Lightweight framing

A lightweight frame is like a skeleton to which exterior wall claddings, internal linings, flooring, roofing, windows and doors are attached. Lightweight framed construction is the most common construction system used in Australia. Framing systems can be varied to suit almost any design or construction system, provided engineering certification is obtained.

Lightweight houses are well suited to above-ground construction to minimise site disruption. Lightweight floor frames can support internal and external wall, floor, and roof loads on low-impact footing systems. Framed structures lend themselves to the creation of houses with diverse window and door openings for passive solar heating, natural light and ventilation. The lightweight house is particularly suited to creating cost-effective, flexible design solutions for challenging sites and reactive soils.

The 2 most commonly used framing materials – timber and steel – can be made into comfortable and appealing homes with good environmental performance:

• Timber from approved sustainable sources provides a renewable building material that takes in carbon from the atmosphere while growing and stores it for the life of the building. Certification schemes for sustainability include Programme for the Endorsement of Forest Certification (PEFC) or Forest Stewardship Council certification (FSC). Timber is vulnerable to termite attack, and rot and mould from condensation are also becoming a threat as we seal our homes and increase insulation levels.
• Steel-framed construction began to be adopted in the 1940s and continues to be popular especially in areas of high termite risk. Typically, the frames have an extra polymer protective coating to minimise rust, which improves durability. Steel production requires large amounts of energy, but steel is 100% recyclable. Note that a steel frame does not remove the need for termite protection to the home. Steel framing must be appropriately insulated or it will act as a thermal bridge, increasing the risk of condensation.

Photo: Getty Images

Photo: Maeli Cooper

Buildability, availability and cost

Lightweight framed construction is a common and relatively simple building system. Trade skills and availability are abundant.

Timber frames are traditionally erected by carpenters who are skilled and tooled up to work with timber. Steel framing requires different skill and tool sets. It is possible to use both materials within the one building, selecting each for its most efficient application, such as timber wall frames and long span steel truss first floor joists. Some manufacturers combine both into the one structural product for maximum efficiency.

Lightweight framing in both timber and steel offers design freedom at a competitive cost. The growing range of engineered solutions continues to extend design possibilities while reducing resource consumption. Lightweight framing also allows future adaptive changes to be made more readily to the building structure with minimal material impact and lower cost.

Appearance

Lightweight framed homes can range in appearance from the traditional weatherboard bungalow to the ultra-modern metal-clad home. With the enormous variety of non-structural claddings, linings, and finishes available, lightweight framed construction can be used to create almost any desired architectural form or finish (for example, reconstituted hardwood panels and boards from forestry waste, fibre cement sheet, plywood, timber weatherboard, brick veneer, rendered external insulation systems, or composite metal panels).

Photo: Bluescope Steel

Photo: Bluescope Steel

Structural capability

Lightweight framing materials have good compressive strength (ability to resist compression). They can also be used for large spans and cantilevers (overhang beyond a point of support). Steel has greater tensile strength than timber (resistance to structural failure under tension), allowing longer spans and cantilevers or thinner structures.

Timber wall frames are typically either 70mm or 90mm deep with 35mm or 45mm thick studs depending on load and spacing – usually 450 to 600mm. Noggins are installed between studs to provide lateral support, typically at either one-half or one-third wall height intervals. Top and bottom plates are typically 90×45mm and can be double thickness depending on the load (for example, first floor, tiled roof, long truss spans) or the spacing of the supporting floor members. Openings require overhead lintels (beams).

Steel wall frames are also typically 70mm or 90mm deep, and additional strength is achieved by using thicker gauge steel or additional folds or bends in the cross-section. Stud spacings and noggin's are similar to timber. Heavier hot rolled steel sections are often still incorporated into lightweight steel structures to take heavy loads over large spans or high point loads.

Engineered timber and steel structural designs maximise structural capacity, while minimising material use:

• Engineered timber products use adhesives to bind strands, particles, fibres or layers of timber to form a stronger composite material. Materials include engineered timber products such as plywood, particleboard, glue laminated timber (glulam and LGL), laminated veneer lumber (LVL), oriented strand board (OSB), and finger-jointed cladding or fascia.
• Engineered steel solutions use a wide range of steel shaping options such as hot or cold rolling, bending or folding to gain maximum structural advantage from minimal steel thicknesses. Thin steel sections are strengthened with folded profiles (for example, Z and C purlins, top-hat battens and C-studs, folded I-beams and others). Hot rolled sections (for example, U-beams, angles, I-beams and channels) have thicker steel in high tensile or compressive load sections (flanges) connected by thinner webs where forces are neutral.

