Roof ballast: a weight-bearing layer and technical protection

Covering the roof surface with gravel is a tried-and-tested method of providing long-lasting protection for the flat roof waterproofing of a garden shed, whilst also creating a clean, mineral-based appearance. Gravel offers an aesthetic advantage over plain black roofing membrane, particularly on properties where the roof is visible from higher windows or terraces.

Functions, materials and construction

A gravel covering fulfils three key functions on a flat roof: it completely shields the EPDM roofing membrane from direct sunlight, thereby preventing premature ageing caused by UV light. It acts as ballast, providing additional protection for the roofing membrane against suction forces in strong winds. And it acts as a mineral ‘hard roofing’ layer, protecting the building against flying sparks or radiant heat from outside.

As standard, we use washed round gravel (river gravel) with a grain size of 16/32 mm. This size ensures good drainage and is at the same time heavy enough not to be blown away by the wind. A standard layer 3–4 cm thick – matching the roof upstand – weighs around 50–70 kg per m². As the structural design of a DESIGA® garden shed is rated for a total load of approx. 150 kg/m², this gravel layer still leaves sufficient capacity for snow loads. Suitable alternatives include basalt chippings (angular, dark, modern-looking), lava mulch (approx. 40% lighter with high water retention) or slate tiles (flat, elegant, higher-priced).

Before applying the gravel, it is essential to lay a protective fleece (at least 300 g/m²) over the EPDM membrane. This fleece prevents sharp-edged stones or mechanical pressure from damaging the waterproofing. If using angular chippings, the protective layer should be particularly robust – a point that is often underestimated in practice.

Pros and cons: Gravel surfacing in practice

For a clean appearance with minimal maintenance, classic round gravel 16/32 is the technically safest recommendation. It is easy to clean, offers the best protection for the underlying EPDM membrane and is significantly cheaper than a green roof. In locations with many deciduous trees, we recommend checking the roof drain at least once a year in autumn and removing leaves from the gravel surface so as not to obstruct water drainage.