I have read how to install tile over plywood flooring at various
They say the maximum floor joist should be 16" on center with 'a
maximum deflection of 1/360 of the span, at midpoint.'
What does this mean? 1/360th of an inch movement (up & down) between
I saw this at Super-Tek.com
It means the floor joist is not suppose to bend more than 1 degree at
the mid point. The industry claims greater deflection will cause the
tiles/joints to crack if more than that. This can be true when you get
into larger tiles.
If deflection is the vertical movement of the floor joist span measured
at midpoint, there needs to be a set (known) weight to obtain this
A small child standing on the center of the joist will give a different
reading than an elephant.
I am sure not an expert on these matters. From what I can read in my
reference books the deflection factor is based on live load (as in a floor)
or dead load (as in a roof).
Children are permitted in residential construction. Elephants are not
I'm lazy or I would get the book and quote the "load per square foot"
Most building codes have the following limits for residental construction.
Total deflection 1/240 (for both dead and live load)
Live load deflection 1/360
The dead load is the mass of the floor and fixed structure.
The live load is what can come and go such as people, furniture, etc.
A typical building code is 40 lbs/sq ft under live load for residental
construction. For some other types of construction the live load requirement
can increase quite a bit. For example a library under current codes has a
maximum deflection of 1/360 with a live load of 150 lbs/sq ft..
I thought that this might boil-down to the usual fire code/building code
type of mumbo-jumbo...
So many people within so many square feet of space... bla,bla,bla
But still.... a 120 inch span (10') of a single floor joist would be
about 1/3rd inch up-and-down play... that seems (to me) like a heck of a
lot of movement... tile fooling or not!
But, I guess, that's withOUT adding the strength of the subflooring
that will tie the joist together.
Am I right or wrong?
Although what they're talking about is deflection, not bounce, it's
still a lot of movement. But that's why many builders work to
L/480 or higher. Like most of the rest of the code, L/360 is a
MINIMUM standard. If you build to just barely meet code is all
respects, you should end up with a crappy house that's livable,
and not excessively dangerous to you or your community.
Slightly wrong. the 1/360 deflection is for the floor as a whole.
But with that said, the live load is *everything* that is not part of the
structure itself. This includes such items as entertainment centers, furniture,
bookshelves, and of course people and animals. It is highly doubtful that
you'll ever encounter a floor that's moving 1/360 every day as people walk
about on it. But like another poster stated, the 1/360 is a minimum requirement
and a lot of builders build to 1/480 or even stiffer.
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