“The cheapest, most reliable source of new energy is saved energy,” says Peter Turnbull of PG & E. The promulgation of SANS 10400 XA requires that insulation should be installed to achieve prescribed R-values in the roof space.
The obvious methods of installing insulation are to lay it under the roof covering or above the ceiling. If laid under the roof covering, the logical response is to lay it over purlin.
However, the use of insulation of any bulk presents issues of stability for the roof sheeting, and the performance of insulation, bulk or rigid insulation in particular is questionable if it is severely compressed.
If used under pierced fixed sheeting such as IBR, the fastener is longer, and is put under high stress. If used under concealed fix sheeting, the concern is that a thick blanket insulation will bulk up under the sheet and could force the sheet off the clip securing it.
Various methods have been developed by the roofing industry to address these challenges:
- U-shaped stirrups or spacers have been used. They press through the insulation and rest on the purlin, providing some structural support to the fasteners and the roof sheet. However, stirrups are laborious to install and are prone to rock. They also do little to reduce the stress on the longer fasteners and do not adequately support the span of the roof sheeting.
- Another commonly used method is to install a row of timber purlins or packers, which are fixed to the lower row of purlins over the insulation. Not only is this practice a fire hazard, but it results in excessive compression of the insulation. And seldom is a timber spacer in perfect alignment or of perfect dimension with all other packers, providing a sub-standard base for the roof covering, compromising the performance of the insulation.
Various other methods include metal straps, secondary metal purlins and more.
Every method above faces the same unresolved issue
For insulation to perform to its optimum level, a continuous layer must be installed with minimal compression or thermal bridging. None of the above methods achieve this, and the performance of excellent insulation material is compromised by the method of installation. Architects and home owners do not achieve what is claimed or being paid for, and the entire thermal rating of the building is downgraded.
However, a properly engineered roof spacer system such as Ashgrid will, by the creation of a defined cavity or roof space, accommodate blanket insulation at its full depth. Such systems are pre-engineered for rapid and easy installation, and have varying leg heights for various depths of insulation. The resulting R-values of the insulating material may therefore be taken at full design value as claimed by the manufacturer.
Tests on the performance of blanket insulation over Purlin
Safintra South Africa recently undertook tests to determine the performance of an insulation blanket installed with their respected Saflok concealed fix system.
A 135mm fibreglass blanket with a density of 12kg/mᶟ was installed under the sheet and over purlin. Straining wires were installed with sufficient slack to allow the insulation to maintain as much loft as possible between purlins, within practical considerations. Saflok 700 was installed as the roof covering.
Two methods were compared in the test. The first test was conducted without a roof spacer system. The second test was conducted using the Ashgrid roof spacer system.
The tests clearly showed that
- Even without a roof spacer system, the integrity of the Saflok system was unimpaired, with the clips securely locking the sheeting over the blanket which had compressed to 15mm
- However, compression of the insulation blanket at the purlin compromised the thermal performance of the insulation by almost 40%, whereas the roof spacer system allowed the insulation to perform to its full design value.
There is no doubt that the cost of roof construction has been increased by the requirements of SANS 10400:XA. This cost increases by the cost of insulation materials and labour at time of construction, but is more than amortised by energy savings over the first seven years of the occupied life of the building.
The cost of a roof spacer system with properly performing insulation is in most instances actually lower than the cost of additional layers of insulation to achieve the required R-value in the roof space.
Speak to Safintra about roofing systems that deliver as they should – for owners, for occupants and for the environment.
Safintra South Africa
Tel: (011) 323 6300
Website: www.safintra.co.za / www.safintra.com
Safintra explains the technical details and installation options when constructing a concealed fix roofing system.
The metal roofing market has seen a spike of growth and product innovation in recent years, driven in large part by design developments within the construction and architecture industry, often based on considerations of sustainability. Concealed fix systems in particular, used for large-span metal roof applications, have improved their engineered performance to provide a high-performing and durable solution.
Given the large scale of investment buildings often covered by a concealed fix roof system, specifiers need peace of mind regarding the engineering performance of the entire roof system if they are to serve the long-term interests of their clients and reduce liability for all concerned.
When a concealed fix roofing system is installed, the sheets are fixed to the purlin with a clip system, and are not pierced with any fasteners. The clips that are needed to fix the sheets to the purlin serve two purposes – they circumvent the need for holes in the sheet (which prevents the possibility of leakage through the perforation) and they allow the roof to expand and contract in a sliding movement over the clips.
By being able to move over the clips, the sheets can rolled in long unbroken lengths which can thermally expand without restriction. Without concealed clips, the sheets would be positively fastened through the sheet into the purlin and the fasteners would be subjected to the entire force of the expansion; this force is considerable, and either the fasteners will tear larger holes in the sheet, or shear and break. The continuous lengths of a concealed fix system also obviate the use of end laps which further reduces the risk of leaks.
