With Sir Ranulph Fiennes as one its past pupils, Sandroyd School was always likely to push the boundaries when it came to building a new performing arts centre. It clearly succeeded and with Zoe Wannamaker there to unveil it, Sandroyd’s timber frame building has been set firmly on the map.
Set in 60 acres of Dorset parkland, with a long driveway approach, Sandroyd sets great store by the quality of its surroundings and classroom environment and, according to bursar Paul Bowen, the latter was coming under strain from the growing pupil roll.
“We previously used the main hall in the house for theatre and drama but, as the number of children increased, it simply wasn’t adequate. The new centre also allowed us to remove some of the earlier extensions and provide a superb facility that creates a real wow factor, cost-effectively.”
Starting with London’s Globe Theatre as a concept, timber frame was high on the list of preferences, but with a keen eye on costs, could it be made affordable? Surely steel would be the prudent option?
The structure takes shape
To ensure the materials choice worked, Brian Watts, of architects Philip Proctor Associates, and quantity surveyor Alex Moriarty called on the experience of Carpenter Oak & Woodland at the outset. Commercial director Andy Parker praised the approach. “Usually we get called at a later stage, when steel or concrete are already progressing as options. Because of Brian Watts’ experience working with us before, they did the right thing and set up an early meeting. This gave us the chance to design from foundations upwards in timber.”
Carpenter Oak & Woodland’s track record goes back over 20 years ago and it now has over 50 staff. “We have a system where our teams see each project through, from initial cutting to on-site erection,” said Parker. “This helps to ensure that our project deadlines are achieved, because the teams know exactly what they’re dealing with.”
The initial meeting also agreed that Douglas fir would be the nominated material, and the trees were felled from the estate that surrounds the school.
When the designs were worked up and costed, timber started to look attractive – around £80,000 for a 400m2 frame that includes 20m clear spans in some areas. “We knew some people would have never thought of timber, simply because they’d reckon it wasn’t achievable, but of course it is,” said Parker.
Traditional oak beam construction combined with a hi-tech jointing system created the finished product
While the overall frame cost was slightly higher in timber, early involvement meant that other savings came through. “You could argue that you need lighter foundations for timber, compared to steel,” said Parker, “then, when you factor in decoration and fire proofing, the numbers get even better. What’s more, you end up with a beautiful structure that has tremendous kerb appeal.”
The main timber baulks were sawn to size and delivered to Carpenter Oak & Woodland’s workshop, where the team uses a range of power and hand tools in a traditional way. “This is because it’s the most effective method and not because of any desire to work to old methods. Each job is an individual project that we cannot set into a production line.”
One of the principal weapons in Carpenter Oak & Woodland’s armoury is their experience in timber jointing. “Many designers and contractors are concerned about using timber, especially in a fresh cut [green] state, because they don’t know how it will shrink and move,” said Parker. “For the Sandroyd project we used a jib and cotter arrangement that takes all of the shrinkage by the timber posts out of the equation. The strings do not shrink in length and hence, across the critical dimensions, we deliver a millimetre-perfect product.”
Each frame section was pre-assembled before being dismantled and delivered to site, where the team erected the structure in under two weeks. It was completed on time, to specification and within the budget.
The finished building is impressive and the large free space of the theatre area looks magnificent. “From the outside you’d never know that it’s a timber frame,” said Bowen. “The acoustics are very good, with little resonance. The timber sections are maturing and will just go on looking better as time passes but, most importantly, the children love being here, for assemblies, quiet reading and theatre works.”
Nuts and bolts and timber breathing
The simple aesthetics of the large free space within the Sandroyd School’s theatre belie a complicated bespoke jointing system that gives the timber frame its structural integrity.
At the heart of the jib and cotter joint is a cleverly-designed steel flitch plate that is slotted into the downward timber members, bolted in place and then anchored using diagonally crossing steel cables. But why was this needed? With the horizontal timber members running the length of the building, and the downward posts intersecting at various intervals along their length, any design solution in timber needed to provide enough rigidity to support the balcony walkway. Because timber shrinks across its width and thickness, if the horizontal beams were jointed to the vertical members by butting up, with the posts’ natural shrinkage, length would be lost in the top and bottom rails. This loss of length would result in the balcony walkway dropping by up to 350mm – something that was clearly unacceptable in such a prestige project.
Steel flitch plates hold the vertical beams in place, creating a continuous length
What Carpenter Oak & Woodland decided to do was create a joint at the intersections with the posts that would essentially create a continuous timber length for each of the top and bottom rails.
As timber doesn’t shrink in its length, it could have been feasible to create a joint where the ends of each of the horizontal timbers met in the middle of the vertical post, using a traditional mortise and tenon joint. But the architect in the Sandroyd School had seen the galvanised steel joint in another Carpenter Oak & Woodland project, and liked the aesthetics it offered.
It was decided to create the bespoke galvanised steel flitch plate to provide the structure’s rigidity. These plates were attached to each of the end of the horizontal timbers, and then slotted into the face of the vertical timber lengths. They were then secured in place using steel wedges, effectively locking the two plates together.
“There are technical challenges of working with green timber, but so are there with concrete and steel,” said Andy Parker. “We were able to provide a solution that took the expected shrinkage out of the equation.”
Keywords: Sandroyd Carpenter Oak & Woodland timber frame
