Gallons, and then pounds
The volume is easy: area times average depth. Four hundred square feet at an average of two inches is 400 × 2 ÷ 12 = 66.67 cubic feet. A cubic foot holds 7.4805 gallons, so that is 499 gallons. At 62.4 pounds a cubic foot it weighs 4,160 pounds, which is 10.4 pounds on every square foot of the deck.
Ten pounds per square foot does not sound alarming until you notice it is applied everywhere at once and stays for the whole hold. Two tons on a residential balcony is a different proposition from two tons on a slab on grade, and the difference is worth establishing before the hose comes out rather than during.
| Line | Default test | Arithmetic |
|---|---|---|
| Area | 400 sq ft | |
| Depth | 1 in to 3 in | average 2.000 in |
| Volume | 66.67 cu ft | 400 × 2 ÷ 12 |
| Gallons | 499 | 66.67 × 7.4805 |
| Weight | 4,160 lb | 66.67 × 62.4 |
| Load | 10.40 lb per sq ft | 62.4 × 2 ÷ 12 |
| Every inch of depth | 249 gal | 400 ÷ 12 × 7.4805 |
| Fill at 5 gal per min | 100 min | 1.66 hours |
| Evaporation over 24 h | 0.150 in | 37 gal, at 0.15 in a day |
| Drain at 20 gal per min | 25 min |
The level drops even when nothing is wrong
Open water outdoors evaporates. On the default numbers, a full day at 0.15 inches per day takes 0.150 inches off the test, which is 37 gallons and 7.5 percent of the average depth — but 15 percent of the inch at the high point, which is the part that goes dry first and the part someone will notice. Evaporation rates for open water vary by a factor of five or more between cool still air and hot dry wind, so the field is on the form rather than baked in.
The clean way to separate evaporation from a leak needs no arithmetic at all: put a bucket or a shallow pan of water on the same surface, in the same sun and the same wind, and mark both. Whatever the control loses, the test would have lost anyway. That trick is old, free, and settles arguments that a calculator cannot. Rain works the other way and is worse, because it can conceal a loss exactly equal to itself.
What this page will not say
It gives no hold time, no minimum depth, no dam height and no criterion for what counts as a pass. Those belong to the specification, the membrane manufacturer and whoever is inspecting, and they differ from job to job and product to product. It also passes no judgement on results: a test that holds is a test that held still water for a while, which is a useful thing to know and is not a statement about how the assembly will behave under wind-driven rain, thermal cycling and traffic.
For the material side of the assemblies that get tested, the shower take-off, the low-slope roof take-off and the below-grade take-off cover the three usual cases, and the wet mil audit is the check worth doing before the water goes on rather than after. If the concern is a deck holding weight rather than holding water, the hot tub deck load calculator works the same load question from the structural side.
Questions people ask
How many gallons does a flood test take?
Area times average depth divided by 12 gives cubic feet, times 7.4805 gives gallons. Four hundred square feet at an average of two inches is 66.67 cubic feet, which is 499 gallons. A useful figure to keep is that one inch of water over one square foot is 0.623 gallons, so an inch over 400 square feet is 249 gallons — and that is the number to divide a measured drop by when you are trying to work out how much has actually gone.
How much does the water in a flood test weigh?
Water weighs 62.4 pounds a cubic foot, so an inch of depth is 5.2 pounds on every square foot. The default test here — 400 square feet at an average of two inches — is 4,160 pounds, or 10.4 pounds per square foot, sitting there for the whole hold. On a slab that is nothing. On a balcony, a suspended deck or a roof it is a real load and worth checking with whoever is responsible for the structure before the test rather than after.
How long should a flood test be held?
This page does not say, and deliberately supplies no figure. Hold times come from the project specification, the membrane manufacturer, or the inspector with authority on the job, and they differ by assembly and by jurisdiction. What the page does is take whatever duration you enter and tell you what evaporation will do to the level over it, which is the practical problem with a long hold outdoors.
How do I tell evaporation from a leak?
Set a control. Put an open container of water on the same surface, in the same sun and wind, and mark the level in both at the same time. Whatever the control loses is what the test would have lost with no leak at all. On the default numbers, evaporation at 0.15 inches a day accounts for 0.150 inches over a 24 hour hold, which is 37 gallons — enough to look like a small leak and enough to hide one. Rain during the hold is worse, because it adds water and can conceal an equal loss.
What do I need to think about before flooding a deck?
Two things this page can only name. First, the weight: several thousand pounds applied to an assembly that may never have carried a load like it, for hours. Second, what is underneath, because a test that finds a leak finds it by delivering water into the space below, and that space may hold finishes, insulation, electrical equipment or people. Plan the release as well — several hundred gallons let go at once goes to the lowest point in the building, and it is worth deciding in advance where that is.