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

Plasterboard

Concrete deck

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Screed

AVCL

Mannok Flat Roof PIR insulation boards

Waterproofing layer

Mannok Flat Roof PIR insulation boards

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Flat roofs with a concrete deck are more commonly found in commercial buildings than domestic builds, which are typically built with timber deck construction. The concrete deck can offer greater speed of roof construction which is an important consideration on many larger commercial projects.

To insulate a flat roof with a concrete deck, we recommend a warm deck construction method, where the vapour control layer is sandwiched between the concrete deck and a layer of rigid PIR insulation boards.

This method eliminates the need for ventilation and the risk of condensation, whilst providing superior thermal performance to cold deck construction.

There are a number of Mannok roof insulation boards which will suit a warm deck flat roof construction built with a concrete deck, each with different facings.

Mannok Therm Roof / MFR-FFR insulation is a foil faced insulation board which helps further improve the thermal performance to help achieve the desired U-Value.

Mannok Therm Roof / MFR-GFR insulation is faced with a mineral coated, perforated, glass tissue.

Mannok Therm Roof / MFR-DPFR insulation is faced with a fleece-finished, bitumen facing on one side and a mineral coated, perforated, glass tissue facing on the other side.

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

1. Apply a layer of screed over the concrete deck and allow to dry fully.

2. Install a vapour control layer over the screeded concrete deck. A polyethylene sheet of a minimum 1,000 gauge (250µm) is suitable for use and should be fitted with 150mm sealed laps turned up at any vertical upstand.

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

4. Apply the appropriate mechanical fixings or using hot bitumen. Generally, a minimum of 4 fixings should be used per 1200 x 600mm board, but this should be calculated in accordance with BS EN 1991- 1–4 to be site specific. Fixings should be placed 50–150mm from the edges of the board with an additional row of fixings along the middle of the board. Refer to the relevant product BBA Certificate for product specific fixing requirements.

To avoid thermal bridging, the roof insulation should meet the wall insulation with no gaps.

5. Install the waterproofing layer over the fixed insulation boards.

What thickness of roof insulation to use in concrete deck warm roof construction

The thickness of insulation required will be dependent on the U-Values you wish to achieve. In this warm deck construction, the number and type of mechanical fixings can change the thickness of insulation required. The below U-Value tables show the thickness of insulation required dependent on which insulation board you’re using, for the detail outlined. These U-Values are based on telescopic tube fasteners with a thermal conductivity of 1.00W/mK or less, the effect of which is insignificant. To get U-Values specific to a different detail, use our free U-Value calculator.

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

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:

Plasterboard on battens, 150mm concrete deck; 50mm sand/cement screed; AVCL bonded to screed with hot bitumen; Therm Flat Roof insulation (Mannok Therm Roof / MFR-FFR shown for illustration); waterproofing layer (built-up felt 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 concrete deck:

Thickness (mm) U-value (W / m 2K)
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
90 + 90 0.13

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

Plasterboard on battens, 150mm concrete deck; 50mm sand/cement screed; AVCL bonded to screed with hot bitumen; Therm Flat Roof insulation (Mannok Therm Roof / MFR-GFR shown for illustration); waterproofing layer (built-up felt 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 concrete deck:

Thickness (mm) U-value (W / m 2K)
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
90 + 90 0.13

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

Plasterboard on battens, 150mm concrete deck; 50mm sand/cement screed; AVCL bonded to screed with hot bitumen; Therm Flat Roof insulation (Mannok Therm Roof / MFR-DPFR shown for illustration); waterproofing layer (built-up felt 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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