Building Use: Residential

Low to medium-height multi-storey residential structures are increasing in popularity in New Zealand. A very good example of this is the recently opened Christchurch City Mission Stage 1 building Whakaora Kāinga, for transitional housing.  

This build is a very good example of hybrid design and construction where using cross laminated timber (CLT) upper floors and stairs instead of concrete results in a really efficient and lower embodied carbon dioxide solution. Additional benefits that come as a result of using CLT also include lower total building mass and a consequent reduction in foundation capacity and cost.

In this case study “snapshot”, the typical concrete floor is swapped out for CLT. CLT for floors and stair systems is recognised as a very efficient use of the material. CLT walls are also a good option for internal structural walls for apartments, student accommodation and aged-care buildings, but in this snapshot, the CLT application is just in the floors and stairs. 

The key benefits of swapping concrete for CLT in the floors and stairs are: 

• Faster installation - A CLT floor on each level was fully laid in 2-3 hours 
  • A concrete floor would take at least a week to install the permanent formwork base, temporary formwork sides, reinforcement steel and concrete placing 
• Faster job progression - A concrete floor would then require 28 days of curing time 
• Cost - significant preliminary and general time-related cost savings are achieved due to faster construction 
• Lower weight - the concrete floor is 100,000 kgs heavier than the CLT floor system in this example.  This enables a lower-cost foundation design and build 

Over the two upper floors, there is a circa $6k saving using the CLT option whilst also achieving a;  

• 59% reduction in embodied carbon dioxide 
• 49% reduction in weight 
• 8 weeks of programme time savings, enabling earlier building access and occupation 

Assumptions (for CLT option include); 

• Cost savings for 8-week programme reduction 
• Cost saving for a lighter foundation 
• Cost additions for acoustic floor system (SounDown™) 
• Cost additions for premium on GIB Fyreline® to underside of CLT

Click through to some additional news updates released in regards to this project.