Flat roof

Basic priciples of roof construction with Eurothane and Powerdeck
Eurothane/Powerdeck applications in flat roofs with bitumen or synthetic roofing membrane


The EUROTHANE and POWERDECK insulation boards are perfectly suited for the thermal insulation of flat roofs with various surfaces: concrete, steel, wood.

These “rigid” insulation boards are chosen as roof insulation especially because of:
• the high insulation capacity - lambdaD (λD) - between 0.023 W/mK and 0.028 W/mK according to the product chosen.
• the excellent stability
• the good walkability
• a high resistance to wind loads
• the light weight
• a good workability
• the compatibility with the various roofing membranes
• the dimensions of the panels
• the outstanding fire behaviour (Powerdeck)

EUROTHANE and POWERDECK insulation panels are always covered on both sides with a facing whose nature and properties vary according to the application.

An appropriate choice of a EUROTHANE or POWERDECK insulation panel (lining) makes the use of separation layers redundant. Furthermore, larger board dimensions can also be used.

EUROTHANE and POWERDECK panels are marked. The installation instructions - included in the packing – clearly explain the laying instructions for the insulation panel.

As a rule, for roof applications:

• where the insulation is bonded to the base layer (vapour control layer), or laid loosely and ballasted, only small format 1200 x 600 mm boards are used;

• with mechanical securing of the insulation, dimensions of 1200 mm x 1000 mm or 1200 mm x 2500 mm are used.


EUROTHANE and POWERDECK roof insulation boards are used only in flat roofs of the “warm” type.

The general guidelines that apply are set out in Technical Information 215 “THE FLAT ROOF” - published in March 2000 by the WTCB.

More specific rules are incorporated in the various EUROTHANE and POWERDECK Technical Approvals ATG 1575, ATG 2262, CTG-258 and CTG-077.

The thickness of the EUROTHANE and POWERDECK insulation boards to be used depends on the intended energy savings and the thermal comfort to be achieved, but a minimum insulation thickness is also necessary for avoiding surface condensation.

According to the current legislation, the U value for roofs must therefore be a maximum of 0.3 W/m2K for both new construction and renovation. From the viewpoint of energy saving, however, a target of U < 0.2 W/m2K is more than desirable.

Internal condensation, this is the conversion of water vapour into water somewhere inside the roof construction, can also lead to problems with warm roof constructions.

Recticel can offer you - on request - a condensation calculation.

If, on the basis of these physical construction calculations, it appears that there is an accumulation of condensation in the roof construction or that the quantity of condensation is becoming unacceptably large, a correctly installed vapour control layer in the roof construction on the winter warm side of the thermal heat insulation will offer a solution.

Various factors determine the need of a vapour control layer, namely:
• the internal climate classification
• the roof deck
• the nature of the insulating material

Summarized on the basis of the guidelines included in Note 215 “The flat roof” and many years of practical experience of Recticel, the following choices of vapour control layers are possible in combination with Eurothane and Powerdeck insulation panels:

Concrete cast on the spot
Concrete precast element (1)
E3 E3 E3 E4
Dampproof boarding or boards deduced from wood (2) E2 (3) E2 E4
Steeldeck (4) - E2 (3) E2 E4

Explanation of the table:

(1) When renovating roofs with an airtight roof deck of dry concrete, no vapour control layer is provided in climate classes I, II and III.
(2) A vapour control layer is not necessary provided that the finish of the joints between the boards is made airtight with the bitumen that may be used for bonding the insulation boards. In the latter assumption, the boarding is covered with a layer of P150/16. The joints between the boards and the perimeter joints are covered with strips of bituminised glass membrane.
(3) The vapour barrier may be left out if the boards are coated with a vapour control facing and provided with a rebate on the 4 sides.
(4) The air tightness of the joint between the metaldeck and the roof edge needs to be secured. Vapour barriers class E4 are placed on a continuous underground and cannot be perforated.

Overview of current materials for vapour control layers and their overlaps:

+ (μd)eq (*)
(> 2 to < 5 m)

PE-foil (thickness = 0,2 mm) with overlaps of min. 100 mm.

Also usable: all materials of class 2, 3 en 4

A bonding layer, even on a continuous underground, may not be considered as a full valuable vapour barrier.
(> 5 to < 25 m)

- PE-foils (thickness > 0,2 mm) and aluminium laminates
- Bitumen glass fibre V50/16
- Bitumenpolyester fleece P150/16

Also usable: all class 3 and 4 materials

Joints in overlap, when present, must be bonded or fixed with the torch-on method against the other building elements.
(> 25 to < 200 m)

- Reinforced bitumen V3, V4, P3 of P4.
- Polymer bitumen APP or SBS (minimum thickness = 3 mm), glass fleece or reinforced PES

Also usable: all class 4 materials.

Joints in overlap, when present, must be bonded or fixed with the torch-on method against the other building elements.
(> 200 m)
- Reinforced bitumen with integrated aluminium foil (ALU 3)
- Multilayered bituminous vapour control barriers of polymere systems (> 8 mm)
Joints, when present, must be bonded or fixed with the torch-on method against the other building elements. Vapour control barrier class E4 demands a realisation on a continuous carrier. Perforations (for instance by screwing mechanical fixations) are not allowed.

(*) (μd)eq is the equivalent vapour diffusion thickness and determines the vapour retaining property of a (vapour control) layer.
[(μd)eq = 1 m] corresponds to a layer of stationary air of 1 m.
(μd)eq > 200 m: “absolute” vapour control layer.


The table below shows which type of board is preferred by the manufacturer according on the one hand to the base layer and on the other hand the method of fixation of the roof sealing.

Concrete Steel profile plate Wood
Method of fixation of the roof sealing roofing membrane partially bonded, using hot bitumen Bi3 Powerdeck F Powedeck F
bituminous roofing membrane - partially bonded with torch-on system Bi3 Powerdeck B Powerdeck B
bituminous roofing membrane fully bonded with cold adhesive Bi3 Powerdeck F Powerdeck F
synthetic roofing membrane, fully bonded with cold adhesive Bi3 Powerdeck F Powerdeck F
adhesive roofing membrane Bi3 Powerdeck F Powerdeck F
loose-lying bituminous roofing membrane, with ballast Bi3 / /
loose-lying membrane, with ballast Powerdeck / /
mechanical fixation of the waterproofing / Powerdeck Powerdeck


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