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I have seen this done both ways. I put my plumber's putty on the sink in a circle that meets the bottom of the sink or top of the ring.

I have seen others attach the putty to the underside of the flange.

What are the positives and negatives of each and which is right?

On the sink

enter image description here

On the Flange

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DMoore
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1 Answers1

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There is no "right way", but the "theory" is:

On a loose fitting (fitting has play inside the hole), placing the putty on the sink will create a better seal on the area actually making contact. On a tighter fitting, placing the putty on the flange will seal the areas closer to the center leaving less room for voids. See this diagram (tight fitting on top, loose fitting on bottom, "risk" for voids/incomplete seal in red):

Updated(2): Diagram of Flange and Plumbers Putty

Honestly, while the "theory" isn't bad, in practice it simply requires that you use enough putty to make a good seal. The net effect is that the putty just needs to be sandwiched between the flange and the sink to prevent water from leaking, either from an (intentionally) stopped sink into the overflow (bathroom) and thus down the drain, or from around the flange into the cabinet below when the flange acts as a reducer or on sinks without overflows (kitchen).

So, whichever way works best for you is the "right way".

Edit: Updated diagram to reflect the proper amount of putty. Edit(2): Updated diagram to reflect proper amount of putty including over-puttying.

Jacob S
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    On the right side pictures - wouldn't that make a case to not do it that way? There would always be water there right? Just thinking that water trapped in there won't evaporate as easily as it would on top surface. – DMoore Jul 30 '13 at 22:26
  • There would always be water there if you didn't use enough putty. This question/answer may help: http://diy.stackexchange.com/questions/30109/what-is-the-required-amount-when-using-plumbers-putty/30110#30110 – John Smith Aug 01 '13 at 01:05
  • Very true @DMoore and it was just a rough sketch -- my main point I was attempting to make was that in either case, you want to use enough to make a good seal. – Jacob S Aug 01 '13 at 14:20
  • update to reflect how it looks with the proper amount of putty (at least). I just follow my personal code of, it's not sealed until putty squeezes out all the way around. – Jacob S Aug 01 '13 at 14:29
  • Great pictures! Really explains the geometry. – Alex Feinman Aug 01 '13 at 14:48
  • The pictures are really good. Would definitely help a person new to plumbing. Which way of putting on the putty gives you the best chance of the top-right or lower-left (which I believe are the two best solutions). Also what does it look like when you over-putty - as a person who has installed a lot of sinks I have found that over-puttying is the #1 cause for leaking in my work. – DMoore Aug 01 '13 at 16:46
  • Updated, let me know if you have any other recommendations @DMoore – Jacob S Aug 02 '13 at 16:15