Flat Roof Insulation: Insulating a flat roof with metal deck: warm deck construction

Lightweight metal deck

Polyethylene AVCL

Mannok Therm Roof / MFR-GFR PIR insulation boards

Waterproofing layer

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Mannok Therm Roof / MFR-FFR PIR insulation boards

Capping

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To achieve the best thermal performance of a flat roof with metal deck, warm deck construction should be used. This is the preferred option wherever possible as it removes the risk of condensation forming and resulting damp. It also removes the requirement for ventilation and significantly reduces the heat loss which can occur with cold deck construction.

The design for warm deck construction in these flat roofs requires the vapour control layer to be positioned between the metal deck and the roof insulation boards. The waterproofing layer is then positioned on top of the flat roof insulation.

Mannok’s flat roof PIR insulation boards are recommended for insulating a flat roof with a metal deck due to the excellent thermal performance and ease of handling and fitting. There are a number of insulation options to choose from in the Mannok flat roof range which are suitable for use in a flat roof with metal deck, Mannok Therm Roof / MFR-FFR insulation, Mannok Therm Roof / MFR-GFR insulation or Mannok Therm Roof / MFR-DPFR insulation. All of these PIR insulation boards can help achieve excellent U-Values. The example build up shown uses Mannok Therm Roof / MFR-GFR insulation boards.

Choosing the right insulation boards

Mannok Therm Roof / MFR-FFR insulation: This insulation board is suitable for mechanically fixed single ply membranes.

Mannok Therm Roof / MFR-GFR insulation: This insulation board is suitable for fully adhered and mechanically fixed single ply membranes, partially bonded built up felt, mastic asphalt and cold liquid applied waterproofing systems.

Mannok Therm Roof / MFR-DPFR insulation: This insulation board is faced with glass tissue on one side and bituminous fleece on the reverse. With the glass tissue face up, it is suitable for fully adhered and mechanically fixed single ply membranes, partially bonded built up felt, mastic asphalt and cold liquid applied waterproofing systems.

With the bituminous fleece face up, this insulation boards is suitable for hot or cold bonded build-up bituminous waterproofing systems.

How to install insulation in a flat roof with a metal deck: warm deck construction

1. Lay the vapour control layer over the structural metal deck with 150mm sealed laps turned up at any vertical upstand. The vapour control layer should consist of a minimum 1,000 gauge (250µm) polyethylene sheet.

2. Lay the Mannok insulation boards across the troughs of metal decks in accordance with the minimum thickness of insulation shown in the table below. Boards should be laid with the long edges at right angles to the troughs with all joints fully supported.

Minimum insulation thickness (mm) Trough opening
25 <75
30 76-100
35 101-125
40 126-150
45 151-175
50 176-200

3. The insulation boards should be mechanically fixed to the deck. The number of fixings required to fix each board will vary with building location, geometry and topographical data and should be assessed in accordance with BS EN 1991- 1–4.

As a general rule, a minimum of 4 fixings should be used per 1200 x 600mm board. Fixings should be placed 50–150mm from the edges of the board with an additional row of fixings along the middle of the board.

Note: Fixing diagram references can be obtained in the relevant product BBA certificates:

Mannok Therm Roof / MFR-FFR BBA Cert
Mannok Therm Roof / MFR-GFR BBA Cert
Mannok Therm Roof / MFR-DPFR BBA Cert

4. Lay the waterproofing layer over the insulation layer.

U-Values and insulation thickness

When calculating U–Values to BS EN ISO 6946 the number and type of mechanical fixings can change the thickness of insulation required. The tabulated U-Values below were calculated based on telescopic tube fasteners with a thermal conductivity of 1.00W/mK or less, the effect of which is insignificant.

Required insulation thickness of Mannok Therm Roof / MFR-FFR on a flat roof with a metal deck:

Thickness (mm) U-value (W / m2K)
90 0.24
100 0.21
110 0.19
120 0.18
135 0.17
130 0.16
140 0.15
150 0.14
80 + 80 0.13

U-value results above based upon flat roof construction shown:

Lightweight metal deck; polyethylene AVCL; Therm Flat Roof insulation (Mannok Therm Roof / MFR-GFR shown for illustration); waterproofing layer (single ply membrane with 5 fixings per m2 shown). Calculations performed to BS EN ISO 6946, taking account of repeating thermal bridges.

*Where different thicknesses of insulation are used, the thicker Therm board should be placed outermost.

When calculating U-values to BS EN ISO 6946, the type of mechanical fixing used may change the thickness of insulation required. The U-values in these tables were calculated using telescopic tube fasteners with a thermal conductivity of 1.00W/m.K or less, the effect of which is insignificant.

Required insulation thickness of Mannok Therm Roof / MFR-GFR on a flat roof with a metal deck:

Thickness (mm) U-value (W / m2K)
100 0.24
110 0.22
120 0.19
125 0.19
130 0.18
140 0.17
150 0.16
80 + 80 0.15
80 + 90* 0.14

U-value results above based upon flat roof construction shown:

Lightweight metal deck; polyethylene AVCL; Therm Flat Roof insulation (Mannok Therm Roof / MFR-GFR shown for illustration); waterproofing layer (single ply membrane with 5 fixings per m2 shown). Calculations performed to BS EN ISO 6946, taking account of repeating thermal bridges.

*Where different thicknesses of insulation are used, the thicker Therm board should be placed outermost.

When calculating U-values to BS EN ISO 6946, the type of mechanical fixing used may change the thickness of insulation required. The U-values in these tables were calculated using telescopic tube fasteners with a thermal conductivity of 1.00W/m.K or less, the effect of which is insignificant.

Required insulation thickness of Mannok Therm Roof / MFR-DPFR on a flat roof with a metal deck:

Thickness (mm) U-value (W / m2K)
100 0.24
110 0.22
120 0.19
125 0.19
130 0.18
140 0.17
150 0.16
80 + 80 0.15
80 + 90* 0.14

U-value results above based upon flat roof construction shown:

Lightweight metal deck; polyethylene AVCL; Therm Flat Roof insulation (Mannok Therm Roof / MFR-GFR shown for illustration); waterproofing layer (single ply membrane with 5 fixings per m2 shown). Calculations performed to BS EN ISO 6946, taking account of repeating thermal bridges.

*Where different thicknesses of insulation are used, the thicker Therm board should be placed outermost.

When calculating U-values to BS EN ISO 6946, the type of mechanical fixing used may change the thickness of insulation required. The U-values in these tables were calculated using telescopic tube fasteners with a thermal conductivity of 1.00W/m.K or less, the effect of which is insignificant.

Need technical support?

For help and advice on any of our products and how they might fit your needs please contact our Technical by Mannok team.

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