I am creating simple roof 'hipped' on a building (mesh cube) and want to give it a precise 'thickness'. What is the correct method? I have tried divided the top surface of the cuve with loop cut, selected the edge and extruded on the Z axis with a keyboard command eg E Z 55. To give it thickness I have tried selecting each (top) face of the roof using a keyboard command E Z 0.2 which does not work for me. I have also tried using the extrude 'tool' but cannot work out how to extrude a precise amount eg 0.2
Extruding using E will by default use an axis that is normal to the faces.
In your situation, as the roof is symmetrical, this will be Z.
If you hit EZ, that will remove this default axis, and let you extrude following the mouse movements.
So you just need to press E then the value you want.
If you want to use the extrude tool, simply use it then go to the operator panel and enter the value you want: