How to Construct the Foundation for Your Pergola

May 6, 2024

For the sake of stability, a pergola requires a reliable foundation. This blog will explore the different types available and help you determine the most suitable one for your PERGOLUX Pergola.

Why is a sturdy foundation essential?

A proper foundation provides a stable, level base for your pergola, making assembly easier and ensuring its stability. Placing your pergola on uneven ground can lead to unbalanced pressure, potentially damaging screw connections, affecting drainage, and complicating accessory installation. Plus, nobody wants a crooked pergola ruining the aesthetic appeal.

Is a foundation mandatory?

The range of pergola sizes and weights often leads to confusion about the need for a foundation. It's wise to familiarize yourself with building regulations applicable to your pergola's location. Understanding these regulations is crucial, even if a building permit isn't required. Additionally, specific regulations may exist in allotment garden clubs, so it's best to consult the board for clarity.

In some cases, a foundation may not be needed, especially if your terrace already has a solid foundation. However, for optimal attachment, consider anchor bolts (expansion bolts).

Planning the foundation:

Evaluate the subsoil to determine its suitability for construction. Is it compact or loose? Are there roots that could pose a problem? Conducting a test excavation can provide clarity.

Consider utility lines for power and water connections. Integrate conduits during foundation construction, unless retrofitting is possible.

Determine the size of the foundation based on the dimensions of your PERGOLUX Pergola's floor plan. You can also request a foundation plan for your chosen model to streamline the process.

Types of foundations:

Depending on site conditions, various foundation options are available:

Slab foundation using path slabs for small pergolas. Point or strip foundations for larger models. Continuously poured concrete slabs for unstable ground. Screw foundations for stable ground.

Different foundation types:

Path slab foundation:

This simple option involves laying path plates, suitable for smaller pergolas.

Pavement tiles come in various dimensions, such as 30x30, 40x40, or 50x50 cm, with a thickness of several centimeters. However, they are not capable of withstanding heavy point loads as they might break under pressure. To ensure stability and straightness, a deep trench needs to be excavated, typically around 30 cm deep. This trench should then be filled with layers of crushed stone and gravel, each compacted before laying the paving stones. Finally, sand is used to fill the gaps between the tiles, ensuring a snug fit.

Pros: Quick construction, relatively low costs. Cons: Suitable only for lighter structures like pergolas.

Strip Foundation:

The strip foundation earns its name from the approximately 40cm wide concrete strips onto which the pergola is attached. When properly constructed, it can support even the largest pergola models, albeit requiring significant effort to set up.

To ensure the strip foundation's frost resistance, trenches need to be dug to a depth of at least 80 cm. Using a mini excavator can greatly facilitate this process. To prevent the trench walls from collapsing during construction, employing formwork boards to support them is advisable before pouring concrete into the trenches.

If the soil is unstable, an additional 10 cm thick concrete slab can be installed on compacted gravel and a PE foil to reinforce the foundation.

Pros: Very stable, frost-proof, requires relatively less concrete without a slab. Cons: Requires significant labor and construction machinery.

Concrete Slab Foundation:

A closed concrete slab provides an extremely stable base for the pergola. However, its construction is labor-intensive and comes with high costs. This option is typically reserved for unstable or loose soil conditions, or in swampy terrain near water where added stability is crucial.

This type of foundation is sometimes opted for even when not strictly necessary because it appears simpler than a strip or spot foundation. However, it demands large quantities of fresh concrete, often not feasible for DIY mixing.

Before pouring this foundation, preparatory work is essential. Initially, a pit 40-50cm deep must be dug to accommodate the pergola dimensions, with some allowance for the boarding. The ground should then be leveled and compacted, while sturdy formwork boards are necessary to support the walls due to the considerable weight involved.

Next comes a 15cm thick layer of gravel, requiring leveling and compaction with a vibrating plate. A thick PE film follows the gravel to shield the concrete from moisture below.

Only after these steps can the fresh concrete be poured, typically applied in two layers with steel mats (reinforcement) between for stabilization. Finally, the concrete is smoothed with a squeegee.

Pros: Maximum stability, particularly useful on unstable ground. Cons: Intensive labor, high costs, and essential need for construction machinery.

Point Foundation:

Requiring significantly less concrete than a concrete slab or strip foundation, a point foundation consists of multiple "punctiform" individual foundations placed beneath the columns of the pergola at regular intervals.

While a point foundation saves on material, it's only suitable for solid, load-bearing soil. Loose soil may cause the weight of the pergola to shift as the foundations aren't interconnected.

Despite material savings, precise measurement, excavation, and boarding of individual foundations are crucial. Using stable pipes with large diameters as casings is a helpful DIY tip for simplifying the process. Once prepared, fresh concrete is poured into the formwork or pipes, with the necessary amount often manageable with ready-to-mix concrete from hardware stores.

Pros: Relatively less material, hence lower costs. Cons: Not viable on loose ground, demands high accuracy in measurement.

Now that we've covered various foundation types, let's discuss which is suitable for a pergola with a slat roof.

For the PERGOLUX Pergola:

To achieve maximum wind resistance, we recommend using reinforced point foundations measuring 85x85x85cm, interconnected with a strip foundation. The pergola can then be securely fixed to the foundation using anchor bolts through the decking.

Alternatively, the pergola can be attached to smaller point foundations like 40x40x75cm or directly onto terrace coverings such as stone slabs or wooden floorboards. Many customers opt for this approach, and wind resistance depends somewhat on individual circumstances.