The Importance of Greenhouse Foundations

The foundation of a greenhouse plays a critical role in supporting the entire greenhouse structure.  Unstable foundations can lead to uneven settling or shifting, putting additional pressure on other parts of the greenhouse.  A strong and well-built foundation is essential for distributing the weight of the structure evenly.  Uneven bases can lead to structural issues such as cracks in the glass, misalignment of doors and even collapse in some cases.  You know the pictures people share of their greenhouse in a heap on the floor?  The main reason for this can usually be traced back to inadequate foundation preparation.

Why Foundations Matter

The foundation of a greenhouse serves multiple critical functions:

• Distributes the structural weight evenly across the ground
• Provides stability against wind and weather
• Prevents settling and shifting that can damage the greenhouse
• Ensures proper alignment of doors, windows, and panels
• Protects your investment in the greenhouse structure

Foundation Options

Recommended Base Types

1. Paved Slabs

• Provides excellent stability and load distribution
• Allows for proper drainage when installed correctly
• Durable and long-lasting solution

2. Concrete Base

• Offers maximum stability
• Ideal for permanent installations
• Provides consistent support across the entire footprint

3. Ring Bases

• Can be constructed from:
  • Pressure-treated timber or railway sleepers
  • Dwarf walls (brick or block)
  • Concrete perimeter
• Cost-effective while maintaining structural integrity

Level vs. Flat: Understanding the Difference

It’s also really important that you know the difference between a flat base and a LEVEL base.  We need a level base, greenhouses should not be installed onto a base with any fall.  The attached picture shows why – the base looked ‘flat’ – which it is.  But look at the gap where the greenhouse would be unsupported. Imagine that with a gust of wind on it.  Also, some contractors will try to build bases with a “fall” – basically a water run-off, but greenhouses need a level base – no fall, no run off.  Level.  Corner-to-corner level.

• A flat surface may still have slopes or variations in height, so it’s really important to check it with a spirit level, corner to corner.
• A level surface ensures the greenhouse sits perfectly horizontal.
• Any deviation from level can create stress points in the structure which will compromise your greenhouse.

The Fish Tank Analogy

Imagine you have a 6ft wide, 4ft deep, and 6ft tall fish tank sitting on an uneven, unlevel base. The tank would naturally rest on the highest point, leaving sections over the lower points unsupported.

Now, if you fill the tank with water, the immense weight would create pressure points on the unsupported areas. While this might hold up temporarily, over time, the strain could cause the glass to crack, making the tank unstable.  Now add some strong winds or storms…

Now, let’s say the base has a slope. The water inside the tank wouldn’t sit level; it would lean to one side. The same principle applies to a greenhouse – if the base isn’t level, rainwater will flow unevenly through the gutters. That’s fine if the slope directs water to your water butts at the back, but not ideal if they’re at the front and the water is pooling elsewhere.

Water Management

Proper leveling affects more than structural integrity:

• Some contractors mistakenly add slope (also called ‘a fall’) for drainage
• Sloped foundations cause uneven gutter performance
• Water collection may occur on the wrong side of the structure
• Level foundations allow proper operation of water collection systems

Installation Requirements

Size Considerations

Any base should extend the perimeter of your greenhouse.  Please remember that a greenhouse will always be bigger than advertised, by a few inches.  It’s important that you follow base plans provided by the supplier of the greenhouse.  For example – a 6×8 greenhouse will usually be around 6’3″ x 8’5″.

Construction Specifications

For different foundation types:

Paving Slabs:

• Must be perfectly level
• Requires proper substrate preparation
• No gaps between slabs

Brick/Block Perimeter:

• Requires appropriate footings
• Must be square and level
• Exact sizing to match greenhouse specifications

Soil Bases:

• Must be thoroughly compacted
• Cannot use freshly disturbed soil
• Requires precise leveling

Professional Assistance

While DIY installation is possible, seeking professional guidance can prevent costly mistakes. Consider consulting with greenhouse specialists for:

• Base design recommendations
• Site-specific foundation requirements
• Installation guidance
• Problem-solving existing issues

Remember that investing in a proper foundation is far less expensive than replacing a damaged or collapsed greenhouse. Take the time to ensure your foundation is correctly prepared before installation begins.