Estimates
joist deflection.
Use to determine suitability of your floor for tile.
Joist material / condition
SYP / Douglas Fir, good condition
Unknown wood, good condition
Unknown wood, cracky or knotty
Joist spacing (inches on-center)
10"
12"
14"
16"
18"
19.2"
20"
22"
24"
Joist height (inches)
3.5" (2x4)
5.5" (2x6)
7.25" (2x8)
9.25" (2x10)
11.25" (2x12)
4"
4.5"
5"
6"
6.5"
7"
8"
8.5"
9"
10"
10.5"
11"
12"
12.5"
13"
13.5"
14"
14.5"
15"
15.5"
16"
Joist width (inches)
1"
1.5" (typical “two-by”)
2"
2.5"
3"
3.5"
4"
4.5"
Unsupported span (feet)
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
Design load (psf)
50 psf (legacy-style)
40 psf (common live load)
60 psf (conservative)
Simplified model; real loads vary by structure and code.
Calculate
Minimums:
L/360
for ceramic/porcelain,
L/720
for natural stone.
Note: This checks joist deflection only. Between-joist deflection depends on subfloor thickness/grade and underlayment.