Truss of the Month

Truss of the Month

A complicated truss this month although the connoisseurs amongst you will immediately recognise a queen post scissor braced truss. Note the short inter-truss spacing and the triple staggered butt purlins. This one is another fine example of Christopher Wren's work, this time at the Royal Hospital,Chelsea over the the Chapel, probably dating from the 1680's. The diagonal scissor braces allow the roof to be carried over a plaster barrel vault.This was a common problem and addressed in a number of ways but here wesee one of the most dramatic examples in a roof with a span of 38 feet. What is unusual is the pairing of the braces. These are bolted using screwbolts and also fixed with rectangular wooden 75mm keys top and bottom (pieces of square section wood inserted like a bolt through several members). This roof also displays a fine set of original iron straps linking the queen posts with the tie beams using an unusual system of pegs.

Queen post roofs allow roof spaces to be inhabited and the nail holesfrom plaster lathes can still be seen in these spaces. They may have beenused as servants quarters in the early eighteenth century although lacking windows they must have been singularly unpleasant spaces, especially in summer. Today we can admire them purely for their structural virtuosity.

James Campbell/98

Last Truss of the Month

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Date: 1998/1/22