The message
from Mike Scott contains
these words:

Should I not even attempt this? Any thoughts welcome, thanks!

Considering how much weight I've got in my loft on the same size joists
I don't reckon you've got anything to worry about.

For a start, do you ever walk in the loft? If so you must be putting
more load than that on the joists as you walk about.

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