The attic is only for
light storage, and I never plan on putting anything really heavy up
there. It works nice for the purpose, and gives me a completely open
Still, if I stand in the middle and bounce up and down slightly, I can
easily feel the flex in the floor. While it may be fine structurally, I
definitely wouldn't accept that much flex for a living area.
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