Main Meter vs Sub-Meters: How to Share the Water Difference

In almost every condominium the total consumption recorded on the individual sub-meters is lower than what the main meter registered in the condominium's name. This difference, often called condominium water or network gap, is not a calculation error but the result of normal technical causes: hidden leaks, measurement tolerances, uncounted common draw-offs and uncalibrated meters. Since the condominium pays for it anyway to the utility, it must be shared among the owners with a clear criterion approved by the owners' meeting.

Why the sub-meter totals do not add up

The main meter measures all the water entering the building; the sub-meters measure only what passes through each dwelling. Several items open up between the two values and explain the gap:

• Hidden leaks on the common pipework, which consume water without passing through any sub-meter.
• Uncounted common draw-offs, such as garden irrigation, stair cleaning, garage washing or taps in common rooms without a sub-meter.
• Metrological tolerances: every meter has an allowed margin of error, and sub-meters tend to under-record small flows (drips, low pressure).
• Old or uncalibrated meters, which lose sensitivity over the years.
• Non-simultaneous readings: if the sub-meters are read on different days from the main meter, the periods do not perfectly coincide.

A modest gap, in the order of a few percentage points, is entirely normal. A high and growing difference, on the other hand, is a warning sign: there is very probably a leak on the common network to find and repair.

Who decides the sharing criterion

The criterion for dividing the difference lies with the contractual condominium regulation, if it provides for it, or with a resolution of the owners' meeting. There is no formula imposed by law for the gap: Article 1123 of the Italian Civil Code sets the general principles, that is, expenses by thousandths (millesimi) or in proportion to use, but the operational choice is left to the condominium's autonomy, provided it is reasonable and not arbitrary.

It is good practice to set the criterion in writing at the meeting and keep it constant over the years, so as to make the balancing charges predictable and hard to challenge. Changing method every financial year generates confusion and distrust.

The most common sharing criteria

In practice three approaches are mainly adopted, each with strengths and limits:

1. In proportion to recorded consumption: the difference is divided among the units based on the cubic metres read on the sub-meters. It rewards low consumers and links the gap to real use, but places part of the losses on heavy consumers who are not at fault.
2. By ownership thousandths (millesimi): the difference follows the millesimi tables. It is simple and stable, consistent with the common nature of network losses, but detached from actual consumption.
3. By fixed quota per unit: the difference is divided equally among the dwellings. Easy to apply, but it penalises small or vacant units.

Many condominiums choose consumption-proportional sharing because it brings the gap closer to use, but the millesimi solution is often more defensible when the difference clearly stems from losses in the common parts. What matters is that the choice is deliberate and minuted.

Normal difference or a leak to hunt down?

Before sharing, the manager should assess the nature of the gap. If the difference is stable over time and contained, it can be considered normal and shared without further action. If instead it grows year after year or clearly exceeds historical values, it should not simply be spread over the owners: it means the building is losing water and the priority becomes fault-tracing. Sharing a large loss without hunting for it exposes owners to recurring, unjustified costs.

Documenting to avoid disputes

The difference is one of the most contested items in the water statement. To prevent disputes the manager should keep and be able to show:

• The readings of the main meter and of all sub-meters, with the dates of collection.
• The calculation of the gap and its percentage against total consumption.
• The resolved sharing criterion and the reference to the minutes that approved it.
• The history of previous financial years, to demonstrate the consistency of the method.

With AmministraPro the manager enters the readings of the main meter and sub-meters, automatically calculates the difference and shares it with the chosen criterion, generating a transparent statement and storing the history year by year. The water accounting features are shown on the /funzioni page, while the available plans are on the /prezzi page.