Waste pipe compression fitting question

[john2k]

I’m installing a Mira shower tray and the waste trap has two outputs and one output is to be capped with a blanking cap that has a blanking washer. The other pipe that connects to the waste pipe only has a conical ring for the compression fitting but it doesn’t have the usual flat washer that usually sits between the fitting and the flat surface of the conical ring. I’ve attached the Mira diagram. At first I thought it’s missing but the diagram only shows the conical ring for the waste connection. All waste fittings I’ve done in the past have all had a solid plastic washer too. Is this normal? Is it ok to just install without the washer?

 

[terryplumb]

It's fine ,don't overtighten it.
The slip ring on other fittings is essentially to prevent the rubber seal from twisting / misforming, providing even pressure onto it.

 

[robinbanks]

Bit of silicone spray on the conical washer would help prevent it wrinkling, but you may be able to press it into place with fingers before you spin the hut on, to get it nice and even..

Or you can borrow one from elsewhere, or make one out of the next OD pipe size up if you have any lying around and not having it it bugs you

Like Terry says it's to stop the thing you turn from grabbing the rubber and twisting it as you tighten, which would ruin the seal