Required Fall for bath waste.

[malc1]

Hi
I`m stripping out my bathroom and doing a complete new instalation.
A whirlpool bath is being used which is deeper than the one that is coming out.This means that the trap is further into the space below floor board level and therefore the fall on the waste pipe has been reduced.The waste pipe runs above the floor level,will be boxed in,is in two lengths(1.5m`s from the trap,a 90 degree bend and then 1.7ms into the soil pipe)The entry hole /rubber seal below floor level and a pair of 90 degree bends are used.
What is the minimum amount of fall I can get away with on this pipe?
I can`t raise the bath any more due to the bath panel used and I need as much access as possible for the pump.(Its 500mm from floor level)
Malcolm

 

[Slugbabydotcom]

I think the official figure is something like 1 in 200 but in the real world so long as you have some fall it should be fine

 

[Nige F]

90degree bend.......FFS use a swept bend

 

[ChrisR]

Wouldn't want to go less than the "book" 1 in 40. Any pipe sag, bits of grot in there, and more grot will build up.