errm, 50x170 - that's 7x2 isn't it? ;-)

175mm x 50mm is 7 inches by 2 inches ... BUT.. you may find you've
been sold 165mm x 47mm instead when you come to actually measure it.
Also I used C16 but found that three out of the fourteen that I'd
bought did warp badly in the 165mm. direction. If I was doing it again
I would use Redwood. It just feels a lot stronger than the C16. Also I
was exposing the underside of the joist by setting the 12mm.
plasterboad back by about 35mm. to give a slightly old fashioned look
to the ceiling.

Chris.