The message
from "Brian Sharrock" contains these words:

but if one
wants to 'construct' a roof using rafters, purlins, and joists of certain
sizes at certain spacing .... how does one start?

Do one, then copy it all down the roof. Select move/copy, choose the
item to be copied, then press and release [Ctrl]. A + will appear beside
the cursor. Now, click on the endpoint, and then type /n [Enter] where n
is now many repeats you wanted. You can change the number as often as
you like until you do something else, then it's locked.

--
Skipweasel
Pay no attention to that man behind the curtain.