Should a Greenhouse Be Watertight?
Should a Greenhouse Be Watertight? (And Why You Shouldn't Seal It)
One of the most common questions we receive from customers regarding our greenhouses for sale is: "Is my greenhouse supposed to be completely watertight?"
The short answer is: No.
A greenhouse should never be sealed like a fish tank or a conservatory. While it needs to be water-resistant (keeping the majority of rain off your plants), a greenhouse must be able to "breathe."
If you are seeing some water ingress, or if you are tempted to reach for a tube of sealant, please read this guide first.
The Golden Rule: Water-Resistant, Not Watertight
There is a vital difference between these two concepts:
- Water-Resistant: The structure effectively sheds 95-98% of rainwater away from your crops while allowing air to circulate.
- Watertight: A hermetically sealed box with zero air exchange.
If a greenhouse were truly watertight, it would be disastrous for your plants. Humidity levels would skyrocket, condensation would drip constantly, and fungal diseases like Botrytis (grey mould) and powdery mildew would destroy your crops.
Why We Do NOT Recommend Silicone Sealant
At Greenhouse Stores, we advise strictly against using silicone sealant on your greenhouse.
While it might seem like a quick fix for a drip, using silicone creates long-term problems:
- The Humidity Trap: Silicone seals the natural, necessary gaps in the frame. This traps excessive moisture inside, leading to severe condensation issues—especially in autumn and winter when you might be using a heater.
- Maintenance Nightmares: Glass breaks and seals perish. If you have glued your glass in place with silicone, replacing a pane or changing a rubber seal becomes incredibly difficult, often resulting in damage to the aluminium frame during removal.
Do Not Seal the Base Plinth
Another common mistake is sealing around the bottom of the greenhouse where the base plinth meets the ground (concrete, slabs, or perimeter).
You must leave the base unsealed. Here is why:
- Internal Drainage: When you water your plants or wash down the greenhouse, that excess water needs somewhere to go. If the base is sealed, you create a swimming pool inside your greenhouse.
- Natural Run-off: Ideally, your greenhouse base should have a slight "fall" or slope to allow water to run off naturally. Sealing the base blocks this escape route, causing water to pool and stagnate, which promotes algae growth and rot.
What is "Normal" Water Entry?
Before you worry about leaks, it is important to know what is considered normal for a horticultural structure:
- Condensation: In spring and autumn, temperature differences cause condensation to form on the glass. This often drips and is mistaken for a leak.
- Storm Ingress: During heavy storms with strong winds, rain can sometimes be driven through roof vents or door gaps.
- Capillary Action: On traditional greenhouses with overlapping glass, small amounts of water can sometimes seep between the panes.
When to Take Action (And How to Do It Correctly)
If you have persistent streams of water running down walls or dripping from roof bars in calm weather, you may have a mechanical issue. Do not reach for the silicone! Instead, check these maintenance points:
- Check the Rubber Seals: The rubber gaskets between the glass and frame can perish after 15-20 years. If they feel brittle or hard, they need replacing with fresh rubber strips.
- Tighten Clips: Ensure your glazing clips (W-clips or Z-clips) are tight and the glass is seated squarely.
- Clear the Gutters: Blocked gutters can cause water to overflow back into the structure.
Summary
Your greenhouse is designed to start seedlings and protect plants from frost, not to be a waterproof living space. By allowing your greenhouse to breathe—and avoiding the use of sealants on the frame and base—you ensure a healthier, disease-free environment for your plants.
Need spare parts or replacement rubber seals? Browse our greenhouse accessories range or contact our team for advice.
