Cost model: Residential timber

A familiar building material now has many innovative and scientifically developed forms, of which cross-laminated timber is perhaps the most compelling. Alex Hyams, Steve Watts and Catherine Harvatt of Alinea detail the costs of building with CLT

01 / Introduction

Timber has been used in construction for thousands of years, and timber framing is one of the oldest known methods of building. In Britain, archaeologists uncovered traces of timber-framed homes from more than 10,000 years ago and, comparatively recently, the building method saw a surge in popularity in Roman times and the Georgian period, encouraged by the sturdy, resilient qualities of the material and relative ease of construction.

Glued laminated timber, known as glulam, has also been used in construction since the mid-1800s, but the latest innovation in timber construction began in the 1990s with the creation of cross-laminated timber (CLT).

CLT is a multi-layered solid timber panel product used for the wall, floor and roof elements of a building’s superstructure.

In 1996, an industry / academic joint research programme was undertaken in Austria, which resulted in the development of what we know as CLT today. The large format, solid engineered, cross-laminated timber panel production was pioneered by companies such as KLH Massivholz, who in 1999 were one of the first companies to open a CLT manufacturing facility in Europe.

Although wood has been used in buildings for centuries, CLT panels offered – for the first time – a wood product to match the structural and technical requirements of more traditional building materials such as concrete. It took several years and extensive product testing for CLT to become recognised and accepted within the European market.

02 / Types of timber products

CLT

This cost model uses a CLT design for a residential building as its basis, but it is not the only way of using timber as a structural solution. Below is an outline of the main options.

CLT – also referred to as XLam, solid timber, engineered timber or massive wood – is a manufactured multi-layered solid timber panel product used for wall, floor and roof elements of a building’s superstructure. Made from kiln-dried finger jointed spruce or fir planks, the board layers (lamella) are laid out, stacked at right angles and glued together in perpendicular layers using high-pressure bonding.

The majority of CLT panels currently manufactured use polyurethane adhesive to glue the sheets together. The polyurethane adhesive is a solvent and formaldehyde-free adhesive “0” emission class), which has no risk of toxic emissions at any stage in the product’s lifecycle.

CLT panels can be manufactured in three, five, seven or more lamella, with typical thickness ranging from 80mm to 300mm, in various widths from 1.25m to 3.00m and lengths of up to 18m. In the UK it is recommended that the panels are no longer than 13.5m for transportation purposes (standard panel sizes vary depending on the CLT manufacturer).

CLT panels can be supplied with three different finishes to the outer layer of the panel: non-visual, industrial visual, and visual.

The number of board layers and panel widths will depend on the purpose and structural requirements of the element of the building for which they are designed. CLT panels are manufactured at full size as “master panels” and then cut to the size and shape of the individual elements required for the building – minimising the waste from master panels is called panel optimisation and clever panel optimisation can reduce CLT panel wastage and therefore cost.

The benefits CLT offers to structural applications include:

Glulam

Glue laminated timber (glulam) is an engineered solid timber product manufactured for load bearing purposes, predominantly used for columns, beams and truss components. Typically made of spruce, glulam is also made by gluing together lamella. Glulam is available in a range of both softwood and hardwood species including spruce, pine, birch, oak and iroko.

Timber lamellas used in the production of glulam are typically finger-jointed into continuous lengths. The thickness of the lamella will depend on the application and species used, typically 40mm with homogeneous and composite build-ups available. Once glued, the members are planed and cut to exact size and shape. The length and shape of glulam sections are only limited by the manufacturing and transport capabilities of the manufacturer, and it can be used in almost any type of building from simple dwellings to major public buildings and bridges.

Compared with CLT, glulam offers opportunity for use in a variety of situations requiring strength and spanning capabilities, because:

LVL

Laminated veneer lumber (LVL) is a high-strength engineered wood product used primarily for structural applications. It is comparable in strength to concrete and steel and is manufactured by bonding together rotary peeled or sliced thin wood veneers under heat and pressure.

LVL is used for structural elements including beams, lintels, purlins, truss chords and formwork. LVL can also be manufactured to almost any length, restricted only by transportation to site.

In LVL, the grain of each layer of veneer runs in the same (long) direction, so it is strong when edge loaded as a beam or face loaded as a plank. This kind of lamination is called parallel-lamination.

The veneering and gluing process of LVL enables large members to be made from relatively small trees, thereby providing for efficient utilization of wood fibre. LVL is used primarily as structural framing (posts and beams) for residential and commercial construction.

Roof cassettes

Roof cassettes can be specifically adapted to meet various physical and loadbearing requirements. The structure consists of loadbearing timber ribs, clad top and bottom and filled with glass wool or mineral wool thermal insulation. The moisture-adaptive vapour barrier, applied seamlessly across the full width of the cassette, ensures the desired performance of the building can be met.

The standard version of the roofing cassette is used for a maximum single span of 7.5m and is 160-280mm thick – depending on the load specification, but cassettes can be produced up to 22m in length, double or even triple spanning across the structure.

