CPD: Single ply roof membranes
• Installation methods
• Environmental considerations
• Different fastening methods
Bauder product manager Peter Stonebridge looks at thermoplastic single ply roofing including installation methods and options.
Single ply roofing has many advantages to the modern building and is a mainstay solution for new construction requiring lightweight waterproofing. This form of roof waterproofing is also increasing its share of the refurbishment sector not only as an overlay sheet to a currently installed system, but also as a complete replacement. Its popularity within green roofs is also growing, particularly where low maintenance extensive vegetation is being specified.
Single ply membranes are strong, flexible sheets composed predominantly of synthetic polymer. Thicknesses range from 1.1mm to 2mm. Some products are homogeneous, others reinforced with glass fibre or polyester, depending upon the application. They are sealed at joints to form continuous waterproofing. Combined with insulation and continuous support they form durable roof systems.
Fundamentally, and like all other roof waterproofing, a single ply system must provide protection from all weather conditions. Such protection may be required before building completion to facilitate rapid fit-out of the interior.
The roof system must also perform satisfactorily against a wide range of other targets and constraints as required by legislation, by the client, by the building insurer, and by the design of the substructure and services.
Synthetic thermoplastic roofing membranes are remarkably thin and are designed to have very specific properties; as a result they represent highly efficient waterproofing systems that are both durable and reliable. PVC (polyvinyl chloride) and TPO (thermoplastic polyolefin) are two of the most commonly installed thermoplastic single-ply membranes.
The seams on thermoplastic single ply membranes can be hot-air welded, or for PVC membranes solvent welding is an alternative to form cohesive joints, known as laps. When thermoplastic membranes cool, their physical characteristics are unchanged. Other forms of thermoplastic roofing are CPE (chlorinated polyethylene) EIP (ethylene interpolymer), CPA (copolymer alloy) and NBP (acrylonitrile butadiene polymer).
Most thermoplastic membranes include a reinforcement layer, usually polyester or glass fibre, which provides increased strength and dimensional stability. These membranes are characteristically light in colour, so provide excellent reflectivity, resulting in potential energy savings.
Thermoset membranes are a different category of single ply system that are compounded from rubber polymers, or materials that cannot be hot-air welded because it will change their physical characteristics. Adhesive is generally used to bond the laps. The most common polymer is EPDM (ethylene propylene diene monomer).
The Green Guide to Specification, published by the BRE in 2008, looks at the environmental impact of building elements and provides generic ratings in addition to specific evaluations of individual manufacturer membranes.
Within the generic ecopoints ratings for thermoplastic membranes the rating for PVC was 14.3; the rating for TPO was 11.4. When placed within the green guide for example, a warm roof of PVC on all insulation types on a profiled steel deck achieves an A+ rating. A similar specification with FPO membrane is also A+; where A+ represents the best environmental performance with least environmental impact and E represents the worst environmental performance with the worst environmental impact.
Recyclability of thermoplastic membranes
The mechanical fix option for either single ply system makes it possible to independently remove all the component parts of the system. If only the membrane needs renewal, but the insulation and vapour barrier are still sound, then this element can be replaced separately.
TPO Membranes This product is recognised as having significant environmental advantages over PVC systems. TPO membranes can be recycled at the end of their serviceable life by returning them to a manufacturing process to be used instead of raw materials. If the TPO membranes have become contaminated by external agents, then their disposal by incineration may be required. In this instance, their combustion does not cause any environmental problems as any gases released are neutral; additionally it now becomes a useful source of energy.
PVC Membranes When PVC waterproofing membranes are recycled they can be down-cycled and converted into other roof components such as protective or separation layers.
Benefits of thermoplastic membranes
• Can accommodate the sometimes unconventional roof designs of modern architecture
• Lightweight, fast track and cost effective installation
• Durable and resistant to the weathering, chemical oxidation and UV radiation and able to support extensive green roofs
• Safe, flame-free installation methods using hot-air welding, adhesives and fasteners
• Cold bridging eliminated by use of thermal break fasteners
• Effective control of air leakage to meet new energy efficiency regulations
• Simple detailing to suit all situations
• Interfaces to other construction elements
• Various colour options with matching profiles for eaves, verges and parapets
How to choose which material to use
Single ply membranes are a viable long term roofing solution and the product best suited to the project will depend on the characteristics that will address the building’s requirements. Because of this, the specific thermoplastic membrane should be chosen not because it has the lowest price, but because it delivers the best value for money and matches the expectations for the building.
