Me too. The flat-roof dormer hangs off a joist that runs the length of the ridge. My builder let me and my mate hammer the girder into place, then everything hangs off it. Looks like this herbert thought he'd take the roof off and do the joist later.
Cause unknown, but:
http://www.standard.co.uk/news/london/roof-collapses-with-scaffolding-on-walthamstow-street-a3171946.html