Substructure for terraces

A terrace substructure is necessary to balance out any unevenness on the subsurface and to ensure a stable fixture for the planks. Terraces can be installed on many different types of subsurface, such as existing terrace tiles or flagstones, a foundation or even on stilts. This type of installation is used if the terrace is well above the level of the garden.

Depending on different requirements, there are various substructures available for terraces.

Requirements for the subsurface

The subsurface and the substructure of a terrace have to fulfil several requirements so that the terrace can be installed professionally.

Most important is that both are sufficiently strong to bear the weight of the floor covering as well as people and furniture on the terrace without any problems. The substructure must also be protected against rotting and insects. Another important aspect is protection against frost, wind and moderate tremors – the subsurface and the substructure should be sturdy enough to withstand conditions such as these. A suitable drain must be provided at all times to ensure adequate drainage and prevent water logging.

A distinction is made between solid and floating subsurfaces. Solid subsurfaces include existing surfaces like an existing terrace or a new concrete foundation. Floating subsurfaces consist of gravel or ballast, concrete pillars or, with roof terraces, a slab.

Different types of substructure

Distinctions between different types of substructures are similar to the differences in subsurfaces: here, too, we differentiate between floating and solid installations. Solid substructures are screwed firmly to the subsurface – but floating substructures cannot be screwed to the subsurface.


Design of the substructure on a solid subsurface

Solid substructures can be split into those that sit on even, full supports and those that are installed on uneven supports that do not cover the full area of the substructure. For example, this includes concrete or installing the terrace on top of an existing terrace. To even out the levels on concrete, we recommend cement, packing plates or packing pieces as spacers.



Design of a floating substructure

With floating substructures the subsurface can be solid or consist of a sufficiently stable, compressed layer of material, such as gravel, ballast, aggregate, etc. A frame is used to form this subsurface. The level of a floating installation on top of a solid subsurface, such as wooden beams, a concrete foundation, steel posts with steel supports or a concrete strip footing, can be balanced out by the substructure itself or by adjustable supports. If the subsurface is gravel, ballast, or aggregate, the level is balanced out with standpipes filled with concrete, concrete slabs or post supports concreted in place.


Special features of a roof terrace

A roof terrace is also supported with a floating substructure. The special feature with a roof terrace is that wind loads also have to be absorbed in addition to the bearing loads. Because of this, additional concrete slab anchors may have to be installed as loading. The level can be balanced out with concrete slabs or bags filled with cement.



Installing a substructure

When you have determined what type of subsurface your terrace has and which substructure you need for it, the next step is to install the substructure. To make the choice easier for you, we've listed the most important selection criteria in a descriptive table.

Selection aid for the perfect substructure for your roof terrace

If you choose one of the innovative Twinson terrace systems, click here to find useful tips and specific instructions for planning, design and installation.