How did mass timber add extra floors – and a whole lot of flair – to the Technique Building? We catch up with Andy Heyne, of structural engineers Heyne Tillett Steel, to find out all the details.
The building at 132 Goswell Road has been many things. Once it was a gin distillery. In the 1960s it became office space with the intriguing name ‘Laser House’. Today, thanks to an extensive timber-framed retrofit and extension, it has been granted a new lease of life as the Technique Building. It has been beautifully transformed by a new superstructure of glulam column-and-beam frame, with new CLT storeys added to the top.
Why timber and retrofit go hand in hand
Typically, clients want a solution that will create as much extra space as possible. And in creating space, a huge consideration is the strengthening of foundations. As soon as you have to do that, it becomes less viable to add more floors. So the lightness of the material becomes crucial.
“You can’t build lighter than with timber,” emphasizes Andy. “Technique’s foundations needed no strengthening.”
Heyne Tillett Steel ran through some carbon optioneering – to see what would come out best with like-for-like comparisons between all-timber and steel and CLT, or steel and joists. The Gramophone Works building they were also working on at the time was further advanced, so they could see how well an all-timber vertical extension was working.
Getting in there early
“We often get appointed quite early, just so the clients can find out how much area we could add to the site, hypothetically, by extending to the sides and top,” Andy confides. “Then architects and planners will bring it back to earth – what can actually be achieved in practice given planning and real-world considerations. But it’s good to start big, then rein it in.”
Quick and clean
Timber, in Andy’s experience, results in a faster build – a lot less disruptive to tenants. This is a big bonus in a densely occupied part of town, as is the case with the Technique Building. The whole build took just over two years – slowed by the coronavirus pandemic towards its end. The programme for the timber aspects was especially quick: around four months from the CLT’s delivery, pre-cut, directly to the site from Europe.
The speed and ease of construction is what makes timber economical, Andy points out. The fact that it’s dry helps bring it to market quickly: office space can be rented out sooner. All things considered, a timber building comes out at the same overall cost as other materials, Andy believes.
Onwards and upwards
The existing building was one storey higher at the front – Goswell Road – than at the long main body of it at the back.
“We removed one storey from the roof of each of the two blocks,” explains Andy Heyne. “It was a concrete slab which was very heavy. Removing it meant we wouldn’t have to strengthen as much below when we added floors.”
“But it was a relatively weak structure anyway: the roof wasn’t designed for a floor.”
“We had to ‘set the building in’ for planning reasons – to give a kind of wedding cake effect. To transfer that on an existing slab we’d have ended up with steel downstands on the floor below. So it made good sense to take the slab off.”
One storey off, two storeys on
“Although we took off one storey, we added two back on – or three, depending on which part of the building you’re talking about. We also added various small extensions around the edges to the lower part of the building.”
The new floors were laid out on a half grid – about 3 metres centre-to-beams. The half grid meant much thinner CLT could be used, which matched the existing half grid of the floors below.”
While the foundations required no reinforcement, some of the columns were strengthened. The existing columns were steel with concrete encasements. These were fibre wrapped: carbon fibres are glued onto the concrete. This increases their strength without changing their size. A thin render coat is added on top. All in all, a very low amount of reinforcement given two floors were added.
Andy is certain they could have added yet another floor to both blocks of the building without further strengthening. But this wasn’t possible due to planning restrictions.
No ceiling on retrofit ambitions
Originally constructed in the 1930s, the existing building’s steel frame and concrete encasement has housed a number of different uses. Over its lifespan it had been a gin distillery and at one point it had been converted into office space. There were suspended ceilings everywhere.
“With most of our retrofits we tend to take the ceilings out, and we don’t tend to put them in again unless the structure is really ugly,” Andy tells us. “Tenants like to see the natural character, the materiality of the building. Taking the ceilings out revealed board-marked concrete and downstand beams on a half grid – all quite attractive when tidied up.”
Air in the floors
A productive MEP solution was developed with Chapmans to run the mechanical ventilation system through the floors rather than the ceiling.
“Not having the ductwork in the ceiling means you don’t have to thread big holes through the beams,” Andy points out. “You’re not instantly lowering the visual datum of the ceiling.”
Having no pipework or cables visible maintains a sense of calm and minimalism. On each floor there are 3 or 4 cupboards which have a mini air handling and chilling plant – known as a CAM system. This pumps out pre-chilled air into the floor. Grills can be placed in the floor wherever you want. This does require a quite deep floorzone – 400mm or so – but this space is saved at ceiling level.
“Floor-to-floor heights can be a concern with timber. If you can’t put the air through beams and don’t have an alternative solution then you’ll end up with a slightly higher floor-to-floor. That may or may not be a problem. If it’s only 4 floors, then adding 200mm to each floor won’t make a huge difference. But over 15 storeys, 200mm per floor will add the equivalent of a whole extra storey.”
An ever-evolving learning process
Andy doesn’t shy away from the many factors that clients consider when timber is on the table – both for and against. Cost, programme, commercial availability, risk and insurance are all considerations.
“The solutions are constantly evolving,” he points out. “And of course, regulations and guidance are frequently changing.”
He recalls that when HTS were designing and getting sign-off from Building Control for the Technique Building the Structural Timber Association’s fire guidance (Structural Timber Buildings: Fire Safety In Use Guidance) had not been published. Nowadays, it is a standard reference.
“On every job we learn something that we might do differently next time. And we reinforce things that were successful last time. There are always challenges. But you meet them with competence, and the right team.”
This article is taken from issue 4 of Designing Timber magazine,