The design of the roofing elements allows for a wide range of coverings to optimally fulfil the specific requirements of the project team. In the construction of flat roofs and flat inclined roofs, it is possible to use PVC, FPO, EPDM membranes (rubber) and bitumen waterproofing (two layers) as well as more traditional corrugated steel or aluminium profile roofing systems.

Wall cassettes

Due to well-established prefabrication techniques, timber wall cassettes offer countless possibilities for interior and exterior wall surface finishes. With various types of panelling available for walls and facades, timber cassettes can be customised to meet each project’s design and performance specifications.

Using wall elements of single-storey height, buildings are erected one floor at a time, known as platform construction, starting with the internal and external walls, followed by the roof. This is a distinctive construction technique, as no supporting frame is required. Each element has a maximum width of approximately 4.5m and a maximum length of 22m. The wall elements retain load transfer and racking resistance capabilities.

Timber concrete composite

Timber concrete composite is a ribbed floor system built from prefabricated concrete slabs on glulam ribs, which can provide thermal mass through the exposed concrete soffit and inherently has good acoustics.

The flat slab concrete system, in combination with CLT, allows for a smooth timber ceiling and a shallow construction depth.

Timber concrete composites can be used:

ZÜBLIN has developed a timber-concrete composite as a prefabricated system, combining the beneficial properties of concrete, as a damper for vibration and as a sound absorbing mass in the compression zone, with dry, fast and precise timber construction.

03 / Benefits of CLT

All the above fundamental approaches to timber buildings share a number of inherent advantages, but given that this cost model is based on a CLT solution, the following is a summary of that product’s benefits:

Airtightness CLT structures typically offer higher levels of airtightness (2-3m3/m2/hr) than traditional construction.

Quality Surface grade finish of boards can be specified for an exposed finish.

Services installation CLT forms an easy surface to fix to and services installation is likely to be quicker.

Secondary structure Secondary framing or brackets are virtually eliminated.

Embodied carbon For every 1,000m2 of building built in CLT instead of steel or concrete, up to 350t of CO2 is saved or stored (representing up to 10 years of operational CO2 emissions).

Sustainability All the timber for CLT panels should be PEFC certified, with the raw material used coming from sustainably managed forests.

Programme Construction programme speeds can be optimised through maximum repetition and rectilinear forms. Early weathertight date allows “vertically stacked” programme.

Loadbearing facade Can be used as a loadbearing CLT facade structure, preferably with a punched window approach. Windows are pre-ordered as CLT tolerances are high.

Roof A simple CLT roof profile also aids an early watertight date.

Foundations Shallow strip foundations are more likely possible if a load bearing CLT wall structure is adopted.

Waste Construction waste associated with the structure is virtually zero.

04 / Technical issues

To ensure the most effective and efficient design, a number of particular technical considerations must be addressed in the design. These include:

Design responsibility Engineers should design CLT with knowledge and understanding of the CLT supply chain and each company’s standard sizes.

Grids and spans should be planned with a 2.35m grid where possible with spans limited to 7.5m where possible. Greater spans will require thicker panels.

Building services Early and detailed design co-ordination is required (before panel fabrication), board layouts should be planned to suit opening where possible

Acoustics Specialist advice should be sought at early design stage.

Floor vibration Design limits must be determined for intended use.

Thermal mass CLT has a significant thermal mass but less than exposed concrete soffits.

Exposed finishes Early attention to details is required and an early decision made on the visual surface grade (with a cost allowance for the upgrade). Possible issues to address are: staining (rain and UV) and drying shrinkage cracking (gradual commissioning of the building heating system can avoid this).

Moisture CLT needs to breathe and specific types of insulation and vapour barrier must be used to prevent moisture condensation against the CLT surface.

Fire requires a fire engineered approach (see box, right). Exposed CLT panels can be engineered for 1-2hr fire resistance, but exposed CLT will require a spread of flame treatment in the UK.

Cost comparison should not be limited to the cost of the frame: consideration must be given to programme, foundations, facades, secondary structures, etc.

05 / Reinforced concrete frame vs CLT

If the cost of a CLT frame is comparable to a reinforced concrete frame (see cost model), and appears to have more advantages than disadvantages, why do we not see more CLT frames being built?

When evaluating structural frame options at concept design stage there will be certain design criteria that will make one structural form more attractive than the other.

If one is looking to utilise thermal mass in a building to control the internal environment then exposed concrete soffits and walls have a distinct advantage over CLT planks. There are also times when more weight is needed in a building structure, most often when there is a basement with two levels or more. The additional weight of a concrete frame will be beneficial in controlling uplift forces.

From a structural engineering perspective there is little difference between the frame types in terms of flexibility on site and the opportunity to makes changes or corrections. In terms of design approach the industry wide adoption of BIM Level 2 means that buildings are more co-ordinated between design disciplines and this is especially true of window and wall openings and MEP builders’ work penetrations. Again, given the changes in design technology, there is no particular advantage of one frame type over the other in terms of the design process.

