You've hit the nail firmly on the head. The drainage system used at T5 does not follow conventional norms. With conventional gravity drainage, there'd be an array of pipes dropping through the precious retail zones immediately beneath the lounges. Instead, suction drainage was installed which directs the drainage horizontally towards service cores. This system while more expensive, provides greater flexibility and adaptability of tenant spaces which in the long term reduces running costs.

The shower room fit out specification is pretty poor though - resin floors with an anti-slip granite top. That is industry speak for a top layer of granite dust broadcast into the floor finish. Problem with this product is while the anti slip finish provides sufficient abrasion to prevent users from slipping and landing on their behind, it is a total bugger to keep clean. The dirt quickly becomes embedded into the surface finish. This unwanted side effect is visible in all BA's T5 lounge washrooms - all of which suffer from tracking marks on footfall surfaces.

I'd recommend BA inspect The Cabin lounge at HKG before commissioning the retrofit of its T5 facilities