CPD articles

CPD: Single ply roof systems

4 April 2014

If you're looking for a roofing solution that combines ease of installation with durability and sustainability, a single ply system could fit the bill. Pete Hollingworth outlines the key considerations in specifying it.

There are a number of considerations that specifiers must be aware of when designing and selecting single ply roofing solutions. This Sika-Trocal CPD module will look at the benefits, the applications, and the methods of attachment, before examining key factors in roof design and product selection such as falls, type of membrane and sustainability credentials. It will also indicate the key standards and guidelines that specifiers should use to help them to assess products.

The benefits

Single ply can provide one of the best all-round solutions to achieve cost effective, durable and sustainable systems. Choosing a single ply system and designing to best practice will provide solutions that offer:

Applications

Single ply can be used and designed into many forms of roof construction. It is most commonly used in new build but is increasingly being selected for refurbishment. Single ply can be adapted into warm, cold and inverted deck roof constructions. A “warm” roof design is the most widely specified form of roof as the risk of condensation is greatly reduced.  Single ply can also be overlaid with pavers, ballast, timber decking and a green roof finish.

Single ply is not only associated with flat roofs but is very often  considered on feature roofs such as pitched, waveform and curved. Furthermore, the aesthetics of single ply can be enhanced to imitate a more traditional look such as standing seam or lead roll.

Methods of attachment

There are different methods to restrain single ply against wind uplift forces: either by mechanical fasteners, the dead weight of ballast, or adhesives. In the case of mechanical fasteners, the number of fixings required will be determined by calculation in accordance with the latest Eurocode standards.

For ballasted roofs, ballast should consist of rounded gravel 20-40mm diameter laid at a minimum rate of 80kg/m2 and 50mm thickness, or pre-cast concrete pavers of 50mm thickness.

The use of timber decking can also be considered as a ballasted application, but due to the lack of sufficient weight, the membrane would need to be secured.

With a green roof, again there is a minimum weight requirement of 80kg/m2 to hold the membrane system below.

Green roofs are increasingly becoming a prerequisite for planning approval, either through biodiversity action plans, flood risk reduction as part of sustainable drainage systems, or legislation. The main types of living roofs are:

In the case of adhered roofs using polyurethane adhesives, special fleece-backed membranes are needed to provide a key for bonding to the substrate. Adhered applications can be limited by the delamination resistance of the selected insulation board or due to wind uplift exposure determined mainly by height and location.

A closer look at the features and benefits of each attachment method will help specifiers choose the right system for their project.

Mechanically fixed solution

The exposed, mechanically fixed membrane roof is the most widely specified option and is generally the fastest and most economical method to use, especially where the disc fixed method are used when both membrane and insulation is secured with the one fixing, where their locations coincide.

This fixing method can potentially minimise the amount of fasteners by up to 50% and reduce labour costs when compared to traditional single ply mechanical fixing methods.

Additionally, because it is lightweight, this will often enable savings to be made on the supporting structure.

The homogenous nature of the membrane also allows the use of “plastic memory”, so that when installed a limited amount of self-tensioning will take place, removing the creases that are impossible to avoid during installation.

Adhered solution

An adhered membrane really lends itself to refurbishment applications where the use of mechanical fasteners is inconvenient because of the type of existing structural deck. For example, fixing into concrete can be slow and tedious, whereas fixing to a woodwool slab may not be possible if the reinforcement channels can’t be found to fix into.

If the existing roof finish is of a bitumen or asphalt nature and is firmly secured but has come to the end of its life, an adhered single ply membrane can be applied directly as an overlay or, more commonly these days, adding insulation with the use of cold applied adhesives – eliminating the use of “hot works” and minimising disruption to the building.

Fleece-backed adhered membranes can also provide an enhanced visual finish to the roof, minimising the “ghosting effect” of insulation board joints coming through on the underside of the membrane.

Ballasted solution

Ballasted single ply membranes are highly resistant to ageing, pollutants and the bacteria that generally flourish at the membrane level. Additionally, the membrane and its joints are root resistant, which makes it suitable for living/green roofs. These membranes will be reinforced PVC which is designed to eliminate shrinkage and provide dimensional stability from the loads imposed.

Clients have the benefit of protecting the membrane in a variety of ways, including pavers, stone ballast, timber decking and living/green roofs. The key benefits of having a ballasted roof would be improved fire, acoustic and life performance.

Other design considerations

Once you have decided on what type of system to go with, there are other design considerations that play a significant role to ensure you get an efficient single ply roof that is maintenance free, in line with current regulations and ensuring safety for those that access the roof.

The standard for the design of flat roofs is BS 6229. To encourage the removal of water from the roof surface, BS 6229 recommends that no part of the finished roof should have a fall of less than 1:80.

All weatherproofing upstands occurring around the roof area, such as doorways, openings, rooflights, vents and plinths, must have a minimum height of 150mm measured from the finished roof surface. In the case of a ballasted roof, you would take the 150mm from the top of the ballast surface.

Achieving current U-value requirements within the Building Regulations is simple within a single ply system, in fact many projects are constructed with thermal performance far above the regulations’ requirements. Single ply systems can accommodate uniform and tapered rigid insulation boards in PIR, mineral wool, expanded polystyrene (EPS) and extended polystyrene (XPS) in thicknesses of 200mm plus, achieving U-values below 0.08W/m2K.

