Waste outlet seal

Why do shower waste outlets only come with one rubber seal that fits to the underside of the shower tray. Why is there no seal for the top? Can water not leak through the top of the waste? Do I need to use silicon sealant on the top of the shower drain. TIA
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wrote:

If it does how's it going to get out? That's what the seal underneath's for.

I always do. ..
SJW A.C.S. Ltd.
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wrote:

If it's the typical type with the mushroom top and the insert which screws through the tray into the trap, then I'd suggest the following assembly order:
Put small smear of silicone sealer around top flange of trap. Lay rubber ring on that. Apply a small amount of silicone sealer on top of washer. Apply rather more sealer around underside of top component Insert top piece through tray and screw down into trap Tighten down ejecting excess sealant. Remove excess sealant mechanically and by wiping with cloth soaked in meths.
You can either do this before locating the tray or if you organise it right and move quickly, you can locate the trap in place put the tray on top, bedding into the mortar if you are using that and screw in the insert in one operation. I just successfully did one like that.
.andy
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