Building London has posted about Guildhall before, see https://buildinglondon.blog/2022/04/01/36-the-simply-gorgeous-guildhall-crypts/, one of London’s oldest, finest and most interesting buildings, and this post is about the magnificent Guildhall roof, covered with rare Collyweston limestone ‘slates’ from Northamptonshire!
And it’s a mystery when it was first covered in these fascinating slates, and that’ll be discussed in part 1, and in part 2 Building London reports back on a visit to the only mine that now produces these rare slates!
Just to re-cap with a focus on the roof: Guildhall was built in the early 15thC, finished in 1440, [roof #1] but was burnt in the Great Fire of London in 1666. It was partially restored and a flat roof [roof #2] replaced the original high pitched one and this was then replaced in 1866 [roof #3] with a roof similar, it is thought, to the original roof #1! This roof was then destroyed in German bombing in 1940 and a temporary roof [roof #4] was constructed which was replaced in 1954 by the current Collyweston slated roof, [roof #5]  
Th Guildhall Historical Association reprint of “The Reconstruction of the Guildhall – In particular the roof 1953/4” from 1968 states that
“The present roof is the fifth to rest upon the ancient columns of the hall. The first, built soon after 1411. was destroyed in 1666 and little is known of this roof. Controversy still persists between theories as to whether the original roof was a simple hammer beam or a stone arch…. the massive stone piers or columns .. may well have originally intend to support a stone roof…The second roof .. attributed to Wren .. served from 1668 to 1864 .. The restoration … by Horace Jones .. remained in position until .. destroyed by enemy action in December 1940.. The fourth roof was of a temporary nature, of factory type, steel and asphalt, which was erected to protect the fabric of Guildhall between 1941 and 1953 .. [and the fifth roof ] is covered in Collyweston stone tiles which were all laid by local craftsmen.” 
So we can be certain that the current roof is of Collyweston slates, see below, and we also know that roof #2 was of steel – “Whilst the roof of Guildhall collapsed, the medieval walls survived intact … a steel roof was erected as a temporary measure … [and then a ] … new roof for Guildhall was erected by Sir Giles Gilbert Scott and covered in green Collyweston slates in 1953-4.” 
Ruth Siddal of London Pavement Geology states of the current roof, “ The Guildhall is roofed with green Collyweston Slate. Part of the Jurassic Lincolnshire Limestone Formation (Inferior Oolite Group), the ‘slates’ are actually cross‐bedded, sandy limestones which split along the cross‐bedding planes. Although named after the village of Collyweston in Northamptonshire, the slates are extracted from quarries along the outcrop of the beds in north Northamptonshire, south Lincolnshire, Rutland, and northwest Cambridgeshire” 
But was the Guildhall ever previously covered in Collyweston slate? That’s unclear. Bradley and Pevsner suggest that the 4th roof may have been: “Giles Gilbert Scott followed Jones in this when he reroofed the shell in handsome green Collyweston slates, 1953-4” though re the original roof they say simply “What Croxton’s roof was like has been much debated …” 
[ I’m not sure why both Bradley and Pevsner and Siddal refer to the Collyweston slate as ‘green’ though, as when it is first exposed it is yellow and blue but turns a golden brown with age. ]
The 2nd roof, of 1666, is clearly not of slate though and is said to have been flat and of lead. London diarist Blome wrote of the replacement roof after 1666 saying “… and overall the flat roof and platform leaded, whereas before, the roof did meet at the top as in common buildings.” 
And pretty much the only thing we know of roof #1 is that it burnt in 1666! “When the fire reached the area forty eight hours after it had started in Pudding Lane, it completely destroyed all the Livery Halls and the roof of Guildhall itself. Fortunately the exterior stone walls of Guildhall survived reasonably intact and a new roof was erected in 1667-71. Although it was intended at the time to be a temporary solution, it survived 200 years. The post-fire work is believed to have been supervised by Wren although it is more likely to have been undertaken by Peter Mills, the City bricklayer…the new roof for Guildhall was erected by Sir Giles Gilbert Scott and covered in green Collyweston slates in 1953-4.” 
There are no close up drawings of it – this below is a modern illustration of what it might have looked like, though it can be seen further away in this illustration of 1653. .
There is one suggestion that roof #1 was tile, and there is a mention of slate but that is it. The use of the word ‘slate’ here is not specific and could refer to stone slates as Welsh slates were not used at that time “The roof of the chapel ( and perhaps the Guildhall ) was probably tiled since a large quantity of dumped roof tile was found in the Guildhall yard and garden .. parts of the roof may have been in slate since a few fragments of this were also found”  so this could mean that Croxton’s roof was roofed with Collyweston, though there are other types of Limestone slates e.g. Purbeck which could have been used, and we know stone from Purbeck was used as flooring and pillars at Guildhall.
There is, as far as I understand, (the church is currently closed for repairs), one piece of the original, old, 15thC roof whose whereabouts is know, in the neighbouring St Lawrence Jewry church where “… Remains of the old roof from the Guildhall have been worked to form the covering for the 17th century font, a relic from Holy Trinity, Minories.” But this is of the roof structure, not the covering. 
Roof #4 we know was ‘slated’, but again it’s unclear what that means. “The true renovation of this great City hall commenced in the year 1864, when Mr. Horace Jones, the architect to the City of London, was entrusted with the erection of an open oak roof, with a central louvre and tapering metal spire. The new roof is as nearly as possible framed to resemble the roof destroyed in the Great Fire. … The roof, which does great credit to Mr. Jones, is double-lined oak and deal, slated.” 
But looking at various photos and postcards of the 4th roof, it is very hard to tell what it is. It is possible to see them as thicker than thin Welsh slates though in the only tinted picture, they are coloured blue, suggesting they are Welsh slate not Collyweston slate.
So while it’s unclear from any documents and photos Building London has seen if Collyweston slate can be confirmed to have been used on Guildhall before 1953, ( and it appears to have been re-reroofed in 2007  ) and it doesn’t seem to have, and while neither the Corporation of London nor Guildhall Historical Association have been able to confirm what the previous roofs were, ( though I suspect there may be documents in the Guildhall library that would confirm purchases and works from the 1866 rebuilding ), it most certainly has been used now for many years and as a very beautiful and historic building material the next blog post will look at Collyweston slate in more depth, what it is and where it comes from.
Many thanks to Julian Kverndal Senior Heritage Estate Officer at the Corporation of London for correspondence re my research for this post.
Guildhall is very easy to get to in London being a short walking distance of a number of Moorgate, Bank and St Paul’s tube stations and bus routes in The City. You can see the roof from the square outside though nowhere up close.
While you’re there check out the old walls of the Guildhall and if you can book a tour to see the basement, do so!
And visit the 2ndC Roman Amphitheatre, re-discovered in 1988, in the basement of the Guildhall Museum!
 from London 1: The City of London by Simon Bradley and Nikolaus Pevsner (1997) quoted in http://www.astoft2.co.uk/london/guildhall.htm
 The Guildhall of the City of London Baddeley 8th Edition Corporation of London 1951.
 Guildhall Conservation Area Character Summary, Corporation of London 2004 https://doczz.net/doc/1184564/guildhall-conservation-area-character-summary
The London Guildhall : an archaeological history of a neighbourhood from early medieval to modern times. MOLA.
 Ivor Hoole A Guide to the Alleys, Courts, Passageways and Yards of Central London
 Walter Thornbury, ‘Guildhall’, in Old and New London: Volume 1 (London, 1878), pp. 383-396. British History Online http://www.british-history.ac.uk/old-new-london/vol1/pp383-396