Look a little closer
See the mechanism
Two entrances, one downstream outlet
Read the diagram
Water reaching the side opening follows the hidden passage into a port beneath the stopper. A blockage after that junction can affect both routes.
A simplified basin section based on a documented conventional layout. No guaranteed overflow capacity is implied; other basins have tubing or no overflow.
Diagram basis: Lavatory overflow layout; Kohler: overflow-compatible drain.
Sources: United States patent specification, hosted by Google Patents · Kohler technical communication · Kohler Assist · Franke Home Solutions
The small hole is the entrance, not the whole system
An opening near the top of a basin is only the visible part of an overflow route. Behind it, water needs a passage to the waste system. In the lavatory design documented by US4218785A, a partition forms that hidden route, and openings in the drain tube admit overflow water below the main drain seat. Water is not supposed to disappear into the ceramic and stay there.
The side opening must connect to an outlet. In this basin layout, water travels down behind the bowl and enters ports in the drain fitting. The stopper sits above those ports, leaving the alternative entrance open even when the main opening is closed.
Sources: United States patent specification, hosted by Google Patents
Why water can leave while the plug is closed
A stopper closes the normal opening where water enters from the bottom of the bowl. It need not close the drain tube farther below. That distinction allows overflow water to enter through side ports beneath the stopper's sealing surface. The two paths share a downstream destination but have different entrances.
Imagine two doors leading into the same corridor, with one door shut. Closing the first does not close the second or the corridor itself. The overflow similarly joins on the outlet side of the stopper. A connection above that sealing point would be blocked by the same plug.
Sources: United States patent specification, hosted by Google Patents · Kohler technical communication
The height decides when the second route begins
During ordinary use, water below the overflow opening has no reason to enter it from the bowl. Once the level reaches that opening, the route becomes available. The small aperture therefore responds to water level without needing a float switch, battery or instruction from the faucet. Its position is part of the control mechanism.
That does not make it an unlimited drain. To hold a steady water level, the water leaving must match the water coming in. If the available outlet flow is smaller than the faucet's inflow, the level can keep rising. This follows directly from balancing water in and water out; no particular manufacturer's capacity is implied. The overflow buys an additional route, not permission to leave running water unattended.
Sources: United States patent specification, hosted by Google Patents
A shared drain means a shared limitation
A closed stopper and a blockage farther down are different problems. In a conventional shared-drain layout, overflow water bypasses the stopper but eventually enters the same waste pipe. An obstruction downstream of that junction can affect both routes. Adding a second entrance cannot remove a blockage in their common exit.
The visible overflow may be open while the system still has nowhere to send water. Conversely, a free main drain does not prove the hidden overflow passage is clear. The side passage and the main opening are separate upstream routes, with a shared outlet farther down.
Sources: United States patent specification, hosted by Google Patents · Kohler technical communication
The drain fitting has to belong to the basin
Kohler specifies drains with overflow, and its support documentation distinguishes overflow and non-overflow sinks. A listed spacer kit can cover an overflow-drain hole when used with certain non-overflow basins. Those examples establish that the openings in the fitting and the basin's actual passages must be considered together.
This is why seeing a side slot on a spare drain fitting is not enough to know how a sink will behave. The slot must communicate with the intended overflow route and be enclosed in the right place. A fitting can look correct at the top while its less visible geometry does not match. The precise selection and installation belong to the product's instructions; this explanation is a route map, not a substitute compatibility list.
Sources: Kohler Assist · Kohler technical communication
A covered opening can still be an opening
An overflow entrance can be disguised by a small trim piece. Franke describes a hygienic overflow whose logo plate can be removed to expose the aperture for cleaning. The design makes the entrance less visually prominent without eliminating the water path. It also shows why appearance alone is a poor guide to whether a basin has an overflow.
Other sinks have no overflow route at all. A decorative indentation or a slot in a spare drain fitting does not establish that the basin has one. Its own passage and compatible drain arrangement determine where water can go. Cleaning or removing trim requires the instructions for that particular basin.
Sources: Franke Home Solutions · Kohler Assist
Questions you might still have
Where does overflow water go?
In the illustrated conventional basin, a hidden passage leads into side ports in the waste fitting beneath the stopper's sealing surface.
Sources: United States patent specification, hosted by Google Patents
Can an overflow prevent every sink flood?
No. Its passage must be clear and its drainage must keep pace with inflow. A blockage in the shared downstream waste can also affect it.
Sources: United States patent specification, hosted by Google Patents
Does every sink have an overflow?
No. Kohler's support information distinguishes basins with and without an overflow, which also affects the compatible drain arrangement.
Sources: Kohler Assist
The thing to remember
An overflow is a bypass around the closed stopper, usually not a separate escape from a blocked waste pipe.
What this example leaves out
Scope: conventional basins with a connected overflow passage. Some sinks have external overflow tubing or no overflow. No capacity measurement, fitting recommendation or guarantee against flooding is claimed.
Sources & further reading
Source access dates are listed below. Manufacturer descriptions and patents support the specific constructions described, not every product.
- US4218785A — Drain assembly for a lavatory basin ↗
United States patent specification, hosted by Google Patents · Retrieved 2026-10-14 · Basin overflow opening connects through a partitioned passage to side openings below the main drain seat; stopper closes the main seat.
Specific disclosed geometry used for the explanatory schematic, not every sink. English specification retrieved at the available /de document URL. - K-7119T — Clicker drain with overflow ↗
Kohler technical communication · Retrieved 2026-10-14 · Manufacturer specifies an overflow-compatible drain and opening/closing function for sink drainage.
Product specification establishes compatibility, not a tested overflow capacity. - Drain Spacer Gasket Kit For Bathroom Sinks ↗
Kohler Assist · Retrieved 2026-10-14 · Overflow and non-overflow basins differ; a manufacturer's spacer can cover an overflow-drain hole for specified non-overflow sinks.
Compatibility examples only. Installation directions are not reproduced as universal plumbing advice. - FAQ — How do I clean the overflow? ↗
Franke Home Solutions · Retrieved 2026-10-14 · Some sinks use a removable logo plate over an overflow aperture so the opening remains accessible for cleaning.
Franke design example, not proof that every decorative plate is removable or every kitchen sink has an overflow. Exact cleaning methods not reproduced.
