Can a roof terrace affect flat roof drainage?

A flat roof is not actually flat. It is designed with calculated drainage falls—typically between 1:80 and 1:40—that direct surface water towards a defined number of drainage outlets positioned to achieve the required hydraulic performance.

The hydraulic design is governed by BS EN 12056-3, which sets out the method for calculating drainage capacity based on rainfall intensity, roof area and fall geometry.

When a deck system is installed above the waterproof membrane, another element is introduced into that drainage environment. The interaction between the deck structure and the roof drainage design should therefore be considered as part of the overall roof terrace specification.

What a deck system does to drainage

Installing a deck above a flat roof membrane introduces three important considerations.

1. Drainage outlets

Drainage outlets remain critical to the long-term performance of the roof.

The deck layout should always take existing outlet locations into account so that water can continue to flow towards them as intended. Future maintenance requirements should also be considered during the design process, recognising that arrangements for accessing outlets beneath a completed deck will depend on the individual project, outlet locations and deck layout.

2. The drainage void

An adjustable pedestal system creates a level finished deck above a membrane that continues to follow its designed falls.

The space beneath the deck becomes a drainage void through which water flows towards the roof outlets.

If pedestal spacing is too tight, structural members interrupt the direction of flow, or pedestal bases create unnecessary restrictions, hydraulic resistance within that void can increase and drainage performance may be reduced.

This is why the geometry of the supporting substructure should be considered alongside the roof's drainage design rather than independently.

3. Ponding

Poorly designed deck layouts can also contribute to localised ponding beneath the deck.

Where components interrupt the natural flow path, or where construction tolerances create isolated low points, water may remain against the membrane for longer than intended.

Because this occurs beneath the finished deck surface, it is rarely visible during routine inspections. Over time, prolonged standing water can contribute to accelerated membrane deterioration and increase maintenance requirements.

The BS EN 12056-3 calculation

BS EN 12056-3 determines the drainage capacity required for a flat roof using rainfall intensity, roof area and available hydraulic gradient.

However, the standard does not specifically assess how an over-deck structure may influence the movement of water beneath the finished deck.

As a result, the roof drainage design and the terrace design are often considered separately, even though they ultimately need to work together.

What a non-penetrative* system designed with drainage in mind provides

BalcoDeck's non-penetrative* pedestal system is designed so that the roof membrane retains its original drainage falls beneath the finished deck.

The pedestal geometry allows water to flow freely around the substructure while maintaining a level finished surface above.

During the design process, the existing drainage layout and outlet positions are considered when determining the deck arrangement. Any requirements for future outlet access are assessed on a project-by-project basis and depend on the specific roof layout and outlet locations—they are not an inherent feature of every installation.

The objective is to ensure that the roof's existing drainage strategy continues to function as intended while accommodating the roof terrace above.

Questions architects should ask

Before specifying any roof terrace system, two questions should be considered:

• Has the proposed deck layout been checked against the existing drainage falls and outlet positions?
• Has the interaction between the deck substructure and the roof's drainage strategy been reviewed?

Considering these issues at design stage helps reduce the risk of creating drainage problems that may only become apparent long after the terrace has been completed.

What do we mean by "non-penetrative"?

A non-penetrative roof terrace system is one that does not drill, bolt or mechanically fix through the waterproof membrane or roof build-up. Instead, the system is engineered to achieve the required structural performance while leaving the waterproofing layer intact. BalcoDeck® is an example of a patented non-penetrative roof terrace system that uses a specially engineered substructure to support decking and balustrades without penetrating the membrane.