Development of the design

In designing St Paul's, Christopher Wren had to meet many challenges. He had to create a fitting cathedral to replace Old St Paul's, as a place of worship and as a landmark within the City of London. He had to satisfy the requirements of the church and the tastes of a royal patron, as well as respecting the essentially mediaeval tradition of English church building which developed to accommodate the liturgy. Wren was familiar with contemporary Renaissance and Baroque trends in Italian architecture and had visited France, where he studied the work of François Mansart.

Wren's design developed through five general stages. The first survives only as a single drawing and part of a model. The scheme (usually called the First Model Design) appears to have consisted of a circular domed vestibule (possibly based on the Pantheon in Rome) and a rectangular church of basilica form. The plan may have been influenced by the Temple Church. It was rejected because it was not thought "stately enough"[63] Wren's second design was a Greek cross, which was thought by the clerics not to fulfil the requirements of Anglican liturgy.[64]

Wren's third design is embodied in the "Great Model" of 1673. The model, made of oak and plaster, cost over £500 (approximately £32,000 today) and is over 13 kaki (4 m) tall and 21 kaki (6 m) long.[65] This design retained the form of the Greek Cross design but extended it with a nave. His critics, members of a committee commissioned to rebuild the church, and clergy, decried the design as too dissimilar to other English churches to suggest any continuity within the Church of England. Another problem was that the entire design would have to be completed all at once because of the eight central piers that supported the dome, instead of being completed in stages and opened for use before construction finished, as was customary. The Great Model was Wren's favourite design; he thought it a reflection of Renaissance beauty.[66] After the Great Model, Wren resolved not to make further models and not publicly to expose his drawings, which he found to do nothing but "lose time, and subject [his] business many times, to incompetent judges".[64] The Great Model survives and is housed within the Cathedral itself.

Wren's fourth design is known as the Warrant design because it received a Royal warrant for the rebuilding. In this design Wren sought to reconcile Gothic, the predominant style of English churches, to a "better manner of architecture." It has the longitudinal Latin Cross plan of a mediaeval cathedral. It is of one and a half storeys and has classical porticos at the west and transept ends, influenced by Inigo Jones’s addition to Old St Paul's.[64] It is roofed at the crossing by a wide shallow dome supporting a drum with a second cupola from which rises a spire of seven diminishing stages. Vaughan Hart has suggested that influence may have been drawn from the oriental pagoda in the design of the spire. Not used at St Paul's, the concept was applied in the spire of St Bride's, Fleet Street.[67] This plan was rotated slightly on its site so that it aligned not with true east, but with sunrise on Easter of the year construction began. This small change in configuration was informed by Wren's knowledge of astronomy.[17]

Final design

The final design as built differs substantially from the official Warrant design.[68] Wren received permission from the king to make "ornamental changes" to the submitted design, and Wren took great advantage of this. Many of these changes were made over the course of the thirty years as the church was constructed, and the most significant was to the dome: "He raised another structure over the first cupola, a cone of brick, so as to support a stone lantern of an elegant figure... And he covered and hid out of sight the brick cone with another cupola of timber and lead; and between this and the cone are easy stairs that ascend to the lantern" (Christopher Wren, son of Sir Christopher Wren). The final design was strongly rooted in St. Peter's Basilica in Rome. The saucer domes over the nave were inspired by François Mansart's Church of the Val-de-Grâce, which Wren had seen during a trip to Paris in 1665.[66]

The date of the laying of the first stone of the cathedral is disputed. One contemporary account says it was on 21 June 1675, another on 25 June and a third on 28 June. There is, however, general agreement that it was laid in June 1675. Edward Strong later claimed it was laid by his elder brother, Thomas Strong, one of the two master stonemasons appointed by Wren at the beginning of the work.[69]

Structural engineering

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.[70] 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.[71] The foundations settled as the building progressed, and Wren made structural changes in response.[72]

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.[73] 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 which 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.[71][74]

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 clerestorey and vault are reinforced with flying buttresses, which were added at a relatively late stage in the design to give extra strength.[75] These are concealed behind the screen wall of the upper storey 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.[71][73]

Designers, builders and craftsmen

During the extensive period of design and rationalisation Wren employed from 1684 Nicholas Hawksmoor as his principal assistant.[76] Between 1696 and 1711 William Dickinson was measuring clerk.[77] Joshua Marshall (until his early death in 1678) and Thomas and his brother Edward Strong were master masons, the latter two working on the construction for its entirety. John Langland was the master carpenter for over thirty years.[58] Grinling Gibbons was the chief sculptor, working in both stone on the building itself, including the pediment of the north portal, and wood on the internal fittings.[58] The sculptor Caius Gabriel Cibber created the pediment of the south transept[78] while Francis Bird was responsible for the relief in the west pediment depicting the Conversion of St Paul, as well as the seven large statues on the west front.[79] The floor was paved by William Dickinson in black and white marble in 1709–10[80] Jean Tijou was responsible for the decorative wrought ironwork of gates and balustrades.[58] The ball and cross on the dome were provided by an armorer, Andrew Niblett.[81]