Drainage System (Pergola and Terrace)

A drainage system is a set of structural elements that carry rainwater and condensation away from a pergola, terrace or sauna roof, clear of the structure and its foundations. In aluminium pergolas, a drainage channel carries rainwater from the closed louvres through the supporting columns directly into the ground or into a connection to the stormwater system. This keeps water from running down the facade or across the terrace floor, protecting surface materials from water ingress and long-term moisture stress. The system includes gutters integrated into the structure, downpipes routed inside the columns and, in some cases, rain sensors that close the louvres when it rains. A correctly sized drainage system is an essential part of the structure, not an optional add-on.

Advantages of the drainage system

• Protection of the facade and terrace from water ingress: Water is routed clear of the structure, which eliminates run-off down the walls and across the floor.
• Integration into the supporting columns: Downpipes routed inside the columns preserve a clean architectural appearance without visible pipework on the surface.
• Protection of the foundations from waterlogging: Controlled water discharge reduces the risk of long-term moisture damage to the surrounding ground and the foundation slab.
• Compatibility with rain sensors: The system can be linked to automatic louvre closing, which limits water ingress from the first drops of rain.

Terrace drainage: what to watch out for

Most problems with terrace drainage and pergola drainage do not arise in the design, but in the connection details and in maintenance. The overview below sets out the places where faults appear most often.

• Where the water ends up. The downpipe in the post is only part of the route – if the water ends up at the foot of the post on the terrace, the problem has simply moved a metre further along.
• The fall of the gutters. An integrated gutter has to fall towards the downpipe. On a structure altered after the fact this is sometimes forgotten and water stands in the profile.
• Access for cleaning. Leaves and pine needles collect in the gutter and in the downpipe inlet. A system that cannot be opened cannot be cleaned either.
• Winter operation. Water that stays in the downpipe and freezes can damage the profile. The answer is usually drainage laid to a fall with no dead spots.
• Connection to the drains or a soakaway. This is dealt with as part of the groundworks, not left until the pergola is installed.

Drainage and the rain sensor

On pergolas with rotating louvres the drainage only works when the louvres are closed. The rain sensor therefore forms a single unit with the drainage system – at the first drops it closes the louvres and the water reaches the gutter instead of falling under the pergola. Without a sensor that job falls to whoever is operating the pergola, which does not work while the owner is away.

Related terms: Bioclimatic louvres, EPDM membrane, Metal roofing, Siberian larch.