Why ribbed valve washers?

[MonsterMonster]

I've noticed that on some close-coupled toilets the syphon valve washer has one side smooth and the other ribbed or, rather, has two concentric grooves. Which way up are these supposed to go? I've always assumed ribbed side down onto the ceramic as if it was up then the outer rib\edge might not make contact with the rim of the syphon.

Just one of things I've never questioned or not seen in instructions.

 

[shackec]

It is 2024 and I still haven't found an answer to this question!

 

[footprints]

I tend to fit them ribbed side down, I think my toilet likes the extra sensitivity!

 

[shackec]

Thanks footprints. Oddly enough I think I have found the answer; on a different subject post. Ribbed side to the ceramic or the side to be sealed. It seems some recommend a bead of silicon in the rib.
It is what I have assumed in the past but good to have confirmation. How lucky you are to have a sensitive loo!

 

[polesapart]

Ribbed rubber products as advertised on certain "niche" websites are circumspect as to which way round they should be worn.

If any female readers of these pages would like to offer their opinions on this issue I'd be grateful.