Step 14 : remove the part of the main rafter that interferes with the skylight

The central main rafter is a PartDesign body. It can therefore be modified by creating a new sketch from a linked shape.

Create a suitable pocket.

 

Step 15 : remove parts of others main rafters and hipsters
               that interferes with the skylight

The elements that remain to be modified are non-parametric copies :
we are therefore going to use tools from the Part WB.

Nothing being selected any more, go to the Sketcher WB to create the profile
of a blade which will be used to cut the elements to be modified.

From the Part WB, extrude the "blade": this is a surface element
(without thickness).

From the Part WB, select in order (while holding down the Ctrl key) the element
to modify then the "blade" and apply the slicing function : each element
will then have two parts, one to keep, the other to hide.