How coated steel expands in roofing systems
Coated steel expands at a nominal 1mm per 1m, and it is apparent that a sheet of a mere 15m will move by 15mm in heat. Best-practice installation is to not install a pierced fix sheet in lengths of more than 15m. If the sheet is over 15m, a pierced fix sheet should be end-lapped to allow for thermal cycling.
Concealed fix roof profiles are aesthetically pleasing and functional
The profiles of concealed fix roofs are both aesthetic and functional. When designing a low-sloped roof, a defined rib and a wide, deep pan are needed in order to deliver excellent water run-off at slopes as low as 2 or 3 degrees. The height of the rib is important as it also provides structural strength, acting rather like a mini I-beam.
The design of the clips is also something that needs to be considered as this is the “anchor” that will secure the sheet to the sub-structure and hold it down in high winds. The strength of the clip will be determined by the material that is used as well as the thickness of the clip, which in turn helps to determine the minimum wind-uplift resistance of the system.
Consider the complete roofing system before specification
The roofing system design will define the manner in which the clip locks into the sheet profile. Every rib of the profile should be engaged by the clip to prevent wind cushioning under the unsecured rib, and distorting the sheet off the clip. Generally, the more ribs that are secured with a positive clasp holding it to the purlin, the stronger the wind uplift resistance. If ribs aren’t positively secured, they should be shaped to provide a strong and stable interlock with the clip at every point. A complete system will also offer a high wind load detailing for particular terrain categories or design parameters such as wide overhangs.
This is not right….
One of the most widely specified systems available in the market is the Saflok™ concealed fix roofing system. The precisely engineered clip and the dimensional consistency of the Saflok profiled sheet provide a robust and reliable roof. Over 10-million square metres of Saflok™ concealed fix roofing products are installed in South Africa alone.
Specifiers and design professionals are encouraged to request full technical details of the system they intend to specify – and request technical support throughout the installation itself. Safintra offers this service free of charge to all customers from the company’s seven branches in South Africa (Johannesburg, Durban, Cape Town, Polokwane, Nelspruit, Port Elizabeth, Bloemfontein). Safintra recently appointed an agent for Namibia, operating from Windhoek. Further operations include Mozambique, Zambia, Malawi, Tanzania, Kenya, Angola, Uganda and others.
Safintra Roofing & Steel
Tel: 011 823 4027
Fax: 011 823 4288
This article is part 1 of a series on sustain-ability in the built environment, aimed at professionals, specifiers and owners looking to walk the talk in using green building practises. They focus specifically on the role played by metal roofing and cladding systems in creating better buildings.
PART ONE: Site-assembled built-up metal roof systems for optimal thermal performance
Steel is one of the ultimate building materials for sustainable buildings: light, very strong, long lasting and design versatile and above all, 100% reusable, it can be fully recycled after its useful life.
A metal roof system made up of a series of everyday components can outperform any alternative systems for bespoke and specific thermal properties. Site assembled systems are built up from their constituent parts on the building site. Critically, this system can be used for a new build as well as a retrofit over an existing metal roof.
The assembly consists of a steel liner sheet, a layer of insulation material, a bar & bracket spacer system and an outer profiled steel weather sheet, as illustrated.
Built-up cladding systems are supported by conventional secondary steelwork (purlins or side rails), at the published rates for spanning of the cladding sheets (typically in the order of 1.5m to 2.3m depending on the applied loading) so no extra sub-structure is required.
- Weather Sheet
- Liner Sheet
Liner sheets are simply a profiled sheet of coated steel with a shallow trapezoidal profile and a sheet thickness of anything from 0.3mm-0.55mm.
The thickness of the liner sheet will depend on the required trafficability, spanning capability, the cladding installation method and the acoustic requirements of the cladding.
Where required, the acoustic performance of the cladding, in particular its ability to absorb internal sound and minimise reverberation, may be enhanced by the use of a perforated liner sheet.
Thinner gauge liner sheets are not strong enough to walk on, so it is essential that the insulation, spacer system and weather sheet are installed from access panels. Access panels are simply a few thicker profiled steel sheets with the same profile as the liner. These are loose laid and nested over the liner sheet and used as a platform. These panels can be equipped with adhesive non-slip strips. These sheets are shifted along the roof plane as work progresses.
Whether trafficable or not, the steel liner sheets still provide an excellent non-fragile barrier against falling once they have been fully fastened.
The Spacer System:
The primary function of the spacer system is to support the weather sheet at the required height above the liner sheet. The components of the system must, therefore, possess sufficient strength to safely transmit the required loading through to the primary purlins, without deformation.
The bar & bracket system consists of steel bars (1), which provide continuous support to the weather sheet, supported at intervals by steel brackets (2) firmly attached to the purlins through the liner. The bracket foot (3)must incorporate an EPDM pad (which acts as a thermal break) to minimise thermal bridging.
NOTE: In the absence of an engineered spacer system, the structural integrity of blanket insulation or rigid insulation boards installed over purlins are entirely reliant on the roofing fasteners and packers. The design lengths of the fasteners are often exceeded whilst the insulation and packers do not offer them adequate support. This places the entire roof assembly at risk.
