Inverted Roofing: a guide to design & specification - Radmat Building Products Ltd


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Welcome to this Radmat CPD presentation on inverted roofing a guide to design and specification.

We have set 5 main learning aims for this CPD they are:

  1. To assist the specifier to understand inverted roof construction.
  2. Identify the features and benefits of inverted roof construction
  3. Know the key design considerations comply with regulations and codes of practice
  4. Be able to evaluate the suitability of product
  5. Specifications for inverted roofs.

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Flat roof definitions

To achieve these objectives let's start with a bit of roofing 101 and go through generic flat roof definitions so we proceed with a common understanding. A traditional cold roof has the insulation placed below the structural deck this is very typical of timber roof construction both flat and pitched roofs. This system relies on a vapour barrier beneath the insulation and incorporating a continuous flow of air to disperse water vapour within the structural void. The waterproofing membrane is exposed to thermal shock UV degradation and maintenance foot traffic due to limitations of design this construction method is unable to meet current thermal regulations and when this doesn't work effectively water vapour can reach 2 point and condense within the structure causing damp and what and potentially decreasing the efficiency of the insulation itself.


A warm roof uses insulation above the structural deck the waterproof covering is placed above the insulation and a vapour control layer is placed below the insulation unlike the cold roof there is no risk of condensation occurring within the structure however the risk of condensation between the vapour control layer and the membrane must be calculated in accordance with BS5250:2011 code of practice for control of condensation in buildings.


Like the cold roof this system leaves the waterproof layer exposed to impact damage and the elements because the membrane is laid above a high performance insulant inevitably the membrane is forced to act as a heat store thermally stressing the membrane and contributing to the urban heat island effect in cities this UV degradation by the sun creates high surface temperatures causing the material to expand and then contract when temperatures drop overtime this puts strain on the waterproofing material and can cause irregularities in the surface is and cracking.


Ultimately these factors will cause the failure of the roofing system typically anywhere between 10 and 40 years into the future depending on the waterproofing material an inverted or protected roof is a form of warm roof construction in which the insulation is placed above the waterproof layer ballast, paving or a green roof is then used to only insulation in place. In inverted roof construction the waterproof layer also acts as a vapour barrier eliminating the risk of condensation within the roof construction. This type of roof construction has the advantage of protecting the waterproof surface from impact damage and the elements as well as insulating the structure its limitation in application tends to be the structural load of the system whether a ballasted roof paved roof or green roof the upper layer protects the insulation and waterproofing membrane beneath significantly reducing urban heat islands effects.


BBA certification

This graph summarises the thermal stability but putting the insulation above the waterproofing provides a key component in achieving a BBA certified for the design life of the roof or structure in which it is incorporated. Minimising the urban heat island effect in this way also makes a positive contribution to our cities and the world's climate it additionally protects waterproofing from mechanical damage and effectively uses the waterproof layer as vapour control to control condensation risk. Now we have established the correct definition of an inverted roof components used in the roofs construction before moving on to design and specification inverted roofing is typically installed on a concrete deck due to the 80KG per square metre loading required to retain the insulation on the roof in situ. Concrete is the easiest to apply to ideally without a screed to falls as this can complicate the application and delay the installation program but how do we achieve the falls without a screed later in the presentation will demonstrate why you can install an inverted roof to 0 falls and satisfy all relevant construction standards.


Precast concrete can also be used with or without a screed but the installation method will alter slightly as you have to consider the panel gaps if not using a screed. Glue laminated timber JLT is also used on occasion whatever deck you have. It's important that the substrate is correctly prepared by the main contractor which is why we have produced this guidance document for specifiers and contractors.


Waterproofing membranes in inverted roof construction

Next in the construction comes the waterproofing membrane and we will look at the main generic types used in inverted roof construction. Traditionally mastic asphalt and reinforced Benjamin membranes were used for inverted roof waterproofing today mastic asphalt has more or less been replaced by hot melt monolithic membranes which we will look at last.


Reinforced Benjamin membranes or RBMs are still used today but due to the complexity to detail and time taken to install they are rarely used by suppliers with better alternative product offers the exception to this are single layer systems based on POC be modified bitumens which can offer advantages when doing a refurbishment or working on small roof areas with minimal penetrations.


Thickness range 6 to 8 millimeters increased resistance to damage by following trades 10 to 30 five years.


Single ply membranes are mostly used in warm roof applications which is probably where many of you have used them suppliers of these products will also specify them in inverted roof applications especially when it is the only product type they offer.


When considering a single ply membrane for inverted roofing check the thickness specified and the puncture resistance offered by that thickness or product. The membrane will be trafficked during installation and it's important to avoid damage for obvious reasons. No single ply is currently certified 40 falls application so will require screed to falls to comply with building regulations, this adds costs and prolongs the program for codes of best practice refer to the single ply roofing Association SPRA design guide.


