The roof is a timber trussed conical form, supported off an inner cast-iron ring beam and columns and an external masonry wall. The truss system relies on the counterbalancing of the 16 principal trusses, with the pin at the top formed by a circular ring beam.

There are also 32 secondary trusses, two between each primary, which are supported by the cast iron ring beam, external wall and a purlin spanning between principle trusses at the top.  Critical to finding out the species of timber and how much timber repair was required, a micro core survey was carried out by Hutton & Rostron Environmental Investigations. It took a sample from every piece of structural timber for analysis and gradually built up a computer model to work out which parts of the structure worked and which didn’t.

It concluded that there were two principal problems: water ingress, which caused decay, and poor quality of timber — knots as deep as the timbers themselves were revealed.  Wherever possible, timber roof members were strengthened by bolting on steel plates to preserve as much of the original appearance as possible. Any retained timber members were cleaned with a light steam and finished with a Danish wood oil.  Where timber had rotted, a minimum length was cut out and a new timber section of the same European redwood was installed and spliced to the original member via bolted steel plates.

No attempt was made to conceal the restoration. Maber’s Ian Harris says the concept of “honest repair” was inspired by William Morris, whose valuing of old and new is the restoration’s guiding principle.  In other instances, key structural supports had to be replaced. At least 75% of one major diagonal member in each of the trusses had to be replaced because they were overstressed.

The original cupola was destroyed and a new timber cupola finished with a lead roof was designed based on historical photographs.  A continuous ring of double glazed rooflights was installed, fitted with actuators to control the windows’ opening and closing.  A sarking board restrains the top flange of the trusses, which creates a diaphragm that distributes horizontal loads and ties the whole roof structure together. About 80% of the sarking board was retained; the rest was rotten and had to be replaced.  A vapour control barrier was added, together with an expanded polystyrene insulation slab, counter battens, a breather membrane, more battens and then reclaimed slates.