Glass fibre or mineral wool blankets, and are favoured due to their light weight, low thermal conductivity, acoustic performance, ease of handling, cost effectiveness and fire-performance.
Glass fibre & mineral wool blankets are flexible which ensures that gaps between the insulation are eliminated during installation, preventing thermal bridging.
The outer ‘weather ‘ sheet:
The outer profiled metal sheet of a double skin built up cladding system is known as the weather sheet. While protecting the building by forming a weather tight envelope, it also is a structural element, as it plays an important role in transferring externally applied loads, e.g. from wind, hail, snow and foot traffic, through to the secondary steelwork and the primary load bearing frame.
The metal weather sheets are available in a wide variety of profiles, finishes and colours.
Benefits of Built Up Systems
Built-up steel systems offer savings of approximately 10% to rigid insulation board applications of equal r-values. They also offer savings by fast tracking the critical path of the project.
- Cost Effective
- Lightweight, easy to install: fast method of construction.
- Secure and waterproof at an early stage of the build programme - internal works can continue within a waterproof environment.
- Efficient: R-values up to 7.0 m2.K/W can be achieved, and the system offers consistent thermal performance for the life of the building.
- Improved structural performance: The liner sheet and engineered bar & bracket spacer system provides restraint to the steel purlins thus ensuring rigidity of the roof structure.
- Fire Performance: Glass fibre and mineral wool deliver an A/A1/1 fire-rating with no restrictions.
- Acoustic performance. The acoustic performance of steel built-up systems far exceeds that of rigid insulation boards installed over purlin and eliminate common problems such as rain drumming.
- Improved security due to a second steel skin within the roof assembly.
- Recyclable and reusable: As the individual components are not bonded, recycling of the materials after the buildings operational life does not present the potential ecological impact presented by foam cored composite panels.
Safintra manufactures metal sheeting in a variety of profiles to suit most applications and designs. Technical advice is gladly offered at design and installation stages of the project.
Standing-seam metal roofing offers a durable and sustainable alternative to other roofing types – and can provide low-maintenance service for 5 to 10 decades. Sadly, this exceptional lifespan is often sabotaged during the mounting of essential rooftop equipment.
It is widely agreed that the best way to prevent roof-related problems is to keep the rooftop clear and let it function as a roof, rather than as a platform for equipment. However, the reality is that HVAC equipment, piping and walkways are common roof mountings, not to mention fall protection, plumbing vents, signage and more.
The first rule about any metal rooftop mounting is to avoid penetrating the membrane whenever possible. While this may seem obvious, it is often violated, particularly with standing seam (concealed fix) roofing on commercial and industrial structures. The norm seems to be to anchor the item to the structure through the roof. This practise not only threatens the sheeting weather integrity, but can also violate the metal’s natural expansion and contraction by inadvertent pinning of the sheet to the structure. Within a short time, such a point of attachment will fatigue and fail from forces of thermal expansion exerted by the metal.
The better practise is to use special seam-mounted clamping hardware that grips the standing seam without penetrating the membrane. Unlike many other roof materials, metal is a rigid high tensile material and the seam creates a beam-like anchorage that can be used to secure most items without harming the roofs weather-proof performance. Mechanical equipment can be safely and cost-effectively secured to these seam clamps, leaving the roof penetration-free, and allowing for its thermal cycling behaviour.
Clamps must be made of non-corrosive materials, typically aluminium and stainless steel, to last the full life cycle of the roof. It is also important to remember that any loads introduced to the clamps will be transferred to the roof sheets and to their anchorage to the structure. The anchorage must therefore be capable of carrying the added load. Best practise is to utilise clamps that have been appropriately tested for material and seam-specific holding strength. The roof panel manufacturer should also be consulted with respect to approval of the devices used.
Safintra are the sole distributors of S-5 clamps for standing seam roofs. They are the widest selling roof clamp brand in the world, proven over decades in the field. S-5 clamps provide penetration-free attachment to Saflok and most standing seam profiles, and are warranted for 25 years. A range of S-5® brackets are also available for virtually any pierced-fix metal sheeting profile.
All S-5 components are made from solid aluminium. Additional hardware, such as bolts and plates, are made of non-corrosive 300 series stainless steel. These materials are totally compatible with aluminium-zinc coated metal sheeting, sold in South Africa as Colorplus, Colorbond, Zincal or Zincalume.
Sally Stromnes, Group marketing manager of Safintra, says the holding strength of the S-5® clamp is unsurpassed, with ultimate load-to-failure rates averaging more than a ton and in some clamps, over four tons. Ask Safintra for their test results for complete peace of mind.
Speak to Safintra about their warranted roofing systems, which deliver returns for the owner and the occupier for the full occupied life of the building.
Safintra South Africa
Tel: (011) 323-6300 (JHB office)
Website: www.safintra.co.za and www.safintra.com
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