Friday, 22 July 2011

Above the Cathedral Vault I

The main vault inside Exeter Cathedral is not only the longest stretch of uninterrupted Gothic vaulting in the world but also crowns one of the most beautiful medieval interiors in Europe.

In the words of the great architectural historian, Sir Nicholas Pevsner, "the luxuriant palm-branch effect is unforgettable". During the summer months the cathedral runs guided tours into the roof space that gives visitors the opportunity to walk through the void above the vault as well as providing access to the top of the 12th century North Tower.

The tour begins in the south-west corner where a very narrow spiral staircase rises through the building from ground level via a short passageway over the aisle to the roof space above the vault. Following the destructive fire in the south transept of York Minster in 1984 the Dean and Chapter at Exeter decided to install some significant fire prevention measures throughout the cathedral, including very sensitive computer-monitored smoke sensors in the roof space and water spray extinguishing systems. Another addition was the division of the roof space above the main vault into five separate compartments using firewalls fitted with fire doors.

Prior to this installation there was a continuous view of the top of the vault from the west end to the east end but unfortunately this is now no longer possible. (Despite these measures insurance for the building still costs around £250,000 a year.) The photograph right shows part of the roof space and the rose window that sits high up in the gable at the eastern end of the cathedral. The central walkway literally runs on top of the middle of the vault and was installed to help firefighters move around the roof in the event of a fire. The humps and hollows to the right mirror the contours of the tierceron vaulting beneath. The floor of the cathedral is 68ft (21m) below.

The vaulting, and the roof that covers it, all date to between c1275 and c1335, a period of around 60 years that saw the cathedral's nave and choir rebuilt almost entirely in the Decorated Gothic style. Apart from the vaulting in the transepts, all of the vaults in the cathedral are made of stone. Exeter Cathedral underwent an extensive restoration, beginning in 1870, under the leadership of Sir George Gilbert Scott.

At some point in the last quarter of the 19th century, the Victorians hit upon the ridiculous idea of coating the top of the vaulting in concrete, allegedly in order to strengthen the stone work. The concrete was applied across the top of the entire vault to a depth of around four inches (11 cms). This could yet prove to be a disaster. Not only did the concrete cover up the original rough-hewn medieval stonework but it also added an enormous extra weight to the vault itself. Over the last century there have been signs of the supporting columns of Purbeck marble in the nave and choir starting to flake with the added stress of the concrete. At the moment it is simply impossible to remove the concrete without endangering the entire structure.

The photograph left clearly shows the concrete-covered vaulting. The total thickness of the vaulting, including the concrete, is only around 9 inches (22cms). The pipework is part of the water sprinkler system, the idea being that the water can pool in the hollows and then run out of the building via the small grated hole set into the wall to the far left. Also visible are the steel supporting brackets that were installed throughout the roof space in the 1920s.

Most of the 14th century timber structure of the roof itself has survived relatively intact. The roof is constructed around a basic A-frame design. Rafters resting on the top of the walls rise diagonally to meet in the centre at the ridge beam. Purlins run parallel with the ridge beam the full length of the building and the rafters are tied together across the width with tie beams, the tie beams supported in turn by curved braces. Above the tie beams are a further set of smaller beams called collars and resting in the centre of the tie beams, connected to the collars, are king posts which help to prevent the tie beams from sagging.

The photograph right shows one of the medieval king posts resting on its tie beam with the smaller collars visible beneath the apex. Also visible, through the slats between the rafters, is the dull grey of the lead that clads the exterior. It's worth remembering that although the cathedral suffered a direct hit on the St James chapel during the bombing raid of May 1942 the vaulting survived unscathed. If one incendiary bomb had pierced the lead and lodged in the dry timbers then the roof void would've acted like a giant chimney and the entire vault would probably have been lost,

The cathedral's archive contains one of the most complete sets of medieval records in the world. These are the so-called Fabric Rolls. Written on vellum, the rolls recorded in minute detail the costs of rebuilding and maintaining the cathedral, the fabric of the building, and cover an almost uninterrupted span between 1279 and 1514.

