The following table provides the weight of water per inch in pounds for various areas/square footages. This should allow you to quickly and accurately estimate the weight of any amount of standing water on your low slope roof. One cubic inch of water weighs 0.036127 of a pound. One square foot of water one inch deep weighs 144 times this, or 5.202288 pounds. Everything else has been calculated from that.

Chronic ponding water on this roof has contributed to permanent deck deflection and discoloration of the white roof membrane.

This table should give you an idea of how much of a live load is being added to your roof by areas of ponding water or clogged drains.

Also see Roof Online’s Roof Drainage page.

Values for areas or depths not covered in the table should be easy to arrive at simply by adding together values for areas or depths which are listed.

Weight of Water per Inch per Square Foot in Pounds
Area in Square Feet
Depth of Water in Inches 1
Square Foot
2
Square Feet
3
Square Feet
4
Square Feet
5
Square Feet
10
Square Feet
20
Square Feet
30
Square Feet
40
Square Feet
50
Square Feet
100
Square Feet
500
Square Feet
1000
Square Feet
0.25 Inch 1.30
lbs.
2.60
lbs.
3.90
lbs.
5.20
lbs.
6.50
lbs.
13.01
lbs.
26.01
lbs.
39.02
lbs.
52.02
lbs.
65.03
lbs.
130.06
lbs.
650.29
lbs
1,300.57
lbs.
0.5 Inch 2.60 5.20 7.80 10.41 13.01 26.01 52.02 78.03 104.05 130.06 260.11 1,300.57 2,601.14
1 Inch 5.20 10.41 15.61 20.81 26.01 52.02 104.05 156.07 208.09 260.11 520.23 2,601.14 5,202.29
2 Inches 10.41 20.81 31.21 41.62 52.02 104.05 208.09 312.14 416.18 520.23 1,040.46 5,202.29 10,404.58
3 Inches 15.61 31.21 46.82 62.43 78.03 156.07 312.14 468.21 624.27 780.34 1,560.69 7,803.43 15,606.86
4 Inches 20.81 41.62 62.43 83.24 104.05 208.09 416.18 624.27 832.37 1,040.46 2,080.92 10,404.58 20,809.15
5 Inches 26.01 52.02 78.03 104.05 130.06 260.11 520.23 780.34 1,040.46 1,300.57 2,601.14 13,005.72 26,011.44
6 Inches 31.21 62.43 93.64 124.86 156.07 312.14 624.28 936.41 1,248.55 1,560.69 3,121.37 15,606.86 31,213.73
7 Inches 36.42 72.83 109.25 145.66 182.08 364.16 728.32 1,092.48 1,456.64 1,820.80 3,641.60 18,208.01 36,416.02
8 Inches 41.62 83.24 124.86 166.47 208.09 416.18 832.37 1,248.55 1,664.73 2,080.92 4,161.83 20,809.15 41,618.30
9 Inches 46.82 93.64 140.46 187.28 234.10 468.21 936.41 1,404.62 1,872.82 2,341.03 4,682.06 23,410.30 46,820.59
10 Inches 52.02 104.05 156.07 208.09 260.11 520.23 1,040.46 1,560.69 2,080.92 2,601.14 5,202.29 26,011.44 52,022.88
11 Inches 57.23 114.45 171.68 228.90 286.13 572.25 1,144.50 1,716.76 2,289.01 2,861.26 5,722.52 28,612.58 57,225.17
12 Inches 62.43 124.86 187.28 249.71 312.14 624.28 1,248.55 1,873.04 2,497.10 3,121.37 6,242.75 31,213.73 62,427.46
24 Inches 124.86 249.71 374.57 499.42 624.28 1,248.55 2,497.10 3,745.65 4,994.20 6,242.75 12,485.49 62,427.46 124,854.91
36 Inches 187.28 374.57 561.85 749.13 936.41 1,872.82 3,745.65 5,618.47 7,491.29 9,364.12 18,728.24 93,641.18 187,282.37
48 Inches 249.71 499.42 749.13 998.84 1,248.55 2,497.10 4,994.20 7,491.29 9,988.39 12,485.49 24,970.98 124,854.91 249,709.82
60 Inches 312.14 624.28 936.41 1,248.55 1,560.69 3,121.37 6,242.75 9,364.12 12,485.49 15,606.86 31,213.73 156,068.64 312,137.28