Is a Greenhouse Supposed to Be Watertight?

Let’s deal with the short answer first:  No, your greenhouse is not designed, nor intended to be watertight.  A greenhouse is built to be weather-resistant rather than waterproof, and a small amount of water entering the structure is completely normal.

Why Does Water Get Into a Greenhouse?

Even high-quality aluminium greenhouses are designed to shed most rainfall, but not all of it.  A greenhouse is not intended to be completely watertight, and a small amount of moisture entering the structure is entirely normal.  The glazing system is designed with slight tolerances that allow air to move through the joints, helping to reduce condensation and maintain a healthier growing environment.  During periods of heavy or wind-driven rain, you may notice the occasional drip where panes meet.  This is expected and not usually a cause for concern.  Other forms of water ingress include condensation, water entering through vents during wind-driven rain, water ingress at eaves (this could be caused by a blocked gutter, so ensure that you check them regularly, especially during autumn/winter when leaves are falling).  In most cases, this is expected behaviour and not a fault.

These characteristics are part of how a greenhouse functions.  Air exchange is essential for regulating temperature and humidity, and completely sealing every joint can actually create more problems than it solves.  Trapping moisture inside can lead to persistently high humidity levels, encouraging mould, mildew, and plant disease, while also making the internal environment harder to control.

Why Greenhouses Are Not Fully Sealed

Greenhouses are designed to “breathe.” Small gaps in the glazing system allow for essential airflow, which helps to regulate temperature, reduce humidity, prevent mould and mildew.   Sealing a greenhouse completely can trap moisture inside, leading to poor growing conditions and increased risk of plant disease.

Although it might seem like a quick fix, using silicone sealant around glazing joints is not recommended.  While it may temporarily stop minor drips, it interferes with the natural movement of the structure and prevents the greenhouse from breathing.  Over time, this can lead to excess moisture build-up and can also make future maintenance, such as replacing broken panes, far more difficult and time-consuming.

If water ingress appears excessive or localised, such as a steady drip from one point or water tracking down a panel, it’s usually a sign that something needs attention.  In most cases, the issue can be resolved by checking that panes are correctly positioned, ensuring glazing clips are secure, and replacing any worn or perished rubber seals.

It’s also important not to seal the base of the greenhouse to the ground.  The small gap at the base is normal and allows excess water to drain away.  Blocking this can lead to standing water inside the structure, which can cause algae build-up.  While it might stop rainwater getting in through the base – it also stops water from watering your plants from getting out.

Your greenhouse should be thought of as weather-resistant rather than waterproof.  A small amount of moisture entering under certain conditions is part of normal operation and helps the structure function as intended.

Should You Use Silicone Sealant on a Greenhouse?

Although some customers choose to use it, it is not considered best practice or something we recommend.  The majority of rain that gets into a greenhouse is coming in during high winds and/or driving rain.  Water collection can also form via condensation.  All of these are completely normal and expected.  We do not recommend the use of silicone to seal your greenhouse and the best thing to remember is that although it’s annoying when rain may get in during rainy conditions, it’s also helping ventilation and humidity through the hotter conditions.   Unbelievable as it sounds, glass does contract and expand with the temperature – if you silicone glass, it will restrict natural expansion and contraction of the glass and could potentially crack the glass.  You also need to ensure that any warranty of the greenhouse does not become void with the use of silicone on either the frame or the glass.  We do not recommend using silicone sealant on your greenhouse.  While it may appear to fix small leaks, it can create bigger issues over time.  Also bear in mind, that any glass that gets broken in the future, will be very difficult to remove and replace if it has been siliconed to the frame.

When Is a Greenhouse Leak a Problem?

A small amount of water is normal, but you should investigate if you notice a consistent drip from one specific point, water entering during light rain or water pooling inside the greenhouse.  These are often caused by worn or perished rubber seals, loose or missing glazing clips or misaligned glass panels.  In most cases, simple maintenance will resolve the issue.

When to replace Gasket/Neoprene/Mastic on a Greenhouse

Rubber seals, neoprene and mastic naturally wear over time.  In all cases, once they are perished, they will need to be replaced to maintain an efficient seal.  However, they are designed to last many years before this happens.  Greenhouse seals tend to go hard and brittle when they are ready for replacement, or they can crack or shrink.   You may notice that one side of the greenhouse perishes before the rest, this is usually down to factors such as sun exposure.

Should You Seal the Base of a Greenhouse?

No.  The base should remain unsealed.

The gap at the base allows excess water to drain away. Sealing it can lead to standing water inside the greenhouse, algae and mould growth and long-term damage to the frame.  A paved base is ideal, as it provides natural drainage, while concrete bases should include a slight slope to allow water runoff.

How to Reduce Condensation in Your Greenhouse

Condensation is one of the main sources of moisture inside a greenhouse.  The most effective solution is good ventilation, particularly roof vents, louvre vents and adding automatic openers to your greenhouse.  Automatic openers are especially useful, as they regulate airflow based on temperature and reduce the need for manual adjustment.  If you are unsure how these work, read our blog here:  “How Do Greenhouse Automatic Vent Openers Work?”

Toughened Glass vs Horticultural Glass – Which Is Better for Water Resistance?

Toughened glass offers better resistance to water ingress.  This is because it uses larger, full-length panes.  Each pane is sealed along the edges, leaving fewer joints where water can enter.   Horticultural glass, on the other hand, uses overlapping panes, which naturally allow small amounts of moisture through the joins.