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

Roof Joists

Timber deck

AVCL

Mannok Therm Roof / MFR-FFR PIR insulation boards

Waterproofing layer

Mannok Therm Roof / MFR-FFR PIR insulation boards

  • Roof Joists

  • Timber deck

  • AVCL

  • Mannok Therm Roof / MFR-FFR PIR insulation boards

  • Waterproofing layer

  • Mannok Therm Roof / MFR-FFR PIR insulation boards

Insulating a timber deck flat roof can be done using warm deck construction method or cold roof construction method. It is recommended where possible to use the warm deck method of insulation, where the principal layer of roof insulation is positioned above the structural deck. This avoids the risk of condensation forming between the structural deck and the insulation boards and the subsequent risk of the timber deck rotting and compromising its structural soundness. There is no requirement for a roof ventilation void and thermal bridging is easy to combat as the roof insulation is laid in a continuous layer.

Timber deck roof construction is primarily found in domestic buildings rather than commercial structures and should be insulated using high performance rigid PIR insulation boards, such as the Mannok flat roof insulation range.

Within the Mannok insulation range, you can choose between Mannok Therm Roof / MFR-FFR insulation, Mannok Therm Roof / MFR-GFR insulation or Mannok Therm Roof / MFR-DPFR insulation. All of these insulation products are suitable for use with timber deck flat roofs and will help achieve excellent U-Values.

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 timber deck: warm deck construction

1. Install the timber deck over the roof joists.

2. Lay the vapour control layer on top of the timber deck to prevent moisture getting to the insulation.

3. Fit the Mannok insulation boards with joints staggered between rows on top of the vapour control layer.

4. Fix the insulation boards using the appropriate mechanical fixings, which should be assessed in line with BS EN 1991- 1–4 to take account of building location, geometry and topographical data. 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.

5. Ensure the insulation boards extend to meet the wall insulation tightly to eliminate thermal bridging.

6. Install the waterproofing layer above the insulation boards.

U-Values and insulation thickness requirements

The below U-Value tables are based on a typical flat roof construction with a timber deck in a domestic build. Note, the type of mechanical fixings used may change the thickness of insulation required. Specification specific U-Values can be calculated using our online U-Value calculator.

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

Built-up roofing: Mannok Therm Roof / MFR-FFR
Thickness (mm) U-value (W / m2K)
75 0.25
80 0.24
90 0.22
100 0.20
110 0.18
120 0.17
125 0.16
130 0.15
140 0.14
150 0.14
80 + 80 0.13
80 + 90* 0.12

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

12.5mm plasterboard; roof joists; timber deck; AVCL; Therm Flat Roof insulation (Mannok Therm Roof / MFR-FFR 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 timber deck:

Built-up roofing: Mannok Therm Roof / MFR-GFR
Thickness (mm) U-value (W / m2K)
90 0.24
100 0.22
110 0.20
120 0.18
125 0.17
130 0.17
140 0.16
150 0.15
80 + 80 0.14
80 + 90* 0.13

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

12.5mm plasterboard; roof joists; timber deck; 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 timber deck:

Built-up roofing: Mannok Therm Roof / MFR-DPFR
Thickness (mm) U-value (W / m2K)
90 0.24
100 0.22
110 0.20
120 0.18
125 0.17
130 0.17
140 0.16
150 0.15
80 + 80 0.14
80 + 90* 0.13

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

12.5mm plasterboard; roof joists; timber deck; AVCL; Therm Flat Roof insulation (Mannok Therm Roof / MFR-DPFR 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.

  • Floor Insulation
  • Pitched Roof Insulation
  • Flat Roof Insulation
  • Wall Insulation