When installing a poured base for a shower do you need to put cement board(dura rock) on top of sub floor under the membrane ?

No... you want to put about 1 inch of a mortar mix at perimeter of shower (high point) and then fill it in and pitch toward the drain. This is required so that any water/moisture that does get past the grout/tile/mud floor over time will drain by gravity to the weep holes in the drain assembly instead of pooling and weakening the floor or creating mold, etc...

You let that dry overnight and then install a roofing paper over the mortar mix and then you install the membrane and the drain. Finish installing membrane and then test the pan for leaks by plugging the drain with a 2" test-ball (sold at plumbing supply house) and filling the pan to within 1 inch of the threshold with water. Hold this test overnight and if no wetness/leaks in AM, pour your pan with a thick floor mud made especially for shower pans (sold at home depot... has additves for strength).

Check out this link for more information:

How to Build a Shower Pan

Come back and let us know if you have any questions...

PS... you could also take a couple socks and a plastic bag and plug the drain off using this... but as a plumber I prefer to test with a test ball!

MARK