Bath drains slowly and small amount of water left in the middle

[Stringman_uk]

Hi all
I have a large plastic clawfoot bath.
It has always been slow to drain- Looking at the side of the bath (with the drain on the left) the waste goes down through a s trap along to the right (about 1m) then down through the ceiling, to the void between the joists. when it comes back on its self to exit the house (about 1.3m) the last run has a very drop of about 10-15cm.
The water is often slow to drain (due to hair etc which I often remove) but there is often a little puddle left in the middle of the bath.
When I installed it I put the bath perfectly level - I presumed they were designed with a slight slope on the bottom, I raised the back legs slightly. But no joy.
I suspect that it is just a large cheap bath and thats the way it is?
I wondered if I could fit a small square of wood (to spread the load)under the middle and then use something to "choc" the middle up?

I also wondered if reroute the waste pipe? if spun the s pipe around 180 degrees it would be about 30cm to the outside wall and a further 30cm drop to the waste collector. I think this would improve the waste flow?
Thanks in advance

 

[Hugh Jaleak]

Bath should be made with an inbuilt fall to allow all the water to drain when it's emptied. Waste run shouldn't have an effect on that, but its always good practice to keep the waste from any appliance as short and direct as possible.