Hybrid products that use timber top and bottom flanges connected by steel webs make best use of the structural benefits of both materials and can result in more efficient and useable product outcomes.

Longer spans and greater structural efficiency can also be gained by using products that combine base materials in innovative ways. For example, I-beam joists may use solid or LVL timber for the flanges, and either plywood or OSB in the web. Likewise, truss web joists are available that use cold formed light steel triangular webs between solid timber flanges, providing exceptional stiffness and spanning ability with the ease of nail-fixing to timber. These also allow services to be installed through the open webs (with additional protection from sharp edges as required).

Bracing can be efficiently provided by fixing a suitable sheet to the inside or outside of the frame. This can be fibre cement, plywood, OSB, or hardboard (reconstituted compressed hardwood pulp), and must be manufactured for the purpose. It must also be installed according to the relevant Australian Standard, which always involves more fastenings than if it were used as a lining board. It has the advantage of providing much more rigidity to a structure in a shorter wall length. Keep in mind that because it is fixed to the face of the frame, it increases the effective thickness of that part of the frame by 5 to 10mm – cladding or linings must take this into account, usually achieved by adding strips of the same material to the remaining wall studs.

Photo: Paul Downton

Durability and moisture resistance

Timber is an organic material that deteriorates with weathering and is subject to mould and rot when exposed to water or condensation. While appropriate design detailing and rainproof cladding can protect timber from weathering, it does not protect it from condensation. Structural grade timber that is treated for termites is also more protected against rotting.

Steel is a very durable material and, if treated to appropriate levels for the corrosiveness of site conditions, steel framing can have a very long lifespan. Contemporary steel frames polymer coat the zincalume for added protection against rust (see Australian Standard AS 1397-2011 Continuous hot-dip metallic coated steel sheet and strip – coatings of zinc and zinc alloyed with aluminium and magnesium). Rust is possible if the coatings get damaged. Steel provides limited nutrients for mould growth but condensation absorbed by insulation can damage linings and cause corrosion.

Water vapour condensing as it passes through walls is a significant threat to lightweight framed structures, both timber and steel. Condensation problems are increasing in Australian homes due to higher insulation and air sealing levels. Steel framing must be appropriately insulated or it will act as a thermal bridge, increasing the risk of condensation.

Thermal mass and insulation

Lightweight framed construction has low thermal mass, which can be an advantage in some climates (for example, hot and humid) or sites with no solar access or cooling breezes.

Highly insulated, low-mass houses can respond rapidly and efficiently to auxiliary heating and cooling. When lightweight framing is to be used in climates where thermal mass storage is desirable, mass can be added (for example, with concrete slab floors, masonry feature walls, or phase-change materials.

Insulation in lightweight framing is typically placed between the structural elements (for example, studs, plates, noggins) and the achievable insulation levels depend on the depth (stud size). Bulk insulation should be placed neatly between structural elements and not be compressed – bulk insulation performance is reduced if it is compressed, because it is the trapped air that insulates, not the material.

While performance and thickness vary between different types of insulation, if using typical bulk insulation batts, then 70mm deep frames generally allow for R1.5 batts, 90mm deep frames provide R2.0 or 2.5, and 140mm deep frames achieve R3 or 3.5 (depending upon specific products). Upgrading the frame width may add to the cost of external frames, but the insulation increase will usually make this a worthwhile investment. The choice you make will affect the comfort and running costs of your home for years to come.

Thermal bridging through the frame between a cold exterior and a warm interior can cause cold spots and condensation. Thermal bridging can also occur between a warm exterior and a cold interior, thereby increasing the cooling load. Timber is a natural insulator and a poor thermal bridge, so will not require any additional thermal break in most climate zones. Steel is an excellent conductor and always requires an effective thermal break to prevent thermal bridging.

Sound insulation

Sound travels via floors, walls, and ceilings in lightweight framed buildings, both directly and by reflection, and each of these elements requires effective sound treatment to ensure acceptable outcomes. This means selecting components with the appropriate sound insulation rating, as well as framing that minimise transmission of sound.

Depth of cavity (the distance between the inner faces of the plasterboard) is a very important factor in the control of sound transmission. The greater the separation distance, the greater the soundproofing – so it is important to find the right balance between wall thickness and soundproofing. Many building material manufacturers have done extensive testing of their wall assemblies and created certified systems that take the guesswork out of it. The Australian Building Codes Board has published Sound transmission and insulation in buildings as a non-mandatory free downloadable guide. Sound absorbing insulation is most effective when the faces are structurally isolated.

