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Inverted Insulated Roofs


Placing the principal thermal insulation above the waterproofing membrane not only keeps the membrane and the structure close to the internal temperature of the building, but also provides protection to the membrane.
This form of construction, widely known as ‘Inverted’ or ‘Upside Down Roof', has been successfully applied to many millions of square metres of roof around the world in both hot and cold climates and is widely accepted by specifying authorities.
Roof designs based on the upside down roof concept have many advantages.

The waterproofing membrane is protected from:

Consequently the life expectancy of the roof membrane is greatly increased. In addition the insulation in an upside down roof can be:

Insulation for use on an upside down roof:

This non-specific but typical graph illustrates two ranges of temperature fluctuation: the red band shows the range of temperatures for an unprotected membrane; the blue band shows the range of temperatures when the membrane is protected by extruded polystyrene insulation.

Specifications temperature graph

Substrates & Surface Preparation

The substrate shall be to falls and cross falls of 1:80 to outlets. The substrate shall be surface dry, clean and smooth; free of voids, protrusions and contaminants. The area is to be primed using a bituminous primer. Internal corners shall be filleted and external corners rounded.

(With reference to: Concrete Roofs, Substrates & Surface Preparation.)


One layer Derbigum SP4 waterproofing membrane, with 75mm side laps and 100mm end laps, sealed to primed surface to falls and crossfalls by “torch-fusion”, followed by 50mm thick extruded polystyrene insulation layer and 75mm clean crushed 19 - 25mm stone as ballast, and all exposed surfaces painted with two coats bituminous aluminium paint. Waterproofing to be installed by an Approved Derbigum Contractor under a ten year guarantee.

Specifications insulation diagram


The waterproofing is to be fully dressed into the drainage outlets so as to ensure a watertight seal. Outlets are to be positioned so that they are easily accessible both for the Waterproofing Contractor at the time of installation and maintenance purposes at some future date. Ensure that the level of the flange is kept below the screed level so as to prevent ponding around the outlet.

(With reference to: Concrete Roofs, Outlets)

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