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Flat roofs with PIR-plywood laminate

Key Properties


  • does not readily absorb water, making it suitable for use in damp environments;
  • is light, robust and easy to handle; it may be worked using a saw or sharp knife;
  • is durable and will perform for the service life of the building.

Meeting Regulations

Required thicknesses of QUINN-therm QPD PlyDeck for different U-values
U-value (W/m2K) QUINN-therm Plydeck thickness* (mm)
  Joists @ 400mm centres Joists @ 600mm centres
0.25 70* + 6** 70* + 6**
0.22 90* + 6** 90* + 6**
0.20 110* + 6** 110* + 6**
0.18 110* + 6** 110* + 6**
0.16 125* + 6** 125* + 6**

* thickness of PIR board
** 6mm WPB plywood
Results based on roof construction of: 3 layer built-up felt roofing, QUINN-therm PlyDeck board, unvented low emissivity cavity, 12.5mm plasterboard. Calculations performed to EN ISO 6946:1997, taking account of repeating thermal bridges.

Design Notes

  • Stagger fixings for adjacent boards to the same joist or noggin.
  • Ensure screw heads finish flush with the surface of the plywood.
  • Minimise the period between installation of QUINN-therm PlyDeck and of the waterproofing system. In poor weather use polyethylene sheeting as temporary protection of QUINN-therm PlyDeck.


QUINN-therm PlyDeck is a composite insulation board intended for forming warm flat roof decks which will be finished with partially bonded built-up felt waterproofing systems.

QUINN-therm consists of a core of PIR foam auto adhered to two composite aluminium facings. A layer of 6mm WPB plywood is bonded to the insulation to give additional strength and rigidity, and to provide a suitable substrate for partially bonded built-up waterproofing systems.

The low thermal conductivity of the insulation minimises the board thickness, while the composite board offers rapid coverage and straightforward installation. QUINN-therm PlyDeck is suitable for new build and refurbishment projects.

Warm roof construction reduces the risk of condensation within the roof structure and eliminates the need for ventilation beneath the deck: it also protects the structure from extremes of temperature.

Installation Guidance

QUINN-therm PlyDeck can be installed on 50mm wide joists at 600mm max. centres on roofs intended for occasional maintenance traffic, or 400mm max. centres where more frequent traffic is expected.

QUINN-therm PlyDeck boards must be supported at all edges, including cut edges at penetrations: 50mm x 50mm cross noggins should be fitted between the joists. The boards must bear at least 20mm on all supporting timbers.

QUINN-therm PlyDeck should be fixed with low profile headed screws, long enough to give minimum 35mm embedment into the timbers. Fixings should be at 200mm centres around the board edges (300mm centres on any intermediate timbers) and set at least 10mm from board edges and 50mm from corners.

As 96mm is the maximum board thickness which is practicable to install over joists, U-values of 0.18 W/m2K and lower should be achieved by fitting additional QUINN-therm QR insulation between the joists.

Thermal bridging at roof-wall junctions must be avoided: at eaves and verges the joist space should be packed with insulation; at parapets vertical edge insulation should be applied to the inner face and the wall insulation carried at least 150mm above the surface of the QUINN-therm PlyDeck.

The foil facing on the underside of QUINN-therm PlyDeck has a very high vapour resistance and can be formed into a VCL by sealing the joints between boards, by setting the boards onto a wide (30mm) bead of vapour resistant sealant applied to the upper surface of all the joists and cross noggins.

Installation Sequence

  1. Plan the layout of QUINN-therm PlyDeck boards. Cut and fit cross noggins.
  2. Apply vapour resistant sealant in a 30mm wide strip to the upper surface of the joists and cross noggins.
  3. Lay QUINN-therm PlyDeck boards with long edges following joists and board edges in broken bond.
  4. Fix with screws at 200mm centres around board edges: 300mm at any other cross timbers.
  5. 5. Lay the waterproofing system.

Product Data
Properties Value QUINN-therm Plydeck
Board width x length mm 2400 x 1200**
Board thickness mm 56, 76, 96, 116, 131**
PIR core density kg/m3 32 - 34
Area per board m2 2.88
Edge profiles available   Butt edged

* includes 6mm plywood facing ** other thicknesses available on request

Performance Data
Properties Value QUINN-therm Plydeck
Thermal conductivity† W/mK 0.022 - 0.023
Water vapour resistivity MNs/gm ~ 400
Compressive strength at 10% deformation kPa >150
Surface spread of flame, foil facing only††   Class 1

† measured to EN 12667 and EN 12939 and declared as a 90/90 value to EN 13165. The value does not include the thermal conductivity of the plywood facing, which may be taken as 0.14W/mK.
†† Measured to BS 476: Part 7: 1997. The fire rating of the roof will depend upon the performance of the waterproofing system and the internal lining.

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