'In cases where it is impractical to drill into the surface of a building for screw or anchor ties, or where the quality of the fabric of the building is not known, or where it is impractical to open windows for tube and fitting ties, the attachment of the scaffold may be made by reveal ties - which rely on friction.'
'When used in scaffolds greater than four lifts the scaffold should be seperately designed although it is recommended that no more than 50% of ties to a facade should be reveal ties.' TG20:08 Volume 1 - 5.4.5 'Reveal Ties'
It was the recommendation in BS5973 - no more than 50% reveal ties in an un-sheeted scaffold and 25% of a sheeted scaffold, although now a sheeted scaffold would need a design anyway.
I thought everyone knew that