Inverted or conventional flat roof: what's the difference

On a flat roof, the order in which waterproofing and insulation are laid isn't a minor detail: it defines whether the roof is "conventional" or "inverted", with direct implications for durability and maintenance.

Conventional roof: insulation under the waterproofing

In the conventional system, the waterproof membrane is the most exposed layer, above the thermal insulation. This leaves it directly subject to temperature changes, UV radiation and the mechanical wear of the roof's use, which shortens its life compared with the inverted system.

Inverted roof: the waterproofing protected

In the inverted system, the thermal insulation (normally extruded polystyrene, moisture-resistant) is laid above the waterproof membrane, protecting it from sudden temperature changes and direct sunlight. This considerably extends the life of the waterproofing, although the insulation must be a specific material that doesn't degrade in contact with water.

Which to recommend

The inverted system is now the most recommended on walkable or planted flat roofs, as it better protects the most critical layer of the system. The conventional one remains valid on non-walkable roofs where cost is the main priority.

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