The Fabric Rolls show that most of the timber for the roof came from English oaks which were sourced from various woods in Devon. There were oak forests that belonged to the Exeter diocese at Newton St Cyres and Chudleigh, and there the oak was matured beneath water at the water mills before being transported into the city. Lustleigh, Canonteign and Langford were just some of the other places that supplied oak trees for the cathedral's rebuilding. The trees must've been vast when felled as some of the beams are enormous and only the inner heartwood was strong enough for construction purposes. The English oaks from which the beams came would've begun as acorns germinating on the floor of a Saxon forest in Devonshire during the 6th or 7th centuries AD. Dendrochronological testing on some of the wood above the crossing has returned a felling date of 1306.

No-one knows when it started exactly, but slowly the roof began to move, a process known as racking. It's possible that the culprit was a missing purlin at the east end (the corbel on which it rested is now empty) removed probably in the 18th century because it was rotten and its structural importance overlooked, but the entire timber-frame structure started to drag itself out of vertical. Like dominoes being knocked over, the top of each A-frame shifted westwards.

The pressure gradually built up on the stone gable end of the West Front and started to shove the gable outwards. It is currently 19 inches (50 cms) out of vertical although the movement has been stopped through the use of the steel brackets under the tie beams.

The photograph above left was taken at the east end. The vertical beam on the right was originally adjacent to the stone wall, the gap which has opened up through the movement of the roof is highlighted in red. The extent of the racking of the A-frames was much more extreme in the timbers located towards the middle of the building and many of them are now an incredible 6ft (2 metres) out of vertical. This is nowhere more apparent than from the top of the transept towers, looking down onto the crossing, where the transepts meet the nave and choir. The photograph above right, taken from the North Tower, shows how the roof over the choir and the nave has moved, dragging both roofs of the transepts with it.

Other areas of the cathedral are also covered by the roof tour, including the roof spaces above both transepts, the lead workshop and the bell-ringing chamber before moving to the top of the North Tower. These are covered in part two of this post.

The photograph below, taken in the nave, shows just one small section of the extraordinarily beautiful vaulting.



Jack said...

Hi Wolfpaw, I am a frequent visitor to your blog and would love to use some of your images in an Exeter tour iPhone app I am currently writing. Would it be possible to discuss image permissions with you in more detail and see if we can work something out?

All the best, Jack

wolfpaw said...

Hi Jack - I'd be more than happy to arrange for you to use some of the images from my blog. If you want to drop me an email then you can use the following address: I check it fairly often as it's where I get notified when people leave comments. I look forward to hearing from you.

AntPDC said...

Interesting blog, thank you. I was intrigued your comments about the use of concrete coatings atop the vault. Apparently the Victorian architects at Exeter revived a long-abandoned practice. From "A history of architectural development" by Frederick Moore Simpson:

"The web [of the vault] is generally of the lightest material available, clunch (i.e. hard chalk) being not uncommon, although courses of harder, heavier stone are sometimes introduced at intervals, as in Westminster Abbey. It varies in thickness from 4_to 8, inches, although in some early vaults a kind of concrete is laid on its extrados, as in Chichester Cathedral and Westminster Abbey nave vaults. In later work this is omitted, probably because it was found that the added strength was dearly bought by the additional weight.

In Chichester Cathedral the concrete is a foot thick, the chalk below 6 inches. In Westminster Abbey the vaults over the eastern part of the church and over about two-thirds of the nave have about 9 inches of concrete above them. The remaining bays to the west, which are a century or two later in date, have none. The Roman tradition of layer upon layer of concrete may possibly have suggested this covering; but it is more probable that the old traditions were lost, and that the early mediaeval builders added the layers because they were afraid of the strength of their thin shells."

wolfpaw said...

AntPDC - thanks for the very interesting comment! I can understand the early medieval builders being a little wary of the thinness of the vaulting. It's easy to forget that they didn't have access to various computer programs to model the spread of weight and to measure the stress on the supporting columns. The Chichester vault must weigh an enormous amount though. Presumably the medieval builders took that into account when they built it. The problem at Exeter is that the columns were never designed to take the extra weight of the Victorian concrete. The sheer audacity of the medieval architects never ceases to surprise me. Obviously there were failures and buildings, towers and spires fell down, but on the whole you get the impression that they tackled enormous difficulties with absolute confidence. At Exeter they threw down one entire side wall in each of the Norman transeptal towers in the full knowledge that the entire structure wouldn't collapse on top of them. It's incredible when you think about it.

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