Cut and rub: Hampton Court Palace chimney repair
How historic brickwork techniques were used to repair failing Hampton Court Palace chimney stacks. Felix Mara reports
‘The most horrible moment with a project like this is working on scaffolding, dealing with a wobbly structure, and having to remove the cement haunching, which requires considerable force inflicted on an unstable structure at height,’ says brickwork specialist Emma Simpson, adding: ‘but there is no alternative.’
The project, a finalist in the 2011 Brick Awards, is the repair of Hampton Court Palace’s Wolsey Chimney, which her firm, Simpson Brickwork Conservation, effected with architect Donald Insall Associates. Although the palace was built for Cardinal Wolsey in the early 16th century, this decorative chimney stack, almost 6m high, is thought to have been built between 1825 and 1850.
‘The shafts of the chimney have lovely, flowing lines and are incredibly ornate, but it is in extremely poor condition, having taken the brunt of the weather,’ says Simpson. ‘It has been repointed and bodged with over-strong cement mortar, causing further damage.’ Movement of the chimney under wind loading had fractured its cap, which had undersized jointing pieces, so its three shafts were no longer restrained at high level. The wind had caused the shafts to rotate independently, so they leaned by 1.5-2 degrees. The bed joints no longer lined through, and there were steps of up to 37mm between shafts. Insall associate Patrick Duerden explains that the decision to depart from the Tudor practice of oversized flanking shafts reduced windload resistance.
Hampton Court is a royal palace, cared for by Historic Royal Palaces, which required the minimum amount of work necessary to return the chimney to good repair. The repairs had no stated design life but, because access is difficult and costly, they had to be effective in the long term and retain existing fabric wherever possible.
‘The chimney is significant for its design and the contribution it makes to the roofscape of the palace says Duerden. ‘We therefore aimed to retain its original appearance as far as possible and accepted that individual bricks whose condition was unsuitable for re-use could be discarded, with new facsimile replacements used for repairs.’ The false mortar joint introduced in the chimney cap was considered necessary to maintain the chimney’s symmetrical appearance in elevation. False joints are part of the repertoire of cut and rubbed brickwork, involving bravura and sleight-of-hand. The strategy would have been very different had the chimney been Tudor fabric, in which case every effort would have been made to retain individual bricks, regardless of condition.
Four options were considered:
1) Do nothing, leaving the chimney in a dangerous structural condition.
2) Insert tension rods in redundant chimney shaft flues, so the entire fabric above roof level could be post-tensioned, and reconnect the flues at cap level. This might have prevented structural movement of the chimney, though not monolithically, but, while solving one problem, it might have caused new ones, such as compression failure of shafts during tensioning or long-term failure of lower-level brickwork under wind loading. ‘Post-tensioning would only have transferred the problem to the base shaft and the chimney would have been too rigid,’ says Duerden.
3) Take down the chimney and re-build it to a new design, incorporating existing decorative elements and brickwork but altering the outer shafts’ design, so the shafts would be properly engaged at cap-level; or rebuilding the cap to a new design, incorporating more brickwork bridging between the shafts, enabling the cap to retain its integrity under wind loads. This was seen as a last resort.
4) Rebuild the cap to the existing design, incorporating structural reinforcement to enable it to act monolithically.
The project team chose the fourth option. ‘In the absence of a structural design standard, this would give the cap a fighting chance of retaining its integrity,’ says Duerden. ‘We can’t say this will make the problem go away, but all the other options involved radical interventions, although it is possible that the chimney might fail again.’ The proprietary brickwork bed joint reinforcement was the only permanent structural intervention.
It was agreed that, because of the project’s complexity, the workshop should be on roof-level scaffolding. Three quarters of the bricks were replaced and, as part of the cut and rub work, Simpson cut 20 basic low-fire brick shapes with high silicate content, known as rubbers. ‘A complex rubber with a roll and corner twist takes over two hours to prepare,’ says Simpson, who worked with oversized rubbers. These were bedded and squared, manually rotating them against a grinding stone. Rubbers were checked for right-angled corners and true flatness by running a set square down their length until no daylight was visible at junctions.
The stone and square eventually wear out and have to be checked for accuracy. ‘We start with the best original brick piece available,’ says Simpson. ‘This is scribed onto plastic and its shape transcribed onto a plywood former used to construct an open box,which in turn is used as a guide to cut the bricks with a wooden bow saw. Bricks were cut in the box, then filed, using the box as a guide to produce a flat surface.
Simpson acknowledges the project was more complex than expected and it has only been possible to gloss over it here. A methodical, collaborative and empirical approach achieved the right balance between appearance, fabric retention and stability.
Emma Simpson demonstrating cut and rub brickwork at the Brick Development Association’s Brick Conservation Day
Start on site June 2008
Date of completion September 2008
Procurement GC Works 2
Client Historic Royal Palaces and Hampton Court Palace
Brickwork specialist Simpson Brickwork Conservation
Architect Donald Insall Associates
Structural engineer Hockley and Dawson
Brick manufacturer The Bulmer Brick and Tile Company
Mortar analysis Peter Ellis
Quantity surveyor Harris and Porter
Project manager Historic Royal Palaces
Contract administrator Historic Royal Palaces Surveyor of Works
Design proposals were developed in consultation with English Heritage’s Government Historic Estates Unit