Alnwick branches out

A labyrinth, Poison Garden, Serpent Garden and grotto...Enter the fairytale world of Alnwick Garden, Northumberland. With Alnwick Castle and its grounds already famed as the scene of Harry Potter’s Hogwarts school, a new feature is set to confirm it as a must-visit destination for all children.

Opened on 12 January, the Alnwick Garden Treehouse is not the average hut secured between tree branches you may remember from childhood. “It was quite a big job and is an elaborate scheme,” stresses Garrie Forster, director of electrical contractor Dougal & Railton.

This treehouse covers an area of over 550 m2 – larger than two olympic-sized swimming pools. It is fully waterproofed and has lighting throughout, plasma screens for teaching and centres around a fully-functioning kitchen and restaurant. All suspended above ground level, it is around 17 m at its highest point.

The £3.3 million project was completed as part of a £42 million regeneration scheme of Alnwick Garden. It is sited around a mile from Alnwick Castle, within the walls of the 40 acre garden and around 1·5 miles from the neighbouring town of Alnwick.

The power of play

ECA firm Dougal & Railton won the contract for the installation of electrical services by competitive tender to the main contractor Sir Robert McAlpine. The largest part of the installation, reports Forster, was the lighting as this features in all areas of the Treehouse complex. It also plays an important role in creating the desired ambience in individual areas, with the controlled use of natural and artificial light giving each its own feel.

The Treehouse comprises a complex of turreted buildings built in and around 16 mature lime trees. These are surrounded and connected by a series of walkways that meander through the trees and allow visitors access to the different areas.

The entire structure was built using wood from sustainable forests that was pre-treated with a pressure injection system to avoid decay. Supported on stilts, the first challenge for the contractor was to provide the electrical supply needed for the facilities being installed.

The solution involved the use of the existing supply for the castle. The main switchroom is around 170 metres from the treehouse and based in an old gun store. “We put in a new sub-mains from the switchroom and run this in a trench to the treehouse,” explains Forster. The power supply is run to the bottom of the main, central tower. Here, the switchpanel is concealed in a room behind the supporting stilts.

Cabling is run from the switchroom up a riser through the structure then distributed in floor voids to distribution boards for individual areas. There are four distribution boards: one each in the kitchen, resource room and restaurant and an external lighting board.

Although the structure appears at the mercy of the elements, again it is much more sophisticated and substantial than a ‘treehouse’, hence standard products could be used in most areas. “The bulk of the installation is inside and the building is watertight,” explains Forster. “There is some external emergency lighting that is IP67-rated, but in general there is nothing out of the ordinary in terms of products.”

“The lighting system is unusual,” he adds. In the restaurant a series of “lighting bars” have been installed. These comprise a track and four spotlights. In other areas the lighting is fixed to open wooden beam trusses. “These were surface-clipped, then the cables painted brown to match the wood,” describes Forster. Recessed downlighters have also been used at various points throughout the complex.

“Inside the building all lighting is fixed to the structural beams at a 3 m height,” explains Forster. This removes any safety issues the children bring. Distribution of power to the lighting is along the beams at high level. Outside, floodlights to light the tree roots are fixed to solid paving slabs and the brackets buried under soil.

Control of the lighting and power is via a main bank of grid switches in the restaurant, which are manually switched on and off. In some areas, such as the toilets, motion detection is included.

Around 30% spare capacity has been included in the electrical system to cater for future expansion. One possibility already under discussion is to tie in Alnwick’s Christmas light display to one at the treehouse.

Completing the scene

Parts of the treehouse are fixed to the trees so must cater for movement and growth. “The growth shouldn’t affect the services,” assures Forster. “All services are fixed within the treehouse and move with it.” One room in particular – the resource pod – needed special attention. “There is a hole cut around the trunk in the bottom of the treehouse,” reports Forster. “This is filled with rubber gaskets, which will contract as the tree grows.” The structure is also built to withstand gale force winds.

The treehouse was completed on a fast track scheme. “The electrical installation was done in about ten weeks,” says Forster. “With about eight to ten operatives on site.” Co-ordination of the individual teams was carried out by Sir Robert McAlpine.

Although the structure is raised, working from height posed no problems during installation. “Inside the structure we were working at no higher than 3 m,” reports Forster. All external work, such as the installation of lights on the underdecking of the treehouse was completed from scaffolding erected by McAlpine for the structural work.

Completion of the Treehouse marked the halfway stage of the Garden redevelopment. Further plans include building an adventure playground below the treehouse. On a larger scale, a 2400 m2 visitors centre and pavilion designed by Sir Michael Hopkins is also currently being built and is scheduled to open by the end of 2005, giving even more reasons to visit.