Need to caulk under-mount sink?

Discussion in 'Shower & Bathtub Forum & Blog' started by k9mlxj, Feb 13, 2011.

  1. k9mlxj

    k9mlxj Member

    Messages:
    104
    Location:
    Bay Area CA
    I have a granite bathroom countertop that comes with undermount sink I am going to install.

    I notice that there's no caulking along where the undermount sink meets the granite top. Do I need to add caulking around it to avoid water leaking thru' that area to the outside when water splashes around? If so, I think it'd be along the inner porcelain surface?


    I had added silicone caulking along the inner porcelain surface of another undermount sink where it meets the granite countertop before. Black mold still develops over time along that line, so I need to clean that up manually with a thin wooden stick (can't use heavy chemicals on granite...). Something I don't enjoy doing frequently... .


    I just wonder which is the best way--and to avoid black mold from developing.
    Last edited: Feb 13, 2011
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,353
    Location:
    New England
    The 3 I've installed in my home, I spread a good glob of silicon on the flange before attaching the sink to the underside of the granite. This does two things: seals it, and helps to hold it in place. The granite fabricator I use mills T-slots in the granite, and you use T-bolts and clamps to hold the sink in place. If the sink is already glued in some manner to the granite, then it probably isn't an issue. But, if it is just clamped with nothing there, I'd take it off and then silicon it in place with the clamps.
  3. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    MUST have caulk between the sink and the underside of the granite. After the sink is clamped in place, before it sets up, rub off any caulk which has squeezed out. Use wet finger and/or damp cloth, and use denatured alcohol for silicone caulk. You should be left with a barely visible caulk line, minimizing ongoing cleanup.
  4. k9mlxj

    k9mlxj Member

    Messages:
    104
    Location:
    Bay Area CA
    > rub off any caulk which has squeezed out.

    I don't think for the last countertop+undermount sink I did a very good job rubbing off all of the caulking that squeezed out, or perhaps that area was hard to get close enough to. Now black mold develops along that edge.


    Would that pose a health hazard? I saw flies around the black mold once or twice (I have a separate drain flies problem from the toilet--but don't know if this is also providing a feeding ground for the drain flies as they come up from the drain). I wonder if I need to take out the undermount sink and redo the caulking again because of that black mold. Or black mold is unavoidable along that area regardless of how little caulking shows up along that area.


    There are 3 T-bolts and clamps that come with the set to hold the undermount sink, so the caulking is mainly to seal off the sink from water going thru' along that edge.
    Last edited: Feb 13, 2011
  5. LLigetfa

    LLigetfa DIYer, not in the trades

    Messages:
    4,196
    Location:
    NW Ontario, Canada
    Good quality tub and tile caulk contains mildewcide but still it is best practice to keep it away from the inside of the rim.
  6. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    It is necessary to CLEAN tubs and showers and sinks regularly. Use appropriate disinfectant/cleaners. Use moldex at the FIRST SIGN of a mold problem. Don't wait until it is a bigger problem.
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