厨房外牆排污管維修是誰負責(業主 or 大厦物業管理處??)

那些地方/設施屬大廈的公用部份, 絕對不是用它們在單位內/外去釐定的.
你應先查看你所住大廈的大廈公契, 裡面一般會列明大廈什麼部份由個別業主負責, 什麼部份屬公用部份由業主立案法團負責, 各大廈的情況未必一樣. 若未有特別說明, 便依據《建築物管理條例》的附表1去釐定.
而一般情況下, 渠管的維修責任是依據發生問題的部份屬公用或個別單位使用去界定. 如果那沙兜蓋所在的是公家渠 (例如外牆上的企身大渠), 那當然由業主立案法團/管理公司負責維修. 但如果是分支渠管 (外牆上橫身那些), 而該渠管只是連接你單位的廚房, 即只供你單位使用的, 那便應由你負責維修.
可參考民政事務總署的指引有關排水系統的部份 (見最後一點): http://www.buildingmgt.gov.hk/tc/daily_operation_of_building_management/6_3_15_2.htm

大厦物業管理處...100% sure.

As long as the damaged factility is outside the wall of the main building, it belongs to the common area and it is the responsibility of ALL OF THE OWNERS, not just individual owner. The management office should fix the problem on behalf of ALL OF THE OWNERS.

What you can do is:
1. Report to management office verbally and get them to notify the suffered neighbour.
2. If not much progress, write a letter to the management for record keeping purpose, then notify the suffered neighbour.

The whole purpose is to get the problem fixed by the right people, but don't upset the neighbourhood.

Good luck.

外牆排污管屬於公用設施, 當然就係大厦物業管理處負責.
屋企試過外牆冷氣機位石屎剥落都係大厦物業管理處維修.
所以凡係屋以外既設施都是公共設施, 係大厦物業管理處負責, 即係所有業主共同承擔責任.