Technical Booklet D Structure·Page 69·Diagram 4C.7, Note 1
Masonry unit compressive strength for solid external walls
Where external walls are solid construction rather than cavity walls, the masonry units must have a compressive strength at least equal to that specified for the internal leaf of an equivalent cavity wall in the same position.
If the external wall is solid construction the masonry units should have a compressive strength of at least that shown for the internal leaf of a cavity wall in the same position.
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