Converting a loft with trusses vs converting 'cut roof' loft

[RomanAbramovich]

Been reading loft conversion advice this evening, noticed a statement repeated on many websites:

"For cut roofs, your loft has potential to be converted provided the available head height measured from the bottom of the ridge of timber to the top of the ceiling joist is minimum 2.2 - 2.4 metres.

For trussed roofs, your loft has potential to be converted provided the available head height measured from the bottom of the ridge of timber to the top of the ceiling joist is minimum 2.4 - 2.6 metres."

None of the websites mention why the extra 20cm is required to convert a loft constructed with trusses - can anyone enlighten me?

 

[big-all]

at a guess trusses 3" timber all round normal roof timbers perhaps 8" up top and 4" below

 

[^woody^]

Stay away from those websites.