Commonly there are 3 reasons to use a waterproof floor with floor drain in combination with a hinoki tub.
Reason 1) The first is as a double safe, to pick up some condensation or a discharge caused by an eventual crack in the wood.
Reason 2) The second is for having a “shower area” to wash the body before entering the tub (which must be used for soaking only).
Reason 3) The third reason is to drain the water overflowing from the top of the tub.
Solution 1) To satisfy the first circumstance, a waterproofing the floor with 1/50 slope should be enough. For small spills the floor can be sweeped. Adding a floor drain would be safer but would probably involve going thru the slab or raising the floor. Another solution would be to build up the tub to create a waterproof pan under the tub only. (see curb detail @ bottom-RIGHT)
Solution 2) The second point does not need to be addressed, if you have a separate
shower area. Otherwise you will need at least a 60mm waterproof pan with
floor drain. (see full waterproofing detail @ bottom-LEFT)
Solution 3) With reference to the overflow: to prevent water overflowing you should keep the water level about 100mm under the top. Anyway for standard tubs (external height = 700mm / internal depth = 600mm) this means having 500mm or less of water. Also, it might be cumbersome to have to adjust water level when one person enters the tub. And again if two people use the tub
together. For this reason japanese bathing usually involves a top overflow facing to waterproof floor area in front of the tub. The waterproof pit does not need to be very deep as long as this area is wide enough. This area in front of the tub can be covered with duckboards in hinoki wood. (see full waterproofing detail @ bottom-LEFT) See also waterproofing details
Hinoki, (Chamaecyparis obtusa), or Japanese Cypress is one of the most prized quality of wood in Japan for few simple reasons: 1) Hinoki has excellent strenght and mechanical resistance 2) Hinoki trees grow high and straight like poles, making it ideal to use for construction 3) Hinoki wood contains natural bactericidal agents so it resists to mold, and insects 4) Hinoki doesn`t fear humidity and does not rot. Hinoki structures last 1000`s of years!
HINOKI & HEALTH:
Hinoki wood oil is highly therapeutic: it contains minerals and essential oils such as Thiol: for this reason it is used as raw materials for soaps, antiseptics, perfumes, cosmetics, and hair restoration treatments. While resins from other evergreens are known for causing skin irritations, hinoki wood oil is gentle on the body and skin: it is traditionally used to treat skin irritations and injuries thanks to its antibacterial and antifungus action. When inhalated, hinoki wood oil acts as a decongestant of the respiratory system, is used to cure asthma and as a tonic for the nervous system. For this reason, Hinoki wood oil is also widely used in aromatherpy to reduce stress.