The paper has a very basic purpose.

It creates a physical layer between the drain opening and the surrounding air.

Depending on how it is positioned, it may also make it more difficult for small insects to move through the opening.

The paper itself isn’t a special plumbing material.

It isn’t absorbing dangerous gases or performing a chemical reaction.

It’s simply being used as a temporary barrier.

The upside-down glass then keeps the paper in place.

This explains why the method looks unusual but doesn’t require any special equipment.

What Is the Glass Supposed to Do?

The glass provides weight.

Without it, a lightweight piece of paper could easily move because of air movement, household activity, or accidental contact.

Putting a glass upside down over the paper helps keep the paper pressed against the sink surface.

The glass also creates a small enclosed space above the paper.

Again, however, this should not be confused with creating a professional airtight seal.

A normal drinking glass placed upside down on a sheet of paper is not designed to make plumbing connections airtight.

It’s simply a simple household barrier.

What About Sewer Odors?

This is one of the main reasons people become concerned about unused drains.

Drainage systems can contain unpleasant-smelling gases.

A properly functioning plumbing system is designed to prevent these gases from entering living spaces through fixtures.

The water inside a P-trap is one of the important defenses against this happening.

If the trap dries out, that defense can be lost.

You may then notice a smell that resembles sewage, rotten eggs, or a generally unpleasant drainage odor.

If you return home and notice a strong sewer-like smell from a drain, running water through the fixture may restore the trap if evaporation was the only problem.

But persistent sewage odors should not automatically be blamed on evaporation.

There can be other plumbing problems, including damaged traps, blocked vents, leaks, or defective connections.

Could Bugs Come Through the Drain?

Another reason people may cover drains is to reduce the possibility of insects entering through plumbing openings.

Certain small insects, including drain flies, can be associated with organic buildup in drains and plumbing systems.

Keeping drains clean and maintaining the plumbing system is more important than simply placing a glass over the drain.

A temporary cover can provide an additional physical barrier, but it doesn’t eliminate the conditions that allow insects to reproduce in organic material inside a drain.

If you repeatedly see drain flies, cleaning the drain and addressing the source is more useful than relying exclusively on a cover.

Is This Necessary for Every Sink?

Not necessarily.

A sink that is used regularly receives water frequently, so its trap is continually replenished.

A sink that hasn’t been used for several weeks is a different situation.

The risk of trap evaporation depends on factors such as:

How long the fixture remains unused
Temperature
Humidity
Air movement
The size and configuration of the trap
Whether the fixture is connected to other plumbing
The condition of the plumbing system

For a short weekend trip, there may be little reason to worry about a normally functioning kitchen sink.

For a home that will remain empty for weeks or months, plumbing preparation becomes more relevant.

There Is a Simpler Solution for the Trap

One of the simplest things homeowners can do before leaving for an extended period is to make sure the traps contain water.

Run water through sinks, showers, tubs, and other fixtures shortly before leaving.

This ensures that the traps begin with water in them.

For fixtures that are rarely used, this can be particularly helpful.

Some homeowners also use methods recommended by plumbers to slow evaporation from traps during extended vacancies, but the appropriate approach depends on the fixture and plumbing system.

If you’re leaving a property vacant for a long time, especially during hot weather, asking a qualified plumber about appropriate preparation can be worthwhile.