A Structured Approach to Roof Specification and Design - SIG Design & Technology


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Today’s CPD covers how to apply a structured approach in the specification and design of roofing waterproofing systems. We want you to go away with a better knowledge of specific generic roof waterproofing systems, including green roofing options and an understanding of the process of how to arrive at the best solution for your client’s application. Along the way, we will help you comply with British Standards and codes of practice, offer a checklist on what you should expect in terms of manufacturer support, provide further sources of information and say where to seek further advice.


Our agender is self-explanatory, we’ve aimed to build a useful structure to the roof specification and design process, which you can apply time and time again to ensure maximum longevity for a variety of roofing systems.


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Who are SIG plc?

But first a very brief introduction to SIG plc and our own specification brand SIG Design and Technology. SIG plc is a FTSE 2,50 listed company, and the UK’s market leading specialist supplier to professionals in the building in construction industries. The key facts speak for themselves; SIG plc focuses its activities into three business sectors; insulation and energy management, interiors and exteriors, and products are distributed through its national supply chain.


Part of SIG exteriors, SIG Design and Technology is part of a UK wide operation which comprises three UK partnering manufacturing sites, 140 dedicated roofing depots and our own extensively stocked warehouse in Dudley. SIG design and technology’s headquarters and training centre is based in Leicestershire at junction 23 of the M1. It’s here that we train roofing installers to meet the high standards required to meet our design and technology accredited roofing contractors’ scheme DATAC.


SIG Design and Technology works in partnership with you the specifier, to deliver full manufacturer support. It’s a complete and impartial design and supply service, built around flat roof products, including green roofing systems, sourced from leading product manufacturers. We help roofing designers and contractors make their roofs save energy and carbon, generate energy and integrate with other technologies, such as solar collection, drainage and heat pumps. With decades of experience we’re familiar with many of the problems designers and contractors face when looking at getting the most from a roof, and have examples of successful projects you can use to satisfy your clients and insurers. Our roofing systems are installed by DATAC member companies and are backed up by SIG plcs’s own guarantee for complete peace of mind.


Design considerations when specifying a roof

What do you think of some of the design considerations you need to make when specifying a roof? So, first of all, what does the client need the specific roof to do? Its design and construction must meet a matrix of complex and strategic variables including; planning points, performance requirements, BREEAM points, BRE ratings, costs, structural requirements and buildability. We’re all familiar with these types of documents, they are vital to correct design and due diligence. But how do we ensure compliance when there are so many different standards for any one building element. Once we understand the brief, we can evaluate the options, choose the materials and define the methodology. The job is to waterproof a roof using engineered solutions specific to the project. The formula is about costs, plus design, plus compliance.


Influencing factors when designing a roof

So, what are the influencing factors when designing a roof? Here are four different examples where a different roofing solution is selected.


Example scenario one, is a warehouse with a steel frame. It’s a low-cost metal deck, there is no roof mounted plan, there is limited roof plan. The project is cost driven and it’s over a short program. The solution is a high-quality single ply waterproofing system.


Example scenario two, is a hot melt waterproofing system of a concrete construction, there are limited falls, it offers circulation area, there needs to be design awareness of heavy live loads on structure, and its low maintenance. The solution is a hot melt waterproofing system.


Example scenario three, pre-formed zinc roofing and cladding, a prestige commercial building has high aesthetic appeal, it represents significant investment, the building has long durability and life expectancy, constructed from robust materials, it’s got kerb appeal, and there’s limited maintenance. The solution is pre-formed zinc roofing and cladding.


Example scenario four, is a cold liquid applied rubber waterproofing system. This plant room requires complex and complicated detailing, there are multiple penetrations in the roof, work is difficult to access, there’s restricted working space, and it needs a cost-effective solution on a metal deck refurbishment. The solution is a cold liquid applied rubber waterproofing system.


Waterproofing buildings

So, let’s look at four different roofing options used to waterproof a building.

  • Single ply membranes,
  • Hot melt,
  • Hard metals,
  • And liquid coatings.


