By Joy Crane

The African construction sector is witnessing a significant shift toward sustainable building materials, with cross-laminated timber (CLT) a frontrunner in the transition to green construction. Researchers at the Universities of Stellenbosch and Pretoria point to mounting environmental concerns and climate adaptation needs as drivers of this transformation.

CLT, an engineered wood product known for its structural strength and environmental benefits, is gaining particular attention in the South African market. The material's ability to sequester carbon, combined with its low embodied energy compared to conventional materials like steel, bricks and cement, is gradually attracting commercial and residential developers.

"We're seeing a fundamental change in how African developers and architects approach material selection. The focus has moved beyond operational energy efficiency to considering the entire lifecycle impact of building materials," says Leander Moons of OMT Architects. 

BURJ Zanzibar

Most notably, the versatility of mass timber is recognised in the ambitious BURJ Zanzibar development. Following the worldwide trend of constructing large timber buildings, the 28-storey BURJ Zanzibar will be Africa's tallest hybrid timber building. At its core is a conventional steel-reinforced concrete structure surrounded by glue-laminated timber columns and beams, and modular CLT panels for floors, walls and ceilings. 

Moons and OMT are the architects. He says the timber tower building will promote mass timber manufactured from sustainably managed African plantation timber as the future building material. 

"Plantation forestry is expanding in East Africa. However, presently, there are not enough sawmills in East Africa producing kiln-dried and graded timber for CLT production", Moons explains. 

DfMA

A significant advantage of building with CLT is its compatibility with computer-aided design and manufacturing software that promotes a just-in-time construction process. The Design for Manufacturing and Assembly (DfMA) approach redefines the construction landscape, particularly in mass timber projects. Focusing on sustainability, precision, and efficiency, it also focuses on standardisation and prefabrication. 

The system streamlines workflows, reduces errors, and accelerates project timelines. Research shows DfMA can cut project costs by up to 37% and reduce assembly time by 62% (www.civilengineer.org). 

In its Built Environment Report 2022-2023, Arup states, "Digital technology and data are transforming how we design and engineer buildings, enabling a deeper level of insight and allowing us to reduce operating costs and optimise the life of buildings. Embedding real-time data also provides our clients with valuable metrics to optimise the in-use performance of their assets". 

Tools like Building information modelling (BIM) improve and speed up construction because project managers, architects, engineers and trades collaborate seamlessly to optimise the allocation of resources and minimise wastage.

84 Harrington Street

A groundbreaking example of the change in thinking in construction is 84 Harrington Street in Cape Town's District Six. It demonstrates how existing structures can be repurposed instead of demolished. The gloomy and musty building has transformed into a modern multi-use hotel and apartments with restaurants, a swimming pool deck and bar, and a commercial ground floor.

The development, now known as Neighbourgood, is South Africa's first mass timber retrofit and the world's tallest hemp building, featuring hempcrete blocks and hemp-based panels in its interior walls. The speed of adding three floors onto the six-storey building was unprecedented, and the combination of mass timber with hemp-based materials set new standards for sustainable building practices in Africa.

The professional team included Wolf and Wolf Architects, Moroff & Partners Consulting Engineers, Hulme & Associates Consulting Structural and Civil Engineers, R+N Master Builders, Holzbau Hess and Pirmin Jung.

The developer approached Christian Hess of Holzbau Hess to design and manufacture the engineered trusses and timber for the project.

Using the latest BIM and DfMA software, Hess spent a month in Germany with Pirmin Jung's timber engineers designing and specifying the project. It included:

• 220 pages of calculations and detailing aligned with digital modelling
• 230m³ of CLT produced by XLam, with a fire rating of 60 minutes, and each panel weighing 1.1 tons
• 100m³ (90m³ SA pine and 10m³ of eucalyptus) timber for the laminated trusses
• 14,000 dowels (8,000 in the trusses)
• 25,000 screws
• 2 tons of customised steel and connection plates.

"We over-engineered all dimensions by 40mm on both sides to encapsulate the connectors and enhance the safety and quality of the construction," Hess explains.

The CLT and laminated trusses were manufactured in Cape Town. It took two weeks to prepare the truss components and to transport them to Hess' factory in Windhoek, Namibia. The beams were sized, shaped, and drilled on his large Homag WBS140 CNC machine. 

The components travelled back to Cape Town for assembly into 5m x 14m long beams at MTO's premises in Retreat. When the trusses were ready, they were loaded onto a truck for delivery to the site.

However, at that point, red tape and the elements put a spanner in the works. It took three days for the municipality to re-approve the route for the abnormally wide load and the temporary closure of Harrison Street. When the truck arrived, a 220-ton crane was ready to lift the massive trusses onto the building; however, six days of strong to gale-force winds made it impossible. 

Although the delays were frustrating for the installation teams, Hess says his 20 years of experience and careful planning for all contingencies kept the project on track. 

He highly values expertise and experience when working with mass timber. "When you connect timber to an existing building, there are always anomalies, and you must be ready for them. The materials are precisely engineered and pre-assembled, leaving little room for error, and the artisan must be able to identify problems like out-of-square existing concrete walls and solve them", Hess explains. 

Increased demand

In research towards his master's degree in architecture at the University of Pretoria, Mishau Azar found a growing interest in the adaptive reuse of inner-city buildings like the Harrington Street development.

"Respondents to my questionnaire identified various benefits of mass timber, including reduced foundation size, speed of construction and the precision offered by CNC technology. 

South Africa's two CLT suppliers, XLam and Mass Timber Technologies (MTT) report increasing demand across multiple sectors. 

"The market has evolved significantly over the past few years," notes Marcus van der Hoven, Director of MTT. "We're seeing interest from individual homeowners, commercial developers, and the eco-tourism sector, where sustainable construction aligns perfectly with their brand values. 

Training

On a cautionary note, Hess and Belinda Duncan of Terraverta Habitats assert that mass timber on-site installation teams need specialist training. The scale of the timber and size of the connectors differ from those used in joinery and furniture manufacturing. Hess believes it is essential for installation teams to be exposed to and mentored by international best practices.