How to Create a Pergola Foundation

Jan 29, 2024

Why is a good foundation important?

A correct foundation provides a stable, level base for your pergola. This not only makes assembly easier but ensures the required stability of the pergola. If you install your pergola on a slope, it will be exposed to an imbalance in pressure that the pergola was not designed to withstand.

This may have consequences:

The pressure on the screw connections is too high, which can cause damage.
The integrated slope of the louvers may be negatively influenced, resulting in improper water drainage.
Accessories such as screens cannot be installed correctly.

Plus, it won’t look so good. And who wants that?

Is a foundation mandatory?

Pergolas are available in different sizes and weight classes. It is often not clear whether a foundation is absolutely necessary or whether, for example, compacted soil with a layer of gravel and terrace slabs on top is sufficient. In any case, it is always advisable to know the building regulations that apply to the planned construction site of the pergola.

In some cases, a foundation is not necessary. This would be the case if you have already built your terrace on a foundation and can thus ensure optimal attachment of the pergola, for example with anchor bolts (expansion bolts).

Plan the foundation

Check the subsoil
The condition of the subsoil plays an extremely important role and should be checked first. Is it pure, solid topsoil or loose, sandy soil? Is the subsoil possibly crisscrossed with roots that make it difficult to build the foundation?

To do this, simply dig in and around the area of construction to be sure!

Supply lines

Do you perhaps need power and water connections in your pergola for an outdoor kitchen, light, or heating, for example?

The empty pipes required for this should already be integrated during the construction of the foundation. Except for point and strip foundations, they can be retrofitted.
Consider whether you do this work yourself or whether you need to hire specialists.

Size

For concrete planning of the foundation, refer to the dimensions in the floor plan of your future PERGOLUX Pergola. In addition, it is possible to receive a foundation plan for the respective model in advance. In this way you can prepare the foundation in advance and do not have to store the pergola for a long time.

Foundation type

Depending on the given conditions, different foundation variants are possible.

A slab foundation made of path slabs is often sufficient for small pergolas.
For larger models, there should be point or strip foundations.
On unstable ground, a continuously closed concrete slab is recommended.
If the ground is stable, the screw foundation, which can be built relatively quickly, is an option.

Different Types of Foundations

The foundation of paving slabs

A foundation made of paving slabs is the simplest variant. It can be built with little effort and is sufficient for smaller pergolas.

Paving tiles are available in various sizes, e.g. 12in x 12in, 16in x 16in or 20in x 20in, and are several inches thick. However, they cannot withstand large loads and would simply break/crack. To ensure that the slabs are stable and straight, a deep pit must first be dug, 12in should be enough here. This pit is then filled with one layer of crushed stone and one with gravel. The individual layers should first be compacted, only then do the paving stones follow. The last step is filling the pit with sand so that the joints between the panels are tight.

Advantage: built quickly, relatively low costs
Disadvantage: only suitable for lighter pergolas

The strip foundation

The strip foundation is so called because 1ft – 1.5ft wide concrete strips are poured into the frame which the pergola can then be attached to. Properly built, it is sufficient for the largest pergola models, but requires a large amount of effort.

To ensure that the strip foundation is also frost-proof, the trenches must have a depth of at least 2.5ft - 3ft. A mini excavator would be extremely helpful here! To prevent the trench walls from collapsing, it is recommended to use formwork boards to support them before the trenches are poured with concrete.

If the subsoil is not stable enough, we recommend installing an additional 4in thick concrete slab - on compacted gravel and a PE foil (Polyethylene foils).

Advantage: very stable, frost-proof, relatively little concrete is required without a slab
Disadvantage: an enormous amount of work, construction machinery necessary

The concrete slab foundation

A closed concrete slab forms a complete and stable base for the pergola. However, the construction is quite labor-intensive and can be associated with high costs. This variant is only really necessary for unstable, loose soil, or in swampy terrain near water.

This type of foundation is also sometimes chosen where it is not absolutely necessary because the construction seems simpler than a strip or spot foundation. However, enormous quantities of fresh concrete are required here, which can be quite laborious to mix yourself.

Before you can pour this foundation, preliminary work is necessary. First, a 16in-20in deep pit must be dug in the dimensions of the pergola - with some allowance for the formwork boards. The ground must be straightened and compacted. The walls must be supported with sturdy formwork boards since several tons of weight are involved here.

This is followed by a 6in thick layer of gravel, which must also be straightened and compacted. A vibrating plate is essential here.

A thick PE (Polyethylene foils) film then follows the gravel to protect the concrete from below against moisture.

Only after these steps can the fresh concrete be poured. The concrete is poured in two layers. Steel mats (reinforcement) are placed between the layers for stabilization. Finally, the concrete is pulled smooth with the squeegee.

Advantage: maximum stability, very useful on unstable ground
Disadvantage: Enormous amount of work, high costs, construction machinery absolutely necessary

The point foundation

A point foundation requires significantly less concrete than a concrete slab or a strip foundation. It consists of several "punctiform" individual foundations strategically placed under the pergola's columns in consistent intervals.

You save a lot of material here, but this variant is only suitable on solid, load-bearing soil. If the soil is too loose, the weight of the pergola may shift the punctiform foundations as they are not connected.

Although one saves on material, great accuracy is required when measuring, digging out and boarding the individual foundations. Shuttering the individual foundations with shuttering boards is an effort that should not be underestimated. As a small "do it yourself tip" at this point, the use of stable pipes with correspondingly large diameters should be used as casings. Once everything has been prepared, fresh concrete is poured into the formwork or the pipes. The amount of concrete that is required can still be managed with ready-to-mix concrete from your local hardware store.

Advantage: comparatively little material, therefore correspondingly lower costs
Disadvantage: not possible on loose ground, very high accuracy required when measuring.

Now we have explained the different variants of the foundations. But what type of foundation is suitable for a pergola with a slat roof?

Our recommendation for the PERGOLUX Pergola

In order to achieve the maximum wind load, we recommend using reinforced point foundations measuring 33x33x33 inches and connecting them with a strip foundation. The pergola can then be fixed to the foundation through the decking with anchor bolts (expansion bolts).

Alternatively, the pergola can also be screwed onto smaller-sized point foundations such as 15x15x30inches or directly into terrace coverings such as stone slabs or wooden floorboards. A lot of customers do that too and the maximum wind load is somewhat dependent on your circumstances.

10x10ft PERGOLUX Pergola

3020mm= 118.8inches

850mm = 33.5in