Cold liquid applied membranes

Most commonly used in refurbishment applications cold liquid applied membranes many are also suitable for inverted roof application but as with single ply membranes assistance will be applied, thickness should be changed similar certification for zero falls application will reject any different types of membrane and it doesn't follow that because one generic type is approved for one manufacturer, it is automatically certified for another many different types can be difficult to compare apples with apples thickness range 1.2mm to 4mm.

  • Reduced resistance to damage by following trades 
  • Dependant installation
  • Quick to detail which accommodates high volumes of penetration easily
  • Easy to install on small roof areas BBA durability typically 20 to 30 five years
  • For codes of best practice refer to the liquid roofing and waterproofing Association LRWA liquid applied code of best practice. 


Hot melt monolithic membranes

Hot melt monolithic membranes are the most commonly used types of waterproofing used for inverted roofs since the mid 1990s they have taken over the market from traditional mastic asphalt thanks to their easier installation and greater flexibility. There are a number of manufacturers and when comparing the product offers do check the product complies with the liquid roofing and waterproofing Association LRWA hot melt code of best practice. Some products marked as hot melt do not comply with this code being installed as a single layer rather than two 3mm thick layers. View different types but beware imitations.

  • Thickness range 8 to 10mm
  • Minimal risk of damage by following trades 
  • Fully bonded to prime substrate, preventing water tracking if punctured
  • Two coat application
  • Simple to apply
  • Not weather temperature dependant
  • Work continues in bad weather
  • Requires heating to apply
  • Air jacketed melter needs to be near application area
  • Simple robust details
  • Accommodates high volumes of penetrations easily
  • BBA durability life of the building


These features and benefits show why hot melt monolithic waterproofing is typically the specifiers preferred choice. 


Hot melt monolithic waterproofing

From here on the presentation will focus on hot melt monolithic waterproofing for these reasons. Hot melt monolithic membranes are supplied in blocks of typically 16 kilograms in weight and these blocks are melted in an agitating air jacketed melter which come in different types and sizes. The key is to have an agitating melter as this keeps the component parts of the hot melt mixed together throughout the installation. When installing hot melt, the melters create the main health and safety concerns, they typically use propane gas to heat the air that melts the block. Its all self contained but requires covering by a suitable method statement.


Radmat have recently developed an electrically heated melter which covers these considerations but this creates other method statement requirements related to the power levels required. The final health and safety consideration is the application of the molten hot melt which is like in between 180 and 210 degrees C, again a comprehensive method statement and suitable PBE are required.


Insulation board options

Later will look at detailing and standards now let's look at the insulation board options. These are the requirements for an inverted roof insulation board, they are specific to the application and rule out the types of insulation typically used in warm roof construction. The table shows the two main generic types of insulation available extruded polystyrene and expanded polystyrene and their suitability for specific applications in accordance with ETAG 0311. The European standard applicable to the insulation in inverted roofing expanded polystyrene insulation EPS should be manufactured to BS EN 13163 thermal insulation products for buildings, factory-made expanded polystyrene products. It was introduced post the Millennium after the development of a special grade of polystyrene bead that doesn't absorb water.


Supplier product checklist:

  • Is BBA four inverted roof applications? not all are
  • Is BBA for green roof or roof garden applications? not all are 
  • Is it fire retardant? not all are 
  • Generally the employer's requirements will require the board to be non flammable when stored.
  • What are GWP and ODP? 
  • Thermal calculation checklist 
  • Is declared Lambda value as per BBA?
  • Is moisture conversion factor used as required by ETAG 031?
  • Has rainwater cooling factor been used as required by etag 031? 
  • The traditional choice more inverted roof systems worldwide, extruded polystyrene is available from a number of manufacturers the same checklist applies as for the expanding polystyrene.


It is also worth noting that some boards are grey in colour due to the incorporation of graphite to improve thermal performance which also aids identification.  Vacuum insulated panels (vip's) offer high performance for a given thickness, the downside is its greater cost per square meter and the need to prevent damage that may break the vacuum.


This second point is easily overcome through education of the contractor and generally helps ensure that the roof area is respected rather than abused, the cost becomes less of an issue when the height benefits are designed into the structure. Above 20 floors it may be possible to gain an extra floor and on most projects its use on small areas only has minimal impacts on overall roofing cost.


Most beneficial is the ability to comply with the thermal requirements of building regulations part L, the access requirements of building regulation part M and the drainage requirements of NHBC chapter Seven point one.  Since the publication of ETAG 031 in 2010 insulation technology continues to evolve,  some manufacturers of extruded polystyrene insulation have introduced technology to improve the Lambda value of their boards to 0.034 Watts per meter. Kelvin enabling 1 205mm thick board to achieve a 0.15 U-value other manufacturers have invested in vacuum insulated panel VIP technology enabling a 40mm to achieve a 0.15 U-value.  The decision as to which type of board to use will come down to available Heights costs and fire performance, more on this later.


