domed shower waste washer

[stuart noble]

From what I can gather the black rubber washer goes on the underside of the shower tray but, with this particular trap, no other washers are provided, so I assume this is the only seal. I've heard of of plumbers putting it on the top side but AFAICS this would raise the level of the drain part so that there would always be a circle of water sitting round it. Any advice welcome

[sm_jamieson]

Silicone sealant on top, rubber washer on bottom. Is how I would do it. Simon.

[stuart noble]

and that's how I've done it too