Drainage of a raised deck terrace - how does it work and how to do it correctly?

In a ventilated system, water:

1. flows off the surface of the pavers or boards
2. enters the gaps between the elements
3. permeates into the space beneath the terrace
4. is drained along the slope of the substrate to the drain

This solution eliminates the problem of standing water and increases the durability of the entire structure.

See also: What is a raised deck terrace

The most important element of the entire system.

Important!

• the slope should be at least 1.5–2%
• it must be created in the substrate layer, not on the surface

This is important because:

• the substrate is responsible for the direction of water flow
• the surface can remain perfectly level

→ See also: What slope should a terrace have for water to drain off? What percentage of slope is recommended for a terrace?

Rainwater gets under the terrace through the gaps between pavers or boards.

Standard gap widths between pavers are:

• 3 mm - for even pavers (gres tiles)
• 5 mm - for irregular pavers (stone, atypical)

The function of the gaps is:

• quick water drainage
• compensation of stresses caused by temperature changes in pavers or boards

See also: Gap or joint on an external terrace - how to choose the right width?

This is the space between the surface and the substrate (between the pedestals).

The function of the ventilation space is:

• allowing water to flow through
• ensuring air circulation
• limiting moisture

Thanks to the void under the terrace, the structure dries faster, which prevents moisture from accumulating.

1 - Airflow

As indicated by point

1 - air enters under the terrace structure and exits through the open joints (gaps) between the terrace pavers. Since the pavers are laid "dry" on the pedestals and are not grouted with mortar, these joints remain completely open, functioning as natural inlet and outlet channels for air.

2 - Air circulation

As shown by the arrows in point 2, an empty space (substructure) is created under the raised pavers. This free chamber allows the air that entered through the joints to circulate freely around and between the pedestals. The continuous air movement prevents heat buildup under the pavers and ensures that the entire underside of the terrace is constantly ventilated.

3 - Moisture limitation

Point 3 (also detailed in the box on the right side) shows that the combination of constant airflow and air circulation leads directly to effective moisture limitation. The frame in the graphic clearly highlights the benefits of this solution:

-Dries faster – the air movement causes moisture under the pavers to evaporate quickly.

-Water does not stand – rainwater does not accumulate under the structure, keeping the substructure dry, which prevents the formation of mold, fungi, and material deterioration.

The final element of the drainage system.

The most commonly used are:

• roof drains
• linear drains
• drainage systems

Drains must be located at the lowest point of the surface and must not be blocked by any structural elements.

Causes:

• formation of "point puddles"
• low terrace aesthetics

In a properly installed system:

• water does not pool on the surface
• it quickly disappears after rainfall

However, small amounts of water may appear:

• on the structural surfaces of the pavers
• during heavy rainfall

If water stands for a long time, it is a sign of an installation error.

See also: 5 reasons why water remains on the terrace after rain

Different materials affect the speed and method of water drainage:

• porcelain tile → fast runoff
• concrete → may retain water on the surface
• stone→ may "absorb" rainwater
• wood / WPC → water drains between the boards

See also: Types of surfaces used on raised deck terraces