Source: Forsythe 2004

Separation of structural wall components is one of the most effective noise reduction methods. Parallel wall frames with staggered studs or separated double stud construction are most effective. Resilient furring channels (for example, top hat steel battens) fixed to every second stud via a resilient clip are commonly used to achieve structural separation. Resilience allows the channel or clip to bend, stretch or compress and then regain its shape, absorbing or ‘damping’ rather than transmitting impact noise. Structural isolation systems or specialist acoustic isolating channels, made from sound absorbing or ‘high acoustic loss’ materials, increase acoustic control. Various systems are available from manufacturers.

Source: Gyprock Soundchek Systems for homes

Fire and pest resistance

Timber maintains structural integrity longer than steel which loses strength rapidly when exposed to heat. Where timber is used extensively in exterior applications and around the house, fire risk standards will apply. Australian Standard AS 3959-2009 Construction of buildings in bushfire-prone areas specifies categories of fire risk and defines compliance measures for each.

Termites are a major concern in lightweight timber constructions. A steel frame does not remove the need for termite protection to the home. Termites can still infest steel-framed homes, consuming paper linings to plasterboard, joinery and even books instead.

The 2 principal methods of dealing with the threat of termites are chemical and physical.

Current building regulations (Australian Standard AS 3660-2000 Termite management) emphasise managing termites through physical barrier systems and inspections rather than the environmentally harmful chemical methods that were common in the past.

Termites attack from underground, and the best risk management strategy is to design the house with physical barriers that prevent termites from concealing their tunnels. It is also important to leave an accessible space to allow for easy inspection of termite activity.

Other pests such as rodents and cockroaches can be controlled by ensuring that all cavities with connection to the building’s interior and within walls, floors and roofs are sealed. Ventilation and drainage cavities (behind the external skin of the building) must remain open to air, but can be protected from pests and birds by appropriately sized mesh. This may also be required by bushfire construction standards.

Toxicity and breathability

Timber requires treatment to limit termite attack and rot. Treatment is generally applied to either the surface or the outer few millimetres (envelope treatment), or all through the whole piece of timber during the milling production process before construction. There are various treatment options available:

• Light organic solvent preservative (LOSP) treatments offer low toxicity preservatives, fungicides and insecticides. Protection levels range from H1 (lowest protection) to H3 (exposed external applications). Treatment may be on the outer layer only, or through the full cross-section. LOSP is unsuitable for in-ground applications.
• Copper azole is an effective alternative preservative, fungicide and insecticide suitable for H1–H4 (in-ground) applications. It contains no high toxicity arsenic or chromium compounds and can be burnt and mulched. It is generally applied to the full section.
• CCA (copper chrome arsenate) solution is not recommended. Contact with CCA-treated timber may be toxic to young children and its use is banned in some product categories. It emits highly toxic smoke if burned and cannot be mulched. Any quantities, including shavings and offcuts, must be disposed of as landfill. CCA is effectively prohibited in Japan and some European countries and is being phased out in the United States.

Identify the risk factors that apply to your project before specifying or ordering timber or framing – there is no point paying for unnecessary protection that will not be useful in the building’s lifetime. For example, if an effective termite barrier is in place, it is worth considering whether extra termite protection on the frame is necessary (see Australian Standard AS3660 Termite management Parts 1 and 2). Likewise, do not save a few dollars under-specifying timber that will not last the lifetime of the building.

Glues and solvents used in internal grade non-structural engineered timber products often contain formaldehyde — a known carcinogen with adverse implications for human health. All structural and external grade engineered timber uses a formaldehyde-based glue that has extremely low emissions, to the extent that they approximate the natural formaldehyde levels in timber and are not of any significant health concerns for the average person. Some products now use non-formaldehyde alternative low volatile organic compound (VOC) glues – refer to Indoor air quality.

Steel products are nontoxic during a building’s life, but emit significant ecological toxins and greenhouse gases during production and processing of raw materials.

Environmental impacts

The main environmental impacts to consider in framing are carbon and the use of energy.

Timber has low embodied energy, and timber growth and production reduce atmospheric carbon levels by taking in carbon dioxide. Manufacturing rough sawn timber uses substantially less energy per unit volume than steel, concrete or aluminium. However, some chemicals used in plantations can adversely affect terrestrial and aquatic environments, and poor forest management can lead to increased methane production, soil and biodiversity loss and soil degradation. Looking for sustainable timber products through certification schemes will reduce these impacts.

