May a meat and dairy sink share a wall, or be attached to each other?[1]

The recommended practice: It is initially proper not to have a shared sink, and that each sink should have its own four individual walls, and not be attached in any way. This especially applies to a metal sink.[2] Having completely separate and detached sinks avoid any possible Kashrus concerns.[3] Nonetheless, the following is the letter of the law regarding this matter and can be chosen to be kept by the individual, if he so desires:

The letter of the law-shared walls [see Exhibit A]: A meat and dairy sink should not share walls with each other if the wall is very thin.[4] This applies irrelevant of the material of the wall. If the wall is metal, then it should not be shared even if the wall is very thick, although those who are lenient have upon whom to rely.[5] If the wall is thick and is made of non-metal material, it may share walls with the other sink, although there is room to be stringent even in such a case.[6]

The letter of the law-attached on top [see Exhibit B]: A meat and dairy sink may be made of a single unit and be attached to each other at their top.[7] This applies with any material sink, even metal, although there is room to be stringent by metal sinks.[8]

Exhibit A

Exhibit B

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[1] See Hakashrus 1/14 footnote 26 regarding a metal sink; Kashrus Bamitbach Hamoderni 6

[2] As by a metal sink, there is the added problem of spread of taste from one sink to the other even without heat on the other side. [See coming footnotes]

[3] The concerns: The concern is that meat/dairy taste will spread through the joint material into the adjacent sink and make it Treif. This can happen if a Keli Rishon hot food [such as a Davar Gush or Iruiy Keli Rishon hot food according to Taz 105/4 and Rashal] was placed into any material sink and caused the other sink to get heated up. Or [according to some Poskim] if a Keli Rishon hot food [or Iruiy Keli Rishon hot food according to Taz and Rashal] was placed into a metal sink, even if it did not cause it to get heated up.

[4] The reason: When hot Keli Rishon meat or milk spills into the sink it penetrates the wall a Kelipa worth and depending on the thickness of the shared wall, can penetrate to the other side of the wall. Furthermore, some Poskim rule Iruiy Keli Rishon can penetrate past a Kelipa and it can hence spread as far as its heat can travel, and by a thin wall, the heat could possibly travel to the other side. [Taz 105/4; Rashal and Perisha according to Rabbeinu Tam, brought in Shach 105/5]

[5] As some Poskim hold that by a metal vessel, taste can travel even to cold areas [See P”M 94 M”Z 1; Kaf Hachaim 94/13 and 18], and hence if a Keli Rishon food falls on the wall of one side [such as a Davar Gush], it can send its taste to the other side, even if it is cold. [1^st opinion in Admur 451/22 and 451/75; Michaber 121/7; Taz 121/7; M”A 451/24; Tur Y.D. 121; Rashba in Toras Habayis Hakatzar 4/4] Furthermore, according to those Poskim who rule that even Iruiy Keli Rishon sends taste past a Kelipa worth [Taz 105/4; Rashal and Perisha according to Rabbeinu Tam, brought in Shach 105/5] possibly, this would apply even if the other side is cold, if the material is made of metal. Accordingly, it is best to be stringent by metal sinks, although those who are lenient have upon whom to rely, which are on those Poskim who rule that taste does not travel without heat, and it is unlikely that by a thick wall the other side will become hot. [2^nd opinion in Admur 451/22; 1^st approach in M”A 441/24; Shach 94/3 and 28; 121/17; Teshuvah Mahram Mitz; Rashal Chulin 41; Peri Chadash 94/3 and 24; 121/15]

