The demand for terraces without a permanent foundation is growing every year. Many people do not want to pour concrete due to the cost, complexity of the work or the characteristics of the site. Others are simply looking for lighter, more environmentally friendly and quicker solutions. Composite decking offers great opportunities in this regard.
However, even the highest quality material will not save the situation if it is installed ‘directly on the grass.’ The foundation accounts for 80% of the structure’s durability. It is especially important to remember this when it comes to non-standard soil types: soft black soil, loose gravel, sloped areas, or areas with high humidity.
Why can't you lay joists directly on the ground?
This solution seems simple and cheap, but in practice it quickly leads to subsidence, loosening of fasteners and moisture accumulation. Without a ventilation gap, even WPC boards begin to lose their geometry, and the structure loses its stability.
Therefore, the first rule is: joists and boards should not come into contact with damp or loose soil, or with a moving base. There are modern solutions that allow you to do without concreting — and still create a technically correct terrace.
Option 1: compacted crushed stone and tiles
This is a basic, affordable and proven solution for summer cottages and private properties. First, the fertile layer of soil is removed, then geotextile is laid to prevent weeds from growing. Next, a layer of crushed stone (5–20 mm fraction) is compacted layer by layer. Concrete paving slabs are laid on the prepared surface in the places where the joists are to be installed.
Important: plastic adjustable supports cannot be placed directly on gravel. They need a stable, solid base — concrete, old screed or tiles.
This method gives good results provided that the surface is properly levelled, the distances between the supports are observed and ventilation is provided under the board.
Option 2: Adjustable supports – only on solid surfaces
If the site already has an old concrete screed, tiles or other solid covering, adjustable plastic or polymer supports can be used. They allow you to set the perfect level, even if the base has unevenness.
This solution is particularly suitable for terraces on roofs, balconies or in city courtyards. Installation is quick, and the system can be precisely adjusted using a laser. Combined with aluminium joists and concealed fastening systems, it is almost a work of art.
It is worth paying attention to ready-made installation systems designed with such conditions in mind. For example, Bruggan Expert is one of the systems adapted for installation on adjustable supports, with precise geometry and minimal tolerances.
Option 3: screw piles as an alternative to concrete
In areas with wet or loose soil, on slopes, or in cases where heavy objects (furniture, jacuzzi, canopy) are to be placed on the terrace, galvanised screw piles are the most reliable solution.
They are screwed into the ground to a depth of 80 to 150 cm, forming a point-like but very stable support structure. After installation, metal caps are placed on the piles, to which the joists are attached. This solution is not affected by frost or soil movement.
In addition, installation takes only 1–2 days and does not require any ‘wet’ work. This is especially convenient for sites where concreting is not possible for technical or weather reasons.
What about installing directly on grass?
Yes, such ‘terraces’ are sometimes made, but as a rule, these are temporary structures for one season. Grass growth, moisture and uneven subsidence make such a terrace completely unstable. Composite decking is a durable material, but even it will not save you if the joists lie directly on the ground.
If there is no other option, at least use geotextile, gravel and point supports under the joists.
A modern terrace without concrete is not a simplification, but an alternative. It is environmentally friendly, fast and technologically advanced. But the foundation of the structure must be designed with the same precision as the finish.
