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The building's water bill jumped. How do you find the cause?

DoorKeeping team

4 min read

Test for a leak before you blame use or bill anyone. Read the master meter when no one is using water: if it moves, there is a leak. Then check toilets with a dye test, because toilets are the biggest indoor use and worn flappers are a common leak. Several states bar billing tenants for water lost to a leak, so fix first and bill after.

Step one: is the jump real use or a leak?

Pull the last 12 months of water bills and compare the same month in each year. Then ask what changed: a unit that came back from vacancy, a new tenant, a rate increase, a change in billing days, or nothing at all. Nothing at all points to a leak.

The EPA says the average household's leaks waste more than 9,300 gallons a year, and that nine percent of homes have leaks that waste 50 gallons or more per day. A building has more fixtures than a home, so small leaks multiply.

Step two: the meter test

The EPA's tip is simple: check the water meter before and after a two-hour period when no water is being used. If the meter changes at all, you probably have a leak. In a building, pick a time when units are quiet, ask tenants not to run water, and read the master meter. Then repeat with the main shutoff closed to a single unit if you have unit shutoffs, to narrow down where the water goes.

  1. Tell tenants in advance and pick a two-hour window.
  2. Read and photograph the master meter, including the leak indicator dial if it has one.
  3. Wait two hours with no water used.
  4. Read and photograph again. Any movement is a leak.
  5. Shut unit shutoffs one at a time and repeat to find the unit or the common line.

Step three: the toilets

Toilets account for nearly 30 percent of an average home's indoor water use, according to the EPA, and old ones can use as much as 6 gallons per flush. The EPA's dye test: put a few drops of food coloring in the toilet tank. If any color shows in the bowl after 10 minutes, the flapper leaks. Flush right away so the tank does not stain. The EPA suggests replacing a flapper at least every five years.

Replacing an old toilet with a WaterSense labeled model uses 1.28 gallons per flush or less, which the EPA says is 20 percent less than the current federal standard of 1.6 gallons per flush. For a family, the EPA estimates a 20 to 60 percent cut in the water used by toilets. Your building's savings depend on how many old toilets you have, so price the swap against your own bills.

Example a made-up fourplex pays $480 a month for water and sewer. A dye test finds two leaking flappers, and a $20 part each fixes them. If the next bill falls to $432, that is $48 a month, or $576 a year, saved for $40 of parts. The numbers are invented to show the arithmetic; yours will differ.

Who pays for water lost to a leak?

If you pass water costs on to tenants, a leak is a billing problem as well as a plumbing one.

  • North Carolina: in the hot-water method, a lessor shall not include water loss from leaks in the lessor's mains, and shall not bill for excess usage from a plumbing malfunction the tenant did not know about or that was reported to the lessor.
  • Massachusetts: the per-unit cost is divided over total consumption, and the landlord must verify that submetered usage does not exceed the building's total on the bill. Unexplained gaps between the master meter and the submeters are the first sign of a leak.
  • Oregon: a utility charge may include only the cost as billed to the landlord, and the tenant is billed for service to the tenant's unit or to a common area as described in the agreement.

The safe course is to fix the leak, estimate the normal bill from earlier months, and pass on only that. Keep your repair invoice and a note of the date the leak was fixed.

How do you catch the next one sooner?

  • Read the master meter the same day each month and log it.
  • Ask tenants to report drips and running toilets, and answer within a day.
  • Replace flappers on a five-year schedule at turnover.
  • Add a leak check to the move-out inspection.

Where DoorKeeping fits

DoorKeeping reads each water bill with AI and codes it to the property, so you can ask Claude or ChatGPT how this month compares to the last twelve. A tenant's leak report becomes a work order, and you can send the plumber the job by link. DoorKeeping does not read meters or detect leaks by itself. This is general information, not legal or tax advice. Check the current statute and ask a local attorney or your tax preparer.

Sources

  1. Fix a Leak Week and checking for leaks (EPA WaterSense), read Oct 31, 2026
  2. Residential toilets (EPA WaterSense), read Oct 31, 2026
  3. Statistics and facts (EPA WaterSense), read Oct 31, 2026
  4. N.C. Gen. Stat. 62-110(g) and (h), water, sewer and electric charges by lessors (North Carolina General Assembly), read Oct 31, 2026
  5. G.L. c.186, section 22, Definitions; submeter installation; testing; water use charges (Massachusetts Legislature), read Oct 31, 2026
  6. ORS 90.315, Utility or service payments; additional charges (Oregon Legislature, Chapter 90), read Oct 31, 2026

Questions

How do I know if the building has a water leak?
Read the master meter before and after two hours with no water used. If it changes, you probably have a leak, according to the EPA.
Can I bill tenants for a leak?
Some states bar it. North Carolina's hot-water method forbids billing for leak water loss in the lessor's mains and for unknown plumbing malfunctions. Check your statute.
Are WaterSense toilets worth replacing?
They use 1.28 gallons per flush or less. Whether replacement pays depends on how old your toilets are and what you pay per gallon; compare the cost to your own bills.

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