The roof of the Sheldonian theatre in Oxford, built from 1664 to 1669, is constructed from timber beams which are unsupported apart from at the walls, and held together only by gravity.
The design of the roof was due to John Wallis, the Savilian Professor of Geometry.
"So effective [was the roof] that for nearly a century the University Press stored its books ..., and for many years it was the largest unsupported floor in existence"
Each beam is a straightforward cuboid with some notches cut out of it. Rather than whittle away at balsa wood, I decided to 3D print some.
To construct the pattern, arrange the beams on top of each other. Every beam goes the same way up - the notches in the middle should be on the top side.
Four beams woven together will show the general principle - rest the free ends on something, and the middle will stay aloft without any support underneath it. The pattern can grow indefinitely, by adding more beams.
There's some more information about the roof of the Sheldonian theatre in this document about the Maths in the City walking tour of Oxford.