extra fascia depth would be dependent on how much you want to spend on extra labour by not using standard steel fascia (which is only about 185mm wide) with all the standard fixings ..A carpenter or plumber should be able to make a wide substitute out of sheets of ribbed colorbond steel and the rest should be standard timber framing and soffit linings but check wit a carpenter or roofer about what you want to achieve (why does it need to be 40cm wide??) and there may be a very simple alternative available
oklouise
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