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External fire spread
Approved Document B, simple methods 1 and 2Permitted unprotected areas against the distance to the relevant boundary.
Method
m
Doubles the permitted unprotected area in both methods.
Façade
m
m
Openings
m²
m²
Combustible cladding, unrated spandrel — anything else that counts.
Within the permitted area
12.8 m²
Unprotected area
8.00 m²
Utilisation
62.50 %
Permitted
16.00 %
The simple methods only, and they are bounded: method 1 applies to small residential buildings within 6 m of the boundary. Neither is a substitute for the full enclosing-rectangle method, and neither says anything about the combustibility of what is on the wall.
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