Through the use of a combination of a long plastic tube and short coated carbon steel, thermally broken fasteners offer a simple yet effective method of reducing thermal bridging and provide economical attachment methods.

With the use of vapour control layers, air leakage at junctions and penetrations can be minimised using simple standard practice.

The fire performance of a roof construction is determined in accordance with  European standard DD CEN/TS 1187:2012 and BS 476 Part 3:2004. Single ply systems can achieve unrestricted use under the Building Regulations but always check with your local Building Control office as they define the fire ratings required.

Where access is required to the roof for maintenance, it is vitally important consideration is given as to whether further protection is needed. Single ply membranes on their own can resist light foot traffic. However, in situations when there is rooftop plant that requires frequent inspection and/or maintenance, then a walkway system is required to contain the effects of that traffic.

Detailing options

The correct detailing of single ply membranes is key to ensuring a long-term durable solution. Manufacturers should be able to provide standard details that cover the most common details architects and designers face including  parapet, penetrations and abutment details. It is recommended that advice is sought on non-standard details. Again, a competent manufacturer should be able to assist and provide guidance.

Quality suppliers

The building owner should expect to be provided with a proven, long term, trouble free, reliable roof. In addition, quality manufacturers of single ply should be able to demonstrate the following:

Manufacturer expectations

Specifiers should expect a high quality service from manufacturers, which should include:

Integrated system components

Manufacturers of single ply membranes should also be able to provide a range of ancillary products that can be integrated into the roofing system. These items, when specified, can then be included under the manufacturer’s product warranty.

These include fasteners and adhesives to attach exposed single ply membranes; vapour control layers, both polyethylene and bitumen based; drainage outlets to drain water through conventional gravity outlets or via parapet walls with the use of scupper outlets.

Also, membrane laminated metal for drip details and to form flashings to abutments, upstands and around penetrations; walkway systems for follow on trades/maintenance; and decorative profiles to mimic traditional roofing.

Specifiers can also check industry accreditations and it is important that manufacturers are associated with them to show a commitment to maintaining standards and codes of practice. 

Sustainability

Manufacturers’ environmental and sustainability credentials are also under scrutiny and this is only going to receive more focus and attention in future. Single ply in a built up system can contribute to a positive BREEAM rating for a building. If referring to the BRE Green Guide to Specification, it can achieve an A+ rating on specific constructions. Single ply manufacturers should all be manufacturing to ISO 9001 and specifically 14001.

Single ply PVC can be recycled back into new production, however, the percentage is very low to ensure and maintain higher quality.

Single ply green roofs are a perfect example of maximising sustainability to roofscapes, reducing CO2 levels in cities and reducing the water management burden in flood risk areas.

Single ply can create cool roofs with white or light coloured membranes that increase solar reflectivity, which in turn reduces electricity usage in buildings running air conditioning. Single ply, if properly protected, can also support a wide variety of solar thermal and solar PV solutions.

Pete Hollingworth is national sales manager of Sika-Trocal. For further information contact Sika-Trocal on 01707 394444, email sika-trocal@uk.sika.com or visit www.sikaspec.com

 

 

 

 

Sika-Trocal provides flat roofing membranes suitable for a wide range of applications including new build, refurbishment and green roof specifications.

Speed, choice and efficiency are at the core of the Sika-Trocal concept. Designed to enable the delivery of buildings with low maintenance roofs and better environmental performance, Sika-Trocal uses lightweight, durable and flexible membranes to ensure a fast, safe and economical installation process that is suitable for most roofs.

Sika-Trocal’s range of roofing membranes have been used on a number of high-profile roofing applications, across a variety of sectors, including the Billingham Forum Leisure Centre and Blackpool Sea Life Centre; Trafford General Hospital and Ronald McDonald Teenage Cancer Trust Ward at Birmingham Children’s Hospital.

Sika-Trocal area technical managers offer design assistance and advice on product selection, while site monitoring is carried out by dedicated Sika field technicians.

Comments

Having worked or being involved in the major repair and maintenance of many such type of roof coverings for 35 years prior to my early retirement in 2010, I found that Trocal was essentially a good product until having to find roof leaks on those roofs where the disc fixing method was used. Every seam had to be closely inspected for a defective adherence of the hot welded joint and after a while we found that the defect was nearly always down to detail areas corners and up-stands etc. Therefor, the reliance on continuous waterproofing was predominantly down to workmanship or design of details etc.
We also found that the roof surfaces had a tendency to be very slippery when wet.
Again, many years ago, used this product to line out and waterproof an old Public swimming pool where a previous GRP lining onto a tiled surface had failed. This proved successful in the first instance but failed shortly after completion again due to poor workmanship.

There is not a great deal of information in the CPD issue regarding walkways etc but it is essential as with all other types of roof access points these walkways must be marked out and highlighted and if possible treated with a slip resistant coating or other compatible product.

R Standaloft, 7 April 2014

Thanks for this helpful article, much appreciated. Just a small point, I believe that XPS is extruded polystyrene and not extended polystyrene.

Jason Kelly, 21 April 2014

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