platdak

Platdak is a term used in Dutch to refer to a flat roof, defined as a roof with little or no pitch. In architectural practice, platdak describes roofs that are horizontally oriented or slope only slightly (commonly 1–3 degrees) to facilitate drainage. The design emphasizes a continuous waterproofing membrane over a structural deck, with insulation and a drainage layer to channel water to gutters or internal drains. In Dutch-speaking regions, platdak is a standard typology for residential and commercial buildings.

Construction typically involves a structural deck made of concrete or timber joists, followed by insulation, a vapour barrier, and a waterproofing assembly. Waterproofing options include EPDM or TPO membranes, bituminous sheets, or liquid-applied coatings, often protected by ballast, a protective layer, or a concrete or metal substrate. Drainage is essential; flat roofs rely on perimeter lines and internal drains to prevent ponding. Flashing and parapets seal transitions at walls and edges to minimize leaks.

Advantages of platdak include the potential for usable roof space, ease of installing mechanical equipment or solar panels, and sometimes lower construction complexity for rectangular plans. Drawbacks include susceptibility to ponding water if drainage is inadequate, higher demands on insulation to prevent heat loss or gain, and ongoing maintenance needs to prevent leaks or membrane damage. Safety and accessibility considerations matter for roofs intended for foot traffic or access.

Green roofs and roof terraces are common evolutions of platdak, offering environmental benefits but requiring careful structural, waterproofing, and drainage design. In the Netherlands and other temperate regions, platdak remains widespread in urban building stock and has influenced roofing practices abroad.