When it comes to the “crunch time” of decision making there is more collective industry experience in building traditional reinforced concrete frames and therefore more comfort of knowing what can go wrong and how to solve problems once construction has started. Notwithstanding the suitability of the scheme’s dimensions, this “safety net” sentiment can have a bearing on the decision.

06 / Supply chain

There are a little over 20 manufacturers of structural timber products in Europe. The most prominent, and their product ranges, are shown in the table below.

Timber products

This list is likely to grow because there are already other companies looking to start manufacturing CLT in the future.

Designing timber for fire

Reasonable questions often arise regarding a timber structure’s performance in fire. Such concerns are shaped by history and are typically rebutted on the basis that timber elements, such as those formed of CLT, can achieve comparable fire resistance ratings as those formed of concrete and protected steel. However, such a comparison can be overly simplistic.

The challenge for timber buildings (particularly those exposed in the design) is not to demonstrate how comparable fire resistance ratings can be achieved, but how adequate performance in real fire events can be demonstrated.

This has been the focus of the research community and is yielding new tools and methods that are allowing the boundaries to be pushed. More importantly, advances are allowing structural fire engineers to design on the basis of sound scientific principles.

Fire resistance periods (and, thus, the fire resistance required of elements) are a proxy for what level of performance in a standard test is required to ensure that elements would survive the burnout (full duration) of a real fire event. These periods are predicated on inert enclosures, for example, where structural elements / enclosure are not a source of fuel.

In “real performance terms” challenges exist with regards demonstrating how a timber structure can resist burnout. It is necessary to characterise: (a) how the combustible elements affect a fire’s development, and (b) how, once involved, structural elements (such as CLT) might self-extinguish.

Specific to CLT, behaviours can also be observed in fire, like delamination, that influence both fire development and load -bearing capacity.

Such has been the rate of emergence and development of engineered timber, such as CLT, the production of design guidance has struggled to keep pace. Interim fire design guidance has been published by European research institutions (SP) and further work is underway within the EN 1995-1-2 committee and aligned research projects (such as COST FP 1404). Irrespective, as boundaries continue to be pushed, engaging timber structural fire specialists will be essential.

Danny Hopkin, associate – structural fire engineering, Olsson Fire & Risk

07 / Cost model

We have prepared two detailed cost models, one each for a CLT and concrete design for a seven-storey private residential building. The scheme has been designed with both a timber and concrete solution in mind at the outset, with a structural layout to suit both. The cost model included here is the CLT version, with the summary table below showing how the affected elements differ in cost between the two.

The cost model shown is based on:

The CLT costs are based upon subcontractor costs received on a design completed by Engenuiti whereby a reinforced concrete frame alternative was also priced for comparative purposes. We have assumed that the internal CLT walls and soffits are boarded in order to provide an accurate comparison to the RC version in respect of acoustic, fire and visual requirements.

Estimates were obtained from two independent specialist CLT contractors, and analysed by Alinea, based upon tender-quality pricing schedules. Concrete costs are based on recent tendered information and are commensurate to a mid-market frame contractor pricing level. These were reconciled and compared with a similarly detailed estimate of costs prepared by Alinea for the concrete frame option. The correlation of the CLT “tenders”, together with their alignment with Alinea’s expectations, provide a relatively high degree of certainty in the numbers.

The summary below shows that the construction cost variance between timber and concrete for this hypothetical scheme is minimal. The higher CLT superstructure costs are offset by the ability to reduce pile quantities because of a lighter frame, hence a saving on substructure. The CLT programme for the frame and upper floors is around 10–15% shorter than the concrete option, resulting in a lower preliminaries cost.

With the quicker frame programme and virtual parity in construction costs the overall appraisal figure with CLT should glean a greater return, with units coming to market quicker than the equivalent RC scheme. Further potential savings could arise from the ability to reduce floor-to-floor heights (CLT floors are thinner than the concrete equivalent), which brings savings not only in frame costs but more significantly in external walls.

Exposing walls and soffits of slabs could also benefit costs and programme, but this would depend particularly upon input from fire and acoustic specialists to ensure a fair comparison.

Concrete is the most prevalent frame material both historically and currently for new-build residential developments. Aside from its inherent design qualities, the costs associated with low/mid-rise residential are in the main more competitive than alternative materials. The scale of competition in the concrete market helps to keep costs competitive, meaning that pricing levels are subject to a high degree of variation. Concrete generally remains the most cost effective residential frame solution; however, the gap has narrowed and as the CLT supply chain grows, this gap will continue to close. CLT will therefore become a more viable alternative to concrete as long as it is considered early and schemes are designed to match the specific dimensional requirements. A building requiring longer spans or a more complex configuration will likely increase costs above concrete and therefore nullify the advantages of CLT.

Acknowledgments

Particular thanks to Catherine Harvatt, Alex Hyams and Steve Watts of alinea, together with Paul Grimes and Clive Fussell of Engenuiti and Danny Hopkin of Olsson Fire & Risk.

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