Insulated Roofs

Foamed Cement Screeds

Foamed cement is a common and well accepted insulation medium as it provides both insulation and the falls to outlets simultaneously. Lightweight screeds however should be installed in two densities, with an upper finishing of a sand and cement screed to provide a firm non-collapsible smooth surface. This creates and acceptable substrate.
Derbigum waterproofing membranes can be laid directly over foam cement screeds provided the compressive strength is sufficient to meet the end use. The use of a 25mm sand and cement screed is highly recommended.

Specification

Under Screed Insulation

Special care must be taken with situations where insulation boards form part of the substrate. Polystyrene and polyurethane boards should be firmly fixed to the concrete structure. The sand/cement screed topping usually has varying thickness (in the creation of falls and crossfalls) and is subjected to uneven temperature distribution throughout its mass. Minimum screed thickness should be 50 mm. Not being bonded to the concrete and being subject to differential movement factors, cracking in the screed may occur - thus not providing a firm and stable substrate for the waterproofing membrane. Creating smaller 'panels' will assist in overcoming this problem.  It is often more suitable, wherever possible, to position the insulation (extruded polystyrene) on top of the waterproofing membrane – See Inverted Roofs.

It is advisable that the insulation is covered with polythene sheeting with sealed laps, and the selected grading material laid to falls over the insulation system. Under screed insulation boards can then vary in density because the grading on top of them will provide a firm solid surface.
This method provides a simple and straightforward solid surface for waterproofing and other installations on the roof. It permits replacement of waterproofing treatments in future years without damaging and incurring the expensive replacement of the insulation.

World wide experience has proven that the lifespan of an exposed waterproofing treatment is reduced when it is installed immediately over an effective insulation material. The reason being that the dissipation of heat into the structure below is eliminated with this method and the full thermal shock is concentrated on the waterproofing treatment.

Insulation Boards (Over Metal roofs)

The insulation boards shall be installed in accordance with the instructions of the manufacturer of the insulation and must be suitable for torch applied roofing systems.
It must provide a firm stable surface free of wide breaks and joints and must be of such a surface and character that the Derbigum membrane may be firmly attached by a practical method of hot sealing, torching or mechanical fixing.

Vapour barriers are normally used where high humidity conditions are to be expected inside the structure for various reasons. The decision to make use of vapour barriers or not is the responsibility of the professional team, the subsequent choice of vapour barrier is also part of that responsibility.

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