Editing St Paul's Cathedral (section)

===Structural engineering===
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|image1 = St Paul's Engraving by Samuel Wale and John Gwynn (1755)..gif
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|caption1 =Cross-section showing the brick cone between the inner and outer domes
|image2 = Dickinson St Paul's.jpg
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|caption2 = William Dickinson's plan for the floor paving (1709–1710)
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Wren's challenge was to construct a large cathedral on the relatively weak clay soil of London. St Paul's is unusual among cathedrals in that there is a crypt, the largest in Europe, under the entire building rather than just under the eastern end.{{sfn|Harris|1988|pp=214–15}} The crypt serves a structural purpose. Although it is extensive, half the space of the crypt is taken up by massive piers, which spread the weight of the much slimmer piers of the church above. While the towers and domes of most cathedrals are supported on four piers, Wren designed the dome of St Paul's to be supported on eight, achieving a broader distribution of weight at the level of the foundations.{{sfn|Fletcher|1962|p=906}} The foundations settled as the building progressed, and Wren made structural changes in response.{{sfn|Campbell|2007|p=56–59}}

One of the design problems that confronted Wren was to create a landmark dome, tall enough to visually replace the lost tower of St Paul's, while at the same time appearing visually satisfying when viewed from inside the building. Wren planned a double-shelled dome, as at St Peter's Basilica.{{sfn|Summerson|1953|p=228}} His solution to the visual problem was to separate the heights of the inner and outer dome to a much greater extent than had been done by [[Michelangelo]] at St Peter's, drafting both as [[catenary]] curves, rather than as hemispheres. Between the inner and outer domes, Wren inserted a brick cone that supports both the timbers of the outer, lead-covered dome and the weight of the ornate stone lantern that rises above it. Both the cone and the inner dome are 18 inches thick and are supported by wrought iron chains at intervals in the brick cone and around the cornice of the peristyle of the inner dome to prevent spreading and cracking.{{sfn|Fletcher|1962|p=906}}{{sfn|Campbell|2007|p=137}}

The Warrant Design showed external buttresses on the ground floor level. These were not a classical feature and were one of the first elements Wren changed. Instead, he made the walls of the cathedral particularly thick to avoid the need for external buttresses altogether. The clerestory and vault are reinforced with flying buttresses, which were added at a relatively late stage in the design to give extra strength.{{sfn|Campbell|2007|pp=105–114}} These are concealed behind the screen wall of the upper story, which was added to keep the building's classical style intact, to add sufficient visual mass to balance the appearance of the dome and which, by its weight, counters the thrust of the buttresses on the lower walls.{{sfn|Fletcher|1962|p=906}}{{sfn|Summerson|1953|p=228}}