Drainage and structural engineering: functional management of rainfall

The drainage system for a DESIGA^® garden shed is designed to preserve the building’s visual integrity whilst maximising its structural integrity. This involves a range of design approaches, which are determined by the size of the building.

Slope design and structural analysis: two design approaches depending on the building depth

As standard, a one-sided slope of approx. 2 cm is incorporated into the roof structure. This slope is not visible from the outside, as it is concealed by the surrounding roof upstand. It ensures that moisture is directed specifically towards the side with the drainage point without compromising the horizontal lines. This solution works reliably and requires little maintenance for standard dimensions – the water runs where it is supposed to.

For projects with a building depth of more than 3 metres, the structural priorities change. To incorporate a slope into the solid timber beams, they would need to be notched or cut at an angle. With large spans, this would weaken the load-bearing capacity of the timber – a compromise we are not prepared to make. In such cases, we opt for a flat roof structure that retains the full cross-sectional thickness of the beams. Spans of up to 5 metres can thus be safely achieved without sacrificing structural reserves.

Should standing water occur in isolated spots due to the flat construction, this is technically harmless. Unlike bitumen membranes, the EPDM roofing membrane is completely impervious to water and permanently resistant to standing moisture and the microorganisms it contains. Puddle formation on an EPDM roof is not a defect – it is a calculated condition that the material can withstand without any problems and on a permanent basis.

Internal drainage: Invisible and efficient

Rainwater is drained via an integrated system that is consistently tailored to the building’s aesthetics. The downpipe is positioned inside the garden shed – not on the façade, not visible from the side, and not as a retrofitted component. The clean look of the HPL façade is fully preserved; no downpipe interrupts the surface.

The standard drain is sized at DN50. This nominal diameter is generously dimensioned and significantly exceeds the calculated capacity required for our roof areas. Even during heavy rainfall, the system drains without any build-up. For large garden sheds with a depth of more than 3 m and a flat roof, we recommend a second drainage point to distribute the volume of water across two outlets and further minimise standing water.

The position of the drainage point is agreed with the customer during the planning phase. Depending on the site conditions, the drain can be connected to the mains drainage system, a rainwater tank or a soakaway. The system is designed to be flexible – it does not prescribe a specific drainage route, but adapts to the local conditions.