Putting a plastic bottle filled with water on your roof: 30+ countries use this unusual trick; here’s what it does and how to use it correctly

A plastic bottle filled with water might not look like much, but when placed through a roof, it can act as a simple daytime light source. The idea, known as the “Moser lamp”, uses sunlight to brighten dark indoor spaces without relying on electricity.

The low-cost technique was developed in Brazil and later became popular in communities across several countries. Its appeal is pretty straightforward: it uses readily available materials and can bring natural light into rooms with few windows or little direct sunlight.

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What is the Moser lamp and how did it start?

According to several reports, the idea is credited to Alfredo Moser, a Brazilian mechanic from Uberaba. He developed the concept in the early 2000s, when parts of Brazil were facing problems with electricity supplies.

The basic design involves filling a transparent plastic bottle with water and fitting it through a hole in the roof. The top portion remains outside, where it can receive sunlight, while the lower section extends into the room.

Instead of generating electricity, the bottle simply redirects daylight indoors. That makes it particularly useful in buildings where adding conventional windows or electrical lighting may not be practical.

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How did the plastic bottle roof lamp spread worldwide?

The concept gained wider international attention in 2011 when the MyShelter Foundation promoted the Liter of Light programme in the Philippines. The initiative used the bottle-and-water system as an inexpensive lighting solution that communities could build themselves.

According to figures cited in accounts of the programme, during its first 20 months, it reached more than 150,000 households in the Philippines and around 350,000 homes across 15 countries.

The idea subsequently spread further, with UNESCO references noting that the initiative had reached more than 30 countries.

Why is the water bottle lamp used in so many countries?

The technique became attractive because it does not require complicated equipment. A transparent PET bottle and water are relatively inexpensive and can often be sourced locally.

There are several reasons behind its appeal:

• It makes use of discarded plastic bottles.
• It can provide daylight without using electricity.
• It can brighten rooms with limited windows.
• The materials are relatively cheap and easy to find.
• Local communities can learn how to make and install the system.

It is worth noting that the lamp works during daylight hours, so it is not a replacement for electric lighting at night.

How does a bottle of water brighten a room?

The science behind the idea is fairly simple. When sunlight hits the water-filled bottle, light passes through the transparent plastic and water.

The water causes the light to refract and scatter, allowing it to spread across a larger area inside the room. Instead of sunlight entering through a tiny point and remaining concentrated, the bottle helps distribute the available light more widely.

That is why the method can be useful in rooms with opaque roofs, few windows or otherwise poor access to natural daylight.

Why do some versions contain bleach or chlorine?

Some versions of the Moser lamp use a small amount of bleach or chlorine in the water. The purpose is to slow the growth of algae and help keep the liquid clear for longer.

The exact installation method and materials can vary depending on the design and local conditions. Any chemical additive should be handled carefully, following appropriate safety guidance rather than being added casually.

How is the plastic bottle roof lamp installed?

Although the principle is simple, installing one involves making a permanent opening in a roof, so it is not quite as straightforward as putting a bottle on top of a building.

A typical installation involves a transparent PET bottle filled with water being partially fitted through an opening in the roof. The section outside the roof is positioned to receive sunlight, while the lower part directs light into the room.

For a proper installation, the roof structure should first be assessed and the opening should be made to fit the bottle securely. The area around the bottle then needs to be firmly sealed and waterproofed to prevent rainwater from entering.

Anyone considering the method should also check local building rules and, where necessary, get help from someone experienced with roof work.

What should you check before installing one?

The biggest concern is not the bottle itself but the roof modification. Cutting into a roof can affect its waterproofing and, depending on the structure, its integrity.

Before installing a bottle lamp, check the roof material and construction, make sure the opening is properly sealed, and look for leaks after installation. It is also important to consider whether the bottle is securely fixed and able to withstand local weather conditions.

So, while the Moser lamp is a clever example of using simple materials to solve a practical problem, proper installation matters just as much as the idea itself.


Inputs from agencies