24 April 2018
Cost gap between CLT and concrete is narrowing
A new report highlights the advantages of using CLT over concrete. Alinea’s influential report Residential Timber: Cost Model suggests CLT is a viable alternative and uses two detailed exemplar cost models to demonstrate the point.
Residential Timber: Cost Model found that the cost of a CLT frame is comparable to one made of reinforced concrete and raises the question: why do we not see more CLT frames being made in the UK?
The report highlights the advantages of using CLT over concrete, including: units coming to market quicker, an ability to reduce floor-to-floor heights (CLT floors are thinner than concrete), as well as sustainability, reduced carbon emission, zero waste, high levels of airtightness, and swifter fixings and service installations.
Alex Hyams, author of the report and associate at Alinea, says: “The cost gap between CLT and concrete is narrowing, and as the CLT supply chain grows, this gap will continue to close. If CLT is considered early and schemes are designed to match the specific dimensional requirements, CLT is a viable alternative to concrete”.
The benefits CLT offers to structural applications include:
The computer numerically controlled (CNC) cutting process used in the manufacture of CLT panels allows the panels to be cut or routed to almost any shape.
CLT is stronger than solid timber as it has a wider distribution of natural defects due to the cross-lamination process and high standard of board strength grading.
CLT is typically used for floor, roof and wall elements of building structures with spanning capabilities of up to 8m or up to 12m when used compositely with concrete or as a fabricated cassette section. CLT has high in-plane strength and stiffness, which also allow it to be used effectively as shear walls, diaphragm plates and deep beams.
CLT is manufactured to strict QA requirements from stress graded timber of known structural capacity.
CLT is manufactured from kiln-dried timber (12% moisture content) and the cross-lamination process makes it less prone to movement caused by changes in moisture content.
The report used two detailed exemplar 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, for ease of comparison, with a structural layout to suit both. The cost model is based on:
- A building in London (Zones 3 – 6)
- 307,912ft² gross internal floor area / 223,286ft² net internal (residential) area
- A small retail shell at ground level
- 294 residential apartments (all for private sale)
- CLT frame, upper floors, roof, core, stairs, external walls (all first floor and above)
- Reinforced concrete ground and transfer slab
- CLT all non-visual grade
- Piled foundations due to poor ground conditions
All rates are base dated at Q2 2017 and reflect a two-stage competitive design and build tender. Exclusions from the cost model include demolitions, fees (professional and pre-construction), external works, incoming utilities, section 106/278 agreements and VAT.
Authored by Alex Hyams, Steve Watts and Catherine Harvatt at Alinea Consulting, with assistance from Paul Grimes and Clive Fussell of Engenuiti, and Danny Hopkin of Olsson Fire & Risk.