[6] The reason: If, in truth the walls are thick, and there is a Kelipa’s worth available for each side, then there is no real concern, as the food in a sink is never an actual Keli Rishon [which can penetrate even past a Kelipa worth] but rather an Iruiy Keli Rishon, which can only penetrate a Kelipa worth irrelevant of how far the heat travels. [Shach 98/13 in name of Toras Chatas Klal 85 in name of Issur Viheter; Shach 105/5] Hence, even if meat and dairy Keli Rishon spills on their perspective side of the shared wall, they remain Kosher, as the meat and dairy taste cannot penetrate more than a Kelipa. Now, although there are Poskim who rule that Iruiy Keli Rishon can penetrate more than a Kelipa worth [Taz 105/4; Rashal and Perisha according to Rabbeinu Tam, brought in Shach 105/5], and certainly by a Davar Gush many Poskim rule it always retains its status as a Keli Rishon, nevertheless, by a non-metal vessel the taste can only travel up until the areas that are Yad Soledes, and it is unlikely by a thick wall that the other side will become hot to the point of Yad Soledes.

[7] The reason: Taste of meat or dairy can only transfer from the meat side to the dairy side, or vice versa, if the material of the sink reaches a heat of Yad Soledes [by non-metal materials] and is the status of a Keli Rishon. Accordingly, an attached dual sink is permitted to be used for several reasons: a) As stated above, a sinks hottest usage is Iruiy Keli Rishon and almost never Keli Rishon, it hence will never travel more than a Kelipa’s distance, irrelevant of how far the heat travels. [Shach 98/13 in name of Toras Chatas Klal 85 in name of Issur Viheter; Shach 105/5] Accordingly, since the hot meat and milk will be spilled only into their respective sinks, it will not spread into the adjacent sink, even if the adjacent sink somehow became hot due to it. B) Even if the usage is Keli Rishon, which can travel more than a Kelipa, and even if we would rule like the Poskim who rule that Iruiy Keli Rishon can penetrate more than a Kelipa worth [Taz 105/4; Rashal and Perisha according to Rabbeinu Tam, brought in Shach 105/5], it is still not an issue by non-metal sinks, as by non-metal material, taste can only travel with heat, and since the hot meat/milk in one sink will never be hot enough to heat up the opposite sink, taste of meat/milk will never travel between them. It is highly unlikely that the attached apart of the sinks will ever reach Yad Soledes due to hot meat or milk of the other side, and certainly it would not make the actual meat or dairy sink hot enough to travel its taste to it. If, however, in truth the food would heat up the opposite sink, then even by non-metal materials it could spread its taste to the hot areas. [P”M 94 M”Z 1; Kaf Hachaim 94/13 and 18]

[8] Even if the sink is made of metal with a usage of a Keli Rishon, the main ruling is that we do not say Cham Miktzaso Cham Kulo even by metal [1^st opinion in Michaber 94/1; Rama 94/1], and many Poskim rule this applies even regarding absorbing taste and not just regarding expelling the taste [2^nd opinion in Admur 451/22; 1^st approach in M”A 441/24; Shach 94/3 and 28; 121/17; Teshuvah Mahram Mitz; Rashal Chulin 41; Peri Chadash 94/3 and 24; 121/15]. However, according to those Poskim who rule that even those who do not hold of the rule of Cham Miktzaso Cham Kulo, it does absorb fully into metal even when cold [1^st opinion in Admur 451/22 and 451/75; Michaber 121/7; Taz 121/7; M”A 451/24; Tur Y.D. 121; Rashba in Toras Habayis Hakatzar 4/4], then in truth taste could spread from one side to another, and according to those Poskim who rule Iruiy Keli Rishon spreads even past a Kelipa [Taz 105/4; Rashal and Perisha according to Rabbeinu Tam, brought in Shach 105/5], this would apply even if one pours hot metal or milk into his sink. Accordingly, it is best to be stringent by metal. However, those who are lenient certainly have upon whom to rely as a) It is highly unlikely that one sink will become hot due to the heat in another sink and b) We do not use actual Keli Rishon in a sink and hence even according to the opinion who holds that metal can spread taste even by cold, would not hold this to be the case regarding Iruiy Keli Rishon as the the main opinion follows that Iruiy Keli Rishon does not spread more than a Kelipa.