The church was designed by James Gibbs and was built between 1714 and 1723. It was part of the Queen Anne Commission for Building Fifty New Churches.
I recently discovered this gem tucked in the the south west corner of the church. It is a tight spiral, with very plain Portland stone treads. There are a couple little tricks in this flight which I haven’t seen before.
The stair tower has windows that face to the South and West, which means in places the treads have to fly across them, with no support from the wall.
At first glance the floating treads don’t seem to have any special support.
Only by peering down on the window side I saw that the treads above and below the flying tread have large overlaps with it, which clamps the tread into place and doesn’t allow it to rotate.
The image below shows how the treads must be put together – from the inside of the flight it looks as if there is a normal tread overlap, but at the outside of the flight the treads have a large overlap. Presumably there must be some hidden cramp or joggle detail that straps the treads together.
The tread below the flying tread is cut back into the wall to be supported as normal.
The other nice detail is the ways the backs of the treads have been shaped to give a little extra head room.
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