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.

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 2Square Feet 3Square Feet 4Square Feet 5Square Feet 10Square Feet 20Square Feet 30Square Feet 40Square Feet 50Square Feet 100Square Feet 500Square Feet 1000Square Feet 0.25 Inch 1.30lbs. 2.60lbs. 3.90lbs. 5.20lbs. 6.50lbs. 13.01lbs. 26.01lbs. 39.02lbs. 52.02lbs. 65.03lbs. 130.06lbs. 650.29lbs 1,300.57lbs. 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