On Tue, 27 Aug 2019 11:06:54 +0100, (Roger Hayter)
wrote:

Jim GM4DHJ ... wrote:

On 27/08/2019 10:26, Roger Hayter wrote:
Harry wrote:

Lee Nowell expressed precisely :
I am installing a new shower tray and the waste it comes with has a very
thin washer that goes on top of the tray and underneath the waste
fitting. Is this enough to seal the top or should I silicone it on to
the tray? Or I guess fit it and silicone around it?

Put some Plumbers Mait around the sealing points, plus long the threads
too, to be sure of no leaks. It's none setting, so comes off easily
where it squeezes out or if it needs to be disassembled later.

If I remember rightly I don't think the washer on top *is* an important
sealing point, as the metal part is open underneath. It just stops
water bypassing the plug, and a slight ooze does not matter. The seal
is to the underneath of the tray, and this is where sealant is best
used.

correct

Except that we're probably thinking about sinks with integral overflows,
and maybe shower trays and baths don't have plugs or integral overflows,
so the seal on top may be more valuable, It still looks messy having
sealant on top, but if you don't then you need to seal the threads of
the back nut as well as the face.

So I withdraw my comment.

I think the issue is with single layer fittings (no integral overflow)
the seal *can* be at the top and without the need for any sealant on
the thread.

With a shower where you don't have a built in overflow, even though
you might never use a plug and therefore don't need to seal the tray
as such, water going under the top flange may still leak past the
lower washer / threads but the difference is you *only* need to seal
the top flange in that case.

Anything with an integrated overflow generally means you *will* have
water around the outside of the waste below the primary surface and
therefore the threads and lower nut-to-basin exposed to water, hence
the need to seal both top (water bypassing the plug but not leaking
out of the system) and bottom / threads (leakage from the system).

I must admit I was surprised to learn that you had to seal such things
with sealant (rather than just rubber washer washers) 'these days'.

You might be able to get a straight mechanical seal where the waste
fitting is designed to match the (say) basin where they could have an
expanding seal [1] on a plain (not threaded) tube.

Cheers, T i m

[1] Like the rubber bushing you get in the bolt up BNC connectors and
the like.