Significantly lower volumes of steel are used in lightweight steel framing, than timber used in timber framing. Additionally, steel is 100% recyclable at the end of its life. Currently around 80% is recycled, a process that requires up to 70% less energy than the original manufacturing. New steel frames currently include 30–40% recycled steel (Carre 2011).

The build process

Typical lightweight construction consists of framed and braced structures with applied claddings. The process of construction may begin with a concrete slab onto which continuous frames are fixed, or with the placement of piers or pad footings to support piers, bearers and joists.

Lightweight construction allows simpler and more flexible footing design. Various proprietary systems can reduce footing costs and allow builders to ‘get out of the ground’ much faster than high-mass construction systems, which typically require more extensive earthworks.

Roof construction can use prefabricated trusses or more rarely these days, conventional framing. Trusses can be easily deconstructed in future for reuse on other projects. With lightweight framing, the bracing and tie-down detailing must be designed and installed to appropriate National Construction Code (NCC) standards, wind-loading standards, and seismic codes.

Timber components may be fabricated on site, but modern construction techniques generally favour off-site fabrication of trusses and wall frames. On-site fabrication of more complex designs can minimise costly mistakes and overcome small set-out errors or dimensional variations in slabs and flooring.

Footings

The building frame is supported by a substructure of piers, piles, stumps, posts, dwarf brick walls, or perimeter masonry walls. The substructure carries the load to the footings which, depending on local practice, may consist of sole plates of durable or treated timber on a concrete pad, or a reinforced concrete strip footing.

Innovative tensile and screw pile foundation structures can accommodate the most challenging of sites. The use of piers and posts can greatly reduce the need for cut-and-fill on sloping blocks.

Frames

Lightweight frames designed and built to the relevant Australian Standard are deemed to comply with National Construction Code (NCC) requirements. Areas subject to extreme wind conditions (cyclonic) or areas subject to seismic activity are subject to separate Australian Standards that detail additional fixing and construction requirements.

Joints and connections

Joints and connections are closely linked to tie-down and bracing requirements and vary substantially between steel and timber framing systems. Accredited wall fabricators usually design and certify these aspects with details that suit their system.

Qualified professional carpenters and joiners can usually choose proprietary fixing systems that will be approved by certifiers. It is critical that frame inspections by licensed, accredited certifiers are completed before the fixing of linings or cladding. Bulk insulation, flashings and condensation detailing are commonly inspected simultaneously.

Services

Lightweight framing systems generally simplify the installation of services through simple drilling to accommodate pipes and cables. Steel frames often include pre-notched openings that are easily removed.

Steel frames require cushioning grommets to protect cable insulation during and after installation, and to limit longer term damage to plumbing due to expansion and contraction or corrosion of the steel. Electrocution danger must be minimised by protecting extension leads from sharp edges during construction.

Care should be taken to ensure that trades do not weaken the structure by drilling near the edge (structural zone) of the stud. The need to centre services in frames can conflict with bulk insulation installation, and can result in the electrical cable capacity being de-rated. This often leads to compression of insulation or increased thermal bridging where gaps are left to meet electrical codes. Advanced cabling solutions and circuit design can overcome this. Alternatively, externally fixed foam insulation allows air space around cables.

Finishes

Finish is generally determined by the choice of cladding.

• Eco-comparison websites
  –    Australian National Life Cycle Inventory Database
  –    Building Products Information Rating
  –    Ecospecifier Global
  –    Environmental Product Declaration Australasia
  –    Global GreenTag
  –    Good Environmental Choice Australia
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• National Association of Steel Framed Housing (2005). NASH Standard – residential and low-rise steel framing, Part 1: design criteria. Hartwell, Victoria
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• National Timber Development Council (2001). Environmentally friendly housing using timber: principles, 1st edn, Forest and Wood Products Research and Development Corporation, Brisbane. 
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• Warnock A and Quirt J (1997). Control of sound transmission through gypsum board walls. Institute for Research in Construction, Canada.

Learn more

• Look at Construction systems to find out more about the options for framing your home
• See Insulation for more ideas on how to slow heat transfer through your home
• Explore Materials for ideas on methods and materials for new home construction

Authors

Original author: Chris Reardon

Contributing author: Tom Davis, Paul Downton

Updated: Chris Reardon 2013, Dick Clarke 2020