Timber is a central material to offsite construction and one of the leading mainstream, renewable, low-carbon building materials. Timber is regarded as potentially one of the most sustainable building materials to be used in a construction project. This is especially true if it is harvested from a forest that has been sustainably managed. This means that when the timber is harvested more is replanted in its place, allowing the forest to regenerate. Many now look for either an FSC® or PEFC™ certification on their timber to ensure that it was sustainably harvested. This is especially true of hardwoods, which if cared for properly can last hundreds of years making them a sustainable building material.
Structural timber has developed hugely in recent years. Timber can be used in several slightly different build systems. These include engineered timber solutions such as cross-laminated timber (CLT) and glulam, as well as timber frame, structural insulated panels (SIPS) and associated open and closed panel systems. Timber has impressive thermal and acoustic performance levels, as well as huge aesthetic appeal to architects and building designers.
The use of solid wood solutions such as CLT and glulam has revolutionised the way that timber is used in a multi-storey setting. Buildings across the globe are now part of the 'tall timber' phenomenon.
For more information on structural timber technology, please visit the Structural Timber Association.
Cross Laminated Timber (CLT)
Cross-laminated timber (CLT) is a structural two-way spanning solid wood panel product that can be used to form walls, roof and floor panels as well as shear walls. It is produced by stacking several layers of timber, known as lamellas, at 90º to the layer below and subsequently glued to create panels of up to 24m in length and 2,950mm in width, which can encompass between three and seven layers.
The structural benefits of CLT include its large bi-axial and flexural loadbearing capacity when used as a wall or slab, together with its superior acoustic and structural performance properties. CLT panels have huge structural potential and are suitable for most building types.
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Glulam is the more commonly used term for glued laminated timber. This is an engineered wood product manufactured from layers of parallel timber laminations. These are normally Spruce or Pine, but sometimes more durable timber species such as Larch, Douglas Fir or even hardwoods are used.
One of the biggest selling points of glulam is that is can be manufactured in a wide range of shapes and sizes. Beams wider than normal can be manufactured which can be very useful. These are made through laying boards of different widths side by side and reversing each layer so there is an overlap and no straight-through vertical joint.
Glulam is frequently used in the construction of all types of building, including supermarkets and swimming pools. With attractive design appeal, glulam beams provide large-span structures without supporting columns. Solid wood products such as CLT and glulam are more natural, renewable and far less energy intensive to produce than concrete or steel.
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Timber framing is one of the oldest known methods of building and was very popular in Britain till the 17th century. We can still see many of those buildings standing today which is a testament to their durability and resilience.
Modern timber frame structures are still today precision engineered, strong and durable. The build method relies on a factory manufactured timber frame as a means of structural support - carrying the loads imposed by the floors and roofs, before transmitting them to the foundations, which can be built at a lower cost due to the buildings being lightweight.
The construction of timber frame based structures utilises factory manufactured wall panels, floor and roof panels. The systems used are classified as either open panel, insulated or closed panel. These panels can include the wall insulation pre-fitted and can include the pre-fitting of doors, windows and service zones for onsite installation of M&E works.
Timber frame currently accounts for around 30% of all new homes being built in the UK. This build method is utilised by every sector of the construction industry including social housing providers, due to timber frame's superb environmental credentials, as well as being quick and easy to construct.
Timber frame can consist of wall panels, alongside floor and roof panels – often referred to as cassettes. These can also be open panel or closed panel.
Timber frame open panel systems, are structurally engineered panels that form the inside load-bearing leaf of the external wall, comprising studs, rails, sheathing on one face and a breather membrane.
The open panel system is made from treated softwood timber framing, over which a structural sheet material of either Ply or OSB board is fixed. Depending on the system, U-values ranging from 0.26 down to 0.15 W/m²K, can be achieved.
Closed panels also include plasterboard linings on the faces of the panel, a vapour barrier and breather membrane. A vapour barrier is provided on the warm side of the insulation and a breather membrane on the outer face of the panel. If desired, closed panels may also include fitted windows and internal service zone battens, for ease of installation and construction. U-values from 0.25 right down to 0.10 W/m²K can be achieved. These solutions have been designed to deliver excellent thermal and airtightness properties and are ideal as the basis of a modern energy efficient home.
Closed panels may also include fitted windows, openings for doors and service routes. Manufactured in factory conditions, these cassettes and panels are brought to the site and fixed together to form a rigid load-bearing superstructure. These consist of timber studs and beams, stiffened on one side with oriented strand board (OSB) and plasterboard.
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Structural Insulated Panels (SIPS)
Structural Insulated Panels are an advanced construction method. They deliver excellent structural and thermal characteristics in one system. SIPS have two parallel faces usually Oriented Strand Board (OSB) that sandwich a rigid core of Polyurethane (PUR) foam. The result is a lightweight system which is quick to erect and free from compression shrinkage and thermal bridging.
SIPS systems are used for walls roofs and floors. These deliver rapid and robust energy efficient buildings with superior insulation, structural strength and airtightness. Structural Insulated Panels offer extremely high thermal performance throughout the lifecycle of the building, whilst minimising wall thickness. The PUR core of rigid insulation and OSB3 facing panels achieve U-values as low as 0.10 W/m²K, making significant savings on operating costs.
This technology when used on residential applications reduces build programmes and enable houses to be built faster.
If the house design facilitates habitable living space in the roof zone – then SIPS is a fantastic solution, as they do not require roof trusses. This then ensures maximum space availability, whilst providing superb thermal performance and limited air leakage.
There are two fundamental applications for SIPs, a full structural wrap or infill walling. In all cases the product will be engineered for load bearing capability, racking resistance and wind loading requirements.
Often SIPs are specified as infill to steel, concrete or engineered timber structural frames. They can sit on either the inside or outside of the frame itself. Infill walling is very quick to install, making it an innovative solution for high-rise residential applications.
Full Structural Wrap
Structural Insulated Panels Systems can be used to form a loadbearing full structural wrap. Currently, five storeys are the maximum practical height for loadbearing SIP buildings. Above this, an additional structure needs to be incorporated into the building to carry the imposed load.
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