Single ply roofing confers many benefits principally, over fifty years of track record; suitable for new build or refurbishment applications; it’s now capital cost and cost in use; installation is safe, rapid and clean; it’s from a well-managed and regulated sector, courtesy of SPRA the Single Ply Roofing Association.


This table highlights the main differences between major types of single ply membrane available in the market today. It also helps to demystify the many acronyms which exist.


Why choose hot melt

So, let’s move on to why choose hot melt. Three key benefits are; its long-life expectancy, water is unable to track under a fully bonded system, so it’s ideal for green roofs on concrete.
Compatible with Bitumen, hot melts life expectancy is twenty-five years, it’s easy to install at temperatures as low as minus eighteen degrees Celsius, and it's highly resistant to impact damage. Hot melt is guaranteed for thirty-five years.


Hard metals

So why choose hard metals? They are for a long life span due to natural patina formation, are largely maintenance free, they are 100 percent recyclable, and offer excellent BREEAM credentials, and there’s dramatic visual impact.


The table illustrates product requirements to DIN EN 988.


Cold applied using not hot works and in temperatures as cold as zero degrees Celsius, liquid coating cures quickly, it’s water repellent on application, can be applied on damp surfaces. This seamless waterproofing system is ideal where complex detailing is required. It’s excellent elasticity and tactile strength make it ideal for both refurbishment or new build projects.


The AH polymer system confers all benefits of the material characteristics of liquid coating.


Green roofing

Sustainability is today's watch word, so why exactly does green roofing make perfect sense, and how do you set about matching the product solution to the application?


It makes good use of space, optimising the structural footprint of a building. A green roof catches carbon and improves air quality by absorbing airborne pollutants through plants, removing them from the environment. A green roof substantially increases the life expectancy of the roof's waterproofing membrane, as it protects from UV degradation and extremes of climatic condition. It provides thermal and acoustic insulation. It provides an additional green space for wildlife. And of course, a green roof is aesthetically pleasing. Green roof membranes require FLL certification and there are two levels for root and rhizome protection.


So, does green roofing actually work? The amount of CO2 a tree will offset depends on many factors such as the type of tree, where it is planted, and the amount of room it has to grow. On average one broadleaf tree will absorb in the region of one ton of carbon dioxide during its full lifetime, of approximately one hundred years. Broadleaf trees are indigenous to the UK, per square meter, it’s hard to measure due to the ranging densities of forests, whereas sedum is planted on green roofs and density is a controlled factor. At this moment we do not know how much CO2 sedum is able to absorb but because of the greater amount of sedum leaves per square meter, we expect the number to be considerable.


However, an initial two-year study carried out by Michigan State University in 2009, found that sedum roof absorbed one hundred and ninety grams per square meter of CO2 over a year. From this we can calculate that fifty square meters of sedum absorbs nine and a half thousand grams per annum.


So, as a medium size car generates 1.64 tons of carbon dioxide per annum, based on a car travelling ten thousand miles per annum, we need approximately nine thousand square meters of sedum per car or roof gardens with broadleaf trees.


Green roofs are an important component of SUDS, or sustainable urban drainage systems. There is often a requirement to attenuate water that has been shed from roofs into the drainage system. It’s a fact that a typical green roof absorbs and evaporates fifty percent of the rainfall that falls on the roof. This rate of absorption reduces the pressure on drains, and reduces the likelihood of localised downstream flooding.


Green roofs can make a considerable contribution to conservation of biodiversity. It’s important to include mixtures of drought-tolerant native wildflowers, plus sedum and grasses. Some London boroughs even stipulate which native plants are to be included by postcode. Specific plants are particularly invaluable to some types of invertebrates such as bees. There is also evidence of rare birds using green roofs such as the black redstarts.


There are two main types; built up or modular. A built-up green roof is constructed on site. Built up systems also accommodate brown and intensive green roofs. They require careful construction because, if the waterproofing is penetrated, repair is costly and a disruptive exercise. Modular systems utilise pre-grown planting, which offers limitless plan choice. They comprise lightweight removable modules, which are easy to install. Green roofs affect de-components, the construction of the roof and the building itself.


