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What is the acceptable deflection for a floor that
will be tiled?
Traditionally, the accepted minimum requirement for
floor rigidity is L/360 - before the tile underlayment
is installed. The L/360 standard means that the floor
should not deflect more than the "span" divided
by 360. If the span of the joists is 10 feet (between
supports), then the deflection should not be more than
1/3" between the center and the end. Frequently,
there is misunderstanding regarding deflection between
joists. For example, while joist manufacturers regularly
meet the standard L/360 criteria for code construction
with 24" o.c. (on center) systems, these floors
often have deflection between the joists exceeding L/360.
Recent research has shown tile to fail, under some
conditions, when the floor is more rigid than L/360
- in fact failures at L/600 have been observed. It is
for this reason that recommendations for floor rigidity
are not based on deflection measurements but on empirically
established methods found to work over normal code construction.
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