the permanent roof
Home. Up. Trade Info. Hoppers. Fascias. Flashings. Gutter Systems. Eco roof. Railing Pads. Joinery. Balconyprep. Rooflights. Mansard. Permadeck DL. Edge trims. Compare. Zero Carbon.

Don’t get lost! These are tech pages only, go to HOME for main page list

Have you watched the Permadeck videos?


Permadeck with  soft single ply systems

E&OE      The text and graphics of this website are Copyright © 1996-2015 all rights reserved by Seamless Roofing Ltd.  Permadeck, Thermadeck & Flexiroof are registered trade marks.  The contents of this website are subject to review and are a guide only and do not represent a binding offer. The BBA logo is Copyright © British Board of Agrément.  

If you found this page informative please tell others by clicking the +1

Thermal Calculations, Condensation Risk, Permeability, Weights & Fire (E&OE)
The following information is a guide only and must be checked by a qualified person.
Avoid FOIL-FACED BOARDS as some condensation risk analyses can give a failure of design. When using a single-sided pre-felted board put the felted side DOWN.  
Recommended boards include: Kingspan Thermaroof TR27 and Celotex EL3000.   
Roof Clause to go on submitted plans:
Notes: 1. The upper deck can be secured to the lower deck using screws countersunk by 2-4mm. Screws are               available in various sizes up to 200mm to suit the insulation and depth of fixing, try: Ebay
          2. Re: ‘cold-bridging’ - it is more cost effective to add a bit to the insulation over all (10mm) than use              thermal break fastenings, which are expensive, slower to fix and can’t withstand the high torques               required. Alternatively, the joist positions should be accurately marked on the upper ply so that the                 fixings stay within timber.
           3. Open ceilings with revealed beams may require more insulation for a given U-value.
A.  Extra low U-values may be obtain by also fitting non-permeable insulation between or below the joists eg Thermal Economics 2L2.  Do not use glass wool or roughly fitted insulation board.
Cold decks:
Cold decks can be used over non-habitable volumes.
Insulated cold decks have many advantages over warm roofs. Firstly, they can be warmer!
Being able to put the insulation down on the ceiling where, conventionally, it always used to be can be a much cosier place since there is no draughty void or convection currents between the ‘warm’ roof and the ceiling.

Secondly, the insulation is
cheaper and the volume forever to be heated is less.

However, to comply with the Bldg Regs you must ventilate the void (if there is a void*) continuously on two opposite sides. The picture shows one architect’s approved solution where the Permadeck trim stands off the roof but is held firm by the small outriggers. The picture doesn’t show the built-in vermin grid, which was fitted later.

See THERMADECK for upgrading existing poorly insulated roofs.

Condensation Risk Analysis (CRA):
Weight of Permadeck for structural calculations:

Permadeck = 2.5kg/sqm

18mm Permadeck plywood = 11.7kg/sqm

*All information, suggestions, advice & data are given in good faith and must be checked by a competent person. E&OE.

*We have also seen many situations where height has been a problem and the joist void has (also) been filled with insulation and kept hard against the deck ie there is no cold void to ventilate. A good vapour barrier is required below the insulation.  Ideal for re-furbs but must be cleared with Building Control in new builds and conversions. Insulation should be increased to compensate for the joist bridging effect. Excellent vapour control is also required, especially when electrics may penetrate.

A condensation risk analysis of a correctly built warm roof using Permadeck shows NO risk of interstitial condensation. This generic analysis was for a house in Edinburgh and refers to a Permadeck warm roof using 100mm roof board insulation on 150mm joists with a plasterboard ceiling*.  Please inquire for the cost of producing bespoke calculations for other geographical areas and roof designs.  All design suggestions and calculations must be checked by a competent person.

Using Permadeck as a substitute for metals, particularly Lead, has many advantages especially on flat roofs.  In 1986 lead was no longer recommended as a waterproofing for warm roofs and the same considerations apply to other sheet metals eg zinc and copper. See report on Under Lead Corrosion on warm roofs:  English Heritage

*A heavy duty vapour barrier (vb) must be included below the insulation (min 0.25mm polythene) and  foil-faced plasterboard should be used on the ceiling.  As a general rule, insulation below the vb and foil-faced insulation above the vb should be avoided.  

Warm decks:

The latest Building Regulations now impact on existing roofs that are part of a Warrant application and advice should be sought from your local authority as some of this is discretionary and has an economic and practicality test, ie can the thermal upgrade pay for itself within 15 years, are there physical restraints etc.

These figures can be used as a guide for refurbishment without Warrants, Grants etc.

New-builds: maximum U-value = 0.15 (Subject to whole building considerations)

Changes of use/renovations/extensions to existing: max U-value = 0.18

Permadeck Permeability:

Permadeck has a free-film permeability of 1.55g/sqm/day @ 25°C & 75% Rel Hum.

Q.  Can insulation also be fitted between the joists?

The roof insulation and waterproofing comprises: Permadeck 20 (Agrément Certification 93/2967) with integral edge trims and wall flashings as required on 18mm pre-treated Permadeck DryPly ext wbp plywood, on (thickness)mm TR27 or EL3000 insulation, on vapour barrier on 18mm ext wbp plywood on firring (dimension) on joists (dimension).

Permadeck passes BS476 Part 3 and is rated Ext FAA.

“The roof of a building, including any rooflights, but excluding any wallhead, fascia, flashing or trim, boxed gutters, soffit or barge boards, should have a low vulnerability if not more than 6m from the boundary and a low to medium vulnerability if between 6m and 24m. “     Building Regulations re. spread of fire.

For the above regulations, Permadeck has a very low vulnerability.






0.34 - 0.25

Refurbishment projects and/or where height is restricted.


0.25 - 0.19







Low energy builds and where extra roof insulation compensates for roolfights, windows etc.

* Using a 120mm PUR insulation board with a K-value of 0.024, the thermal resistance = 0.12/0.024 = 5.0

NB Resistance depends on type of insulation.  Rest of roof standard values:  Ext surface = 0.045, Permadeck = 0.06, upper deck = 0.13, lower deck = 0.13, joist void = 0.32, int surface = 0.105.    Giving a total of 0.79

So the whole roof resistance is 5.0 + 0.79 = 5.79, and the U-value = 1 / 5.79 = 0.17

Eco roof

See our highly insulated Eco roof designs