It is helpful to have a checklist of criteria as the most appropriate membrane for the project will depend on a wide range of factors. The Single Ply Roofing Association (SPRA) is an independent organisation that represents the UK’s single ply roofing industry, the association has a checklist that can be used to evaluate the quality of a manufacturer’s membrane and so enable an informed decision to be made on the selection of a membrane.
Important factors to consider are cost of materials, installation time and scheduling. It is valuable to consider whole-life costs and not just a purchase cost or even purchase plus installation cost.
A lightweight green roof solution
Extensive green roofs
Specifying a single ply system beneath an extensive green roof provides a lightweight and cost-effective solution for projects delivering environmental benefits. PVC and TPO membranes perform well beneath an extensive green roof as their formulations can easily incorporate compounds that provide root and micro-organism resistance and UV protection.
Extensive green roofs are designed to be lightweight and support a low maintenance vegetation landscape which is wind, frost and drought resistant. They are not intended for general access or for leisure purposes and are primarily used for their ecological benefits or aesthetic appearance. There are a number of options available offering different substrate and drainage/reservoir board depths to support the widest possible range of plant species within the scheme.
Sedum or wildflower blankets are a lightweight option with pre-cultivated vegetation for instant planting of the roof. Substrate-based systems are also a lightweight option, usually incorporating sedums with other self-sufficient plants.
Three principle options are available for fixing thermoplastic single ply membranes
This method allows the membrane to be fixed accurately and quickly, it is ideally suited for metal and timber deck constructions and can also be used on concrete decking, though one with a high proportion of stone can be difficult to drill through efficiently, but not impossible. The aesthetic appearance of the underside of the structural decking can be compromised, but the inclusion of an internal ceiling readily solves this issue.
A wind uplift calculation in accordance with the UK National Annex to Eurocode 1 — Actions on Structures : General Actions: Wind Actions BS EN 1991-1-4:2005 will ensure the correct fastener density is specified as installing a roof with an insufficient number of fasteners can increase the chances of detrimental wind action and any resultant damage.
This option can be used effectively when the structural decking is either difficult to mechanically fasten into, or the use of fasteners will compromise the air leakage or internal aesthetics of the project. The adhered system can be used with either a fleece-backed membrane or the standard non-fleece-backed membrane.
With a fully adhered system the membrane is glued to the substrate which can be straight to the deck in a cold roof construction, or to the insulation in a warm roof construction, or to an existing system in a refurbishment overlay situation.
Adhered systems still require an amount of mechanical restraint around the perimeter of the roof area, at changes of slope and around details to assist in the resistance of wind uplift.
The finished appearance of an adhered system is generally more uniform than that of a mechanically fastened system as the membrane is fully restrained over the whole roof surface. However, the appearance will of course be affected by the substrate it is being installed on to.
Hot air welding
Hot air welding
There are principally two types of welding equipment, hand held and automatic machines. Membranes will fuse together at temperatures of 380-500°C to provide homogenous joining as they cool.
Hand-held equipment requires 110v power supply and is used at all detailing on the roof as it allows for flexibility of positioning. It is not appropriate for the span of a large roof due to slow application times.
Automatic welding machines are larger and incorporate a heavy roller which follows the welding point and applies pressure to the lapped membranes to ensure a secure weld. The machines require a 240v power supply and can weld an average of 2.5 metres of lap join a minute.
This finish is specified in an inverted roof construction or where wind uplift is a risk as the membrane is loose-laid and then held on the deck by the weight of the ballast, paving or green roof with its associated soil/substrate and vegetation. If a ballasted system is to be incorporated, it needs to be considered at the design stage as provision needs to be made for the supporting structural deck and the associated cost implications.
Where loose-laid membranes are secured against wind uplift by ballast, the areas beneath the ballast may require different properties from the exposed areas. The covered membrane will need to be resistant to bacterial attack but will not need resistance to ultra violet light, while the reverse is true for the exposed membrane.
Securing the laps
Seam strength has always been a feature of thermoplastic membranes. The application of heat through hot air temporarily changes the material from a solid to a semi-solid state so that when pressure is applied the two adjacent pieces will fuse together and return to a solid upon cooling to form a monolithic seam. This has proved a reliable method of welding a watertight joint.
An alternative for PVC membranes is solvent welding where the solvent is wiped between the two pieces of membrane and chemically reacts to fuse the joint.
Temporary protection of the completed roof to following trades
Full temporary protection of the roofing is essential if following trades are to use the finished roof as a working platform or access walkway. This is the point of greatest risk affecting the integrity of the waterproofing system, through damage before hand-over or early failure. The responsibility for ensuring this must be agreed between relevant parties during the course of the building operations.
In general, no building work should be carried out from a completed roof. Paint, cementatious materials, plaster and solvents should not be allowed to come into contact with the completed roof surface.