Thermal sheet - water control layer

The final key component in the evolution of inverted roof systems has been the addition of the thermal sheet or water control layer, this rolled material typically manufactured from spunbonded polyethylene is loose laid over the insulation board with 300 millimetre overlaps. It provides the means by which 90% plus of incident rainfall is drained from the roof, the key to it's effectiveness in use is the detailing more of which later having moved into design considerations now would be a good time to address what can often be the elephant in the room namely whether or not you can design and install a roof to zero falls.


BS6229:2003 is the document that the building regulations refer to when it comes to falls on flat roofs and BS6229 specifies minimum finished falls at running 80 achieved by designing to one in 40 but 2003 was a long time ago and in 2016 BS 6229 will be updated to include zero falls applications as defined by the British board of agreement in building bulletin for it's basically a precis of the requirements of ETAG 031 the European standard we talked earlier when discussing insulation. So not only have Europe recognised at zero falls construction is acceptable then BBA have produced a simple guidance and will certify products for zero falls application. In turn the NHBC have recognised 0 falls construction for being acceptable in their chapter 7.1 standards application information was updated in November 2015, also find reference to the zero faults application in the LRWA hot melt code of practice and the grow green roof code.


When checking the suitability of a roof for zero falls application it is important to ensure the manufacturer assesses the deflection plan for the deck to identify any low points that are not near rainwater outlets. If the outlets cannot be moved to areas of high deflection the options of increasing ballast, paving or green roof loading locally to disperse the water can be considered, another option is to use a vacuum insulated panel to create internal gutters in the inverted roof insulation.


Threshold details

We will now move on to looking at some common detailing starting with threshold details. Here the requirements to comply with building regulations part L building regulations part M & Nhbc chapter Seven point one can cause design and construction issues when putting balconies over heated space. Typically the key conflict is the thickness of EPS or XPS insulation required to meet part L impacting on the level threshold required by part M and or the drainage zone required by chapter 7.1. In these circumstances the introduction of vacuum insulated panels has proven to be the only solution that avoids the condensation issues insulating in ceiling void creates.


Detailing parapet walls is straightforward with the upstands being installed, first overlapping onto the deck of flat area is then installed to overlap the horizontal element of the wall flashing once the hot melt is installe. An access protection sheet is installed to prevent the operative and the insulation sticking to the hot melt where the upstand is formed from a dissimilar material to the deck such as timber, or a metal section an uncured neoprene strap is installed before the upstand is water proved. This UN strap accommodates differential movement between the dissimilar materials the vertical board insulates the wall and has a cementitious face for aesthetics and robustness, at rainwater outlets UN strap is again used to accommodate the dissimilar materials


Here you can also see the thermal sheet above the insulation being taken down into the outlet to ensure effective drainage we have already looked at the potential conflict threshold details create.


Internal up stand

Here we are looking at a typical internal up stand beneath the cladding system in this application the hot melt waterproofing should extend a minimum of 150mm above the finished roof level in this case the top of the gravel ballast and paving. Again if the upstand and deck are dissimilar materials are UN strap will be used to accommodate differential movement. Where there is a small or unusually shaped penetration in the roof hot melt comes into its own for ease of detailing, a galvanised steel curve is formed around the penetration as shown here on a glass balustrade bracket. UN strap is then put around the entire curve and the resulting boxes simply filled with hot melt prior to installation of the access protection sheet.


Environmental profile of an inverted roofing system

We have now completed our overview of inverted roofing systems. Before we review the learning aims let's review the environmental profile of an inverted roofing system. Starting with the green guide to specification most of the inverted roof insulation boards have an A or a plus rating and as the insolent and deck type have the biggest impact on a green guide rating these will drive the achieved level as generally an inverted roof is on a concrete deck, an inverted roof will achieve a C or D rating.


The green guide feeds breeam ratings and here the biggest additional contribution will be the ISO 14001 certification for the manufacturing of waterproofing and the insulation over the past few years the BRE have let there BES 6001 responsible sourcing of construction products assessment to BRE out currently no roofing system manufacturer has spent the money to get a BES 6001 rating. It remains the domain of glass brick and steel suppliers. Lastly our hot melt monolithic waterproofing its accessory components and pro therm insulation was chosen to supply Google at their New London office building following a thorough assessment of the product's environmental profiles.



That completes this Radmat CPD presentation giving you an understanding of inverted roof construction showing you the key features and benefits at grasp of key design considerations, the relevant regulations and codes in practice plus the knowledge to evaluate the suitability of product and specifications for inverted roof construction. 
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