Roof construction and waterproofing: technical safety and material quality

The roof structure of a DESIGA® garden shed is designed to withstand long-term loads and extreme weather conditions. Both the structural load-bearing capacity and the choice of roofing material play a crucial role in the longevity of the entire building.

Structural analysis and snow load safety

The load-bearing capacity of the roof is based on precise structural analysis in accordance with applicable standards. The structure is designed to withstand a snow load of 1.5 kN/m² – equivalent to a load of approximately 150 kg per square metre. This means the garden shed meets the requirements for regions with heavy snowfall, such as those found in the Alpine foothills (snow load zone 1a). This margin is not a theoretical buffer, but a deliberate design choice: a DESIGA® garden shed stands in Scherstetten, Bavaria – we design for the conditions we face right on our doorstep.

The solid beam structure made of C24 solid structural timber forms the load-bearing frame. The wall posts are dimensioned at 140 × 50 mm, significantly exceeding conventional construction methods in the garden shed sector. These cross-sections are not calculated to meet minimum requirements, but are designed with structural reserves that can withstand even extreme weather events – wet snow, freezing rain, strong winds – without any deformation of the load-bearing structure.

For customers planning a green roof or gravel roof, this design means that the additional roof load (50–70 kg/m² for gravel, 80–120 kg/m² for a green roof with saturated substrate) is already factored into the structural calculations. There is always a calculated reserve for snow load.

EPDM waterproofing: Seamless, elastic, durable

For the roof membrane, we use EPDM (ethylene propylene diene monomer) – a synthetic rubber characterised by extreme elasticity and UV resistance. We use a 1.5 mm thick EPDM roofing membrane, which is manufactured from a single sheet to fit every garden shed perfectly. Unlike conventional bitumen roofs, which are welded together from several sheets, our system has no seams. As experience shows that seams are the weak points of a waterproofing system, the risk of leaks is technically eliminated by the single-piece sheet.

The membrane is not laid loosely, but is bonded over the entire surface to the substrate. This prevents a suction effect during heavy storms and stops the roof membrane from flapping – a detail that is often overlooked in cheaper constructions and leads to audible differences in quality in everyday use. Full-surface bonding also ensures that no condensation can collect beneath the membrane, which further protects the lifespan of the timber structure underneath.

The material properties of EPDM allow for an estimated service life of approximately 50 years. The material remains flexible throughout this period, does not harden and does not become brittle – unlike bitumen membranes, which become brittle and crack over time when exposed to UV radiation. We underpin this durability with a 20-year material warranty on the roofing membrane. Should mechanical damage occur despite the high resistance – for example, due to falling branches – repair is straightforward: much like a bicycle tyre, the area can be permanently sealed using self-adhesive EPDM pads. Extensive renovation is not necessary.