Sunday, 31 January 2016 08:50

BE THE CHANGE: Simple, Clever sustain-ability in the built environment

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.

  1. Weather Sheet        
  2. Bar        
  3. Bracket
  4. Liner Sheet        
  5. Insulation    
  6. Purlin    

Liner sheets

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.

Insulation:

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.

  1. Cost Effective
  2. Lightweight, easy to install: fast method of construction.
  3. Secure and waterproof at an early stage of the build programme - internal works can continue within a waterproof environment.
  4. 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.
  5. 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.     
  6. Fire Performance: Glass fibre and mineral wool deliver an A/A1/1 fire-rating with no restrictions.
  7. 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.
  8. Improved security due to a second steel skin within the roof assembly.
  9. 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.