752 High St Construction

25th  November 2020                     

Milestone: CLT panels being installed

An update on site and some insight into the benefits of using CLT construction

The prefabricated CLT panels that are currently being installed on site, have allowed construction to proceed very quickly. We’re seven weeks into the installation and already on the fourth and final level. This highlights the main difference between mass timber and typical construction methodologies that involves props, formwork, the pouring of slabs and then the waiting time to strip the slabs, which is a lengthy process to work through. In this form of construction, once a level is stripped, trades come into the completed floor while the floor above commences.

With CLT, the whole structure goes up without any other trades present. It gets screwed off with extra connections added where needed. After this is completed, the services trades commence. This difference is what allowed the construction of four floors to fit into an eight week program, which is far quicker than traditional construction. The advantage to the builder is a separation of trades and when services trades do start on site, they have a clear, clean space without the typical array of props present for the upper concrete slab. This also allows for added efficiency in their work. Another additional benefit is that overall, the CLT construction method produces much less wastage.

Composite CLT construction

From a technical point of view, most CLT buildings are composite buildings. At High St, 95% of the structure is CLT but some steel beams and columns had to be incorporated to achieve certain architectural solutions. We wanted the apartments, to feel open and spacious. To avoid posts we have steel beams running the length of the open living areas. It’s common and more efficient to add a piece of steel where you have more load rather than increasing the volume of the CLT. The CLT can be cut to fit so when the steel arrives on site it can be simply slotted into position. The steel beams can sit on the CLT due to its thickness while in a stick frame building, a steel post or a double post would also have to be added.

It is possible to have the structure completely out of CLT but it does make sense structurally and in terms of cost to introduce a few steel elements where needed. We don’t see implementing a composite system to have any diminishing effect on the environmental credentials of a CLT building.

To expose or not to expose?

 While it’s often preferable to expose timber as it can have a biophilic effect on occupants, through the design process we gradually realised its not the only way to build with CLT. At High St, the CLT wall and ceiling panels are the structural components rather than the finished exposed material. We chose CLT because it has all sorts of advantages in terms of production, environmental credentials and thermal performance but it’s not the finished material. CLT can’t be left bare externally as it’s not weather protected. Internally, there are various fire regulations in multi-residential buildings that make it difficult to expose internally. In typical construction practices, the structure is often covered up so it’s really no different in this respect though with CLT there’s the one solid structural system rather than a conglomeration of steel, concrete and timber framing. This gives a building solidity and much better air tightness that in turn enhances the thermal performance.

At High St, the CLT wall panels will have layers of fire protective plasterboard internally. Externally, the walls will have a build up of added insulation, waterproofing and then a final cladding material, which for a majority of the building is brick tiles.  Fundamentally, mass timber is just another construction system, so if covering it up for design, waterproofing or fire regulation requirements allows its use then we see this as acceptable. Inevitably, the CLT constructed building will still be more comfortable to live in due to the qualities intrinsic to the form of construction.

 We look forward to sharing with you the next phase of construction once the erection of the CLT has been completed. Next up we’ll show the CLT panels being clad and services being installed on site.