I have a small cabin 20x24. The floor joist system is comprised of
2x8x10 joists resting on outside wall and a center beam. The ends of
each joist are notched with the equivalent of a 2x4 tongue resting on the
walls and center beam.
Joists are spaced 24" o.c. for the most part. Subfloor is 3/4" plywood.
I would like to reduce/eliminate floor deflection sufficiently that I
will be able to install ceramic tiles with an appropriate backer board.
I have thought of installing additional 2x8x10 joists with all joists 12"
o.c. Unfortunately the availability of good 2x8's just is not the case.
I'm looking for options:
1. Instead of 2x8's positioned between existing joists, could I use
2. Could I sister 2x6's with existing 2x8's?
3. Would sistering 2x4's with 2x8's provide sufficient stiffening?
Answer to all probably, you might look at blocking to. Blocking between
joists also help to reduce deflection. If you can sister existing then that
would be the answer. All that you describe above would be better then what
you have now.
"You can lead them to LINUX
but you can\'t make them THINK"
W/ the likely exception of the 2x4 sistering option, I doubt it would
make much difference altho I didn't go do the calculations to confirm my
My initial and still over-powering reaction was what is so hard about
obtaining 2x8-12's altho they may be overkill.
What OP needs to do to get a guesstimate is to use one of the online
beam deflection routines and get a feel for the differential deflection
computed for the various options.
I also didn't do a calculation but I suspect the blocking option alone
won't make a real big difference either owing to the fairly short span
and relatively short vertical difference--it would be more effective for
2x10 and larger and longer spans. Will make some difference, true, but
again my hunch is not as much as might be desirable.
Is this over a crawl space? The joists are notched on the bottom edge
and reduced in depth from 7.25" to 3.5"? That's an excessively large
notch and it represents a potential failure point. Is there any
splitting of the joists at the inside corner of the notches?
One simple option, depending on clearances, would be add two more
girders (support beams), to reduce the joist span in half. But this
would only be a good option if you could avoid further notching of the
Actually, that's a most excellent idea, Wayne... :)
And, if it is crawl space, could simply pier it in a few places (every
other or every third, say) and not need to worry about the height issue.
Would need an adequate base for each pier, however, of course...
On Tue, 27 May 2008 17:12:22 +0000, Wayne Whitney wrote:
There is a full basement in the building. In fact, a wood foundation
I have not noticed any checking of notched ends.
There is some blocking midway between existing joists.
Building was built about 25 yrs ago. Most likely by a reader of Mother
Earth News who smoked more than he read.
As far as the notches go, if there is no splitting at the insidec
corner of the notches, then there is not currently an imminent
problem. But it is a poor condition you might wish to consider
As for stiffening the floor system, simplest and easiest would be to
interleave 2x8x10s with existing ones, ending up with 2x8s at 12"
o.c. and halving your deflections.
Interleaving 2x6s wouldn't work very well--a 2x8 is 3 times stiffer in
bending than a 2x6 of the same grade and species. So assuming equal
deflections of the two members, the 2x8s will take up 3/4 of the total
load, and you'll only reduce the deflection by 25%.
Wit care, you could sister a 2x4 or 2x6 to the 2x8s. First, you
should put them at the top edge or bottom edge of the existing joists.
This will be stiffer than centering them. Given that the existing
joists are notched on the bottom edge, you should choose the top edge
for the sisters.
Second, you would need to somewhat unload the existing joists before
attaching the sisters, using a temporary wall or jacks. Otherwise the
initial nail slip in the connection between the two members means that
you won't gain very much marginal stiffness in response to live load,
which is what you want to avoid tiles cracking. By unloading the
existing joists first, when you then reload the sisters with the dead
load, the initial nail slip should be taken up by the dead load, so
that for additional live load you will gain the full stiffness
Lastly, as another sometimes poster here (BobK) has convinced me, you
should use a construction adhesive between the new and old members,
and attach them together with many small nails. E.g. 0.131" x 3"
nails. This will allow the two members to work together as a
composite section more effectively.
Hope this helps.
On Tue, 27 May 2008 20:30:26 +0000, Wayne Whitney wrote:
Thank you for the ideas.
By the time I finish this project, methinks it would have been better to
raze and rebuild. There's nothing as frustrating as trying to undo and
improve what someone else has done.
I've been saying something like that quite often lately digging out my crawl
space and adding supports under the existing floor... It would have been so
easy to do this stuff during construction. Why didn't they leave room to
crawl in the crawl space? Arrrggg!!! (I wish I could magically lift up the
house for a couple of days...)
1) 10' isn't examply a "super long" span. You might actually run some
tests whereby you actually measure the deflection. You might consider
added another 1/2" to 3/4" of plywood on the top. This would give you
"bullet proof" support in the 24" between the joists.
2) This isn't "standard" but you might consider adding a 2x4/10 to the
bottom of your existing joists. This isn't "sistering" but actually
converting the bottom of the joist into a "half I-beam." The connection
between your added 2x4x10 and the existing joist has to be good. You might
consider "glue and screws." The 2x4x10 has to be good stuff without
significant knots. Just increasing the debth of the joists will reduce
deflection by more than 1/3 (the strength increase with the equal of the
height). With the extra wood, there might be even less deflection.
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