Built-up green roof

A built-up green roof is constructed on site. A built-up bespoke roof offers a multitude of permutations, to include indigenous plant life, or existing turf sods from the locality. It blends into location or hillsides, for example at this RIBA award winning home at Manaton Devon, which has three grass roofs.


These renders show the construction of three types of built-up green roof: 

  • intensive,
  • extensive
  • and biodiverse.


The alternative is a modular system which has dramatically increased in popularity for a number of reasons. Its lightweight structure allows more roofs to be greened than a built-up system. It’s a very fast effective method of creating an extensive sedum green roof that is installed in four easy steps. Modular provides an instant green roof on the day of completion because it's delivered with 90/95 percent mature sedum plant cover for instant results. The sedum plants are cultivated in a volcanic growing medium that is rich in nutrients and grown within a recycled polypropylene tray, to create a seamless green roof finish. Unlike traditional systems this modular tray allows the natural sharing of water, nutrients and beneficial organisms across the entire roof top. A modular system can be installed on new and refurbished roofing, commercial or domestic roofing, flat roofs, pitches up to twenty degrees with suitable retention measures, and the system can be installed over most waterproofing systems.


Our next section covers what sort of service levels you should expect from suppliers.


Service levels from suppliers

You should receive comprehensive technical support from a manufacturer or supplier, and they should comply to all relevant standards. Technical information should include standard details, NBS specifications, cut to fall insulation design, wind load and thermal calculations, and third-party accreditation. SIG Design and Technology now has several products as BIM objects available in the NBS national BIM library. Early involvement in a project by a supplier will make the membrane system choice and technical support process simpler in the long run. Suppliers can offer advice, not just on which products to use, but more importantly, when those products are not suitable, and an alternative should be sought.


Again, a manufacturer or supplier should offer onsite support to protect the long-term integrity of the chosen waterproofing system. This is not just important to ensure the long-term performance of the waterproofing system chosen, it’s also a requirement of British standards BS6229 2003, to maintain a roof and guidance on how this should be done, can be demonstrated during a site support visit.


Examples of service

Here are two examples of service, which should bring about peace of mind. The first is an application form for membership of the design and technology accredited contractor scheme. Without being a member, a roofing contractor cannot buy our materials. It's just one more way that we bring about quality control. The second document is a guarantee which covers the materials, and our is backed by SIG plc.


So, here’s a checklist covering the essential manufacturer support you should receive.


In our experience, problems in-situ occur when detailing and compatibility and interfaces have been not properly considered, or executed.


A modern single ply roofing system is more than just a waterproofing membrane and incorporates a number of key system accessories. This slide illustrates the main additional components that should be considered by the specifier to ensure the integrity of the membrane is not compromised.


These two images show where the integratory of the membrane can compromise the NBS specification by puncturing the membrane, ultimately leading to water in grass.


However, by using the correct materials, in the case of this membrane with an FLL certificate and standard detailing to avoid puncturing the membrane, these roof membrane systems are not punctured at interfaces, and do not risk water in grass.


Poor installation leads to failure and it happens all too often. In a study by Dutch organisation TNO, it was found that poor workmanship and inexperience were costing the EU sum two billion euros per year. The key problems are preventable and comprise punctured membranes, water in grass and wind and condensation problems. There is often evidence of poor preparation, inadequate framework, and poor detailing at cable entry. The picture at the top is a classic on how not to install solar thermal panels. The membrane is punctured. The image underneath shows PV panels, which need no penetration of the membrane as they are held in place by their own weight.



So, in summary, hopefully you will go away equipped with three sets of knowledge; a better understanding of specific generic roof waterproofing systems including green roofing and new technologies, an understanding of the process of how to arrive at the best solution for your clients application, and the role that a manufacturer or distributor can fulfil from the point of specification to insulation and guarantee.


Here are some further sources of references which you may find useful.


Thank you very much for your interest and attention in our CPD today. We hope you feel much better informed on how to apply a structured approach in the specification and design of roofing waterproofing systems. We’ve given you guidance on what to expect from a manufacturer or supplier and how to factor out common problems, through simple best practice.

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