I need to get a 7m 254*102*25 RSJ ridge purlin into place, this is
gonna weigh in at at least 350kg, so me and the girlfriend wont be
able to lift it onto the roof.

The "normal" way I guess is with a crane, but I can't get a crane to
the house, nor can I get a big enough teleporter or excavator in.

There is a window below the RSJ at both ends, the tops of the windows
are not build yet.

The current plan is to get the thing in through the window (we can
drag it with the bobcat), then using scaffolding and pullies lift it
up, building the scaffold underneeth to stop it dropping. When we get
to the top of the windows we can put the bricks in under it.
My worry with this plan is that the scaffold will have to hold if for
some days while all the brickwork sets, and a big storm may not be
good for the situation.

Is this the way is normally done, or is there some trick that I have
not thought of yet ?

Thanks
Rick