The First Angel Roof - Westminster Hall (1395-98)
The earliest known angel roof is that at Westminster Hall, London, which was erected mainly between 1395-1398. (The building is now part of the Houses of Parliament, and is used on major ceremonial occasions).  
The angel roof at Westminster is the work of Hugh Herland (c.1330-1411), Master Carpenter to Richard II. It was installed as part of the enlargement and restoration of the Hall for the image-conscious king. 
Herland’s roof is a masterpiece of art and engineering. It covers a a span previously unprecedented in England (67ft wide x 92 ft high) and uses a “belt and braces” combination of timber arches and hammerbeams to provide rigidity without the need for columns, so leaving the floorspace in the Hall entirely open. (For the likely source of Herland's inspiration see "Westminster Hall: Herland's Inspiration").
It has been estimated that the roof timbers alone weigh 660 tons, while the lead with which the roof was originally covered weighed a further 176 tons.
In the early C20th restorers found that 70% of the roof timbers were by then rotten, and yet the structure still stood, a testament to the degree of redundancy which Herland had built in. It is unlikely that he himself precisely understood the physics of the roof and so the over-engineering is understandable, given the status of his patron, and the unprecedented scale of the challenge. 
The hammerbeams at Westminster Hall are carved into the shape of full-length angels bearing shields blazoned with the arms of Richard II. They project horizontally from the wallplates, intersecting the arch ribs, and stretching beyond to support vertical hammerposts which again connect with the curved ribs higher up. 
Even today, with the benefit of computer modelling and pressure sensors, architectural experts disagree on exactly how Herland's roof works, but its strength is clearly based on the combination of arches (arch ribs) and braced right-angles (hammer posts and hammerbeams) with load also dispersed through vertical tracery (spandrels). 

These are all techniques subsequently deployed in many of the C15th angel roofs of East Anglia. 

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