DIY Doctor UK DIY Forums

Hi, my shower waste is slow to drain, the structure is there is a little white plastic pot which acts as the trap… if I remove this then drainage is perfect, which to me suggests the problem is the waste/trap rather than the plumbing from there onwards, is that right? I was thinking of drilling some holes towards the top of the plastic white cup so it will still act correctly as a trap but just improve the flow rate of water out of there into the drain. Any ideas, does that sound good? Any thoughts at all gratefully received?

That cup acts as your water seal as it is known and if you reduce its effective depth, then you risk siphoning that will break the seal and allow smells from the drain.
Is it a high flow trap that you have i.e. 90 mm as opposed to 50 mm? Do you know what the flow rate is at the particular trap that is used? What is the output flow rate of your shower?
90mm traps can generally shift between 25 and 30L per minute, which is adequate for these low profile trays that are all the rage. 50mm traps are used for the deeper, traditional, trays that don’t require the water to be removed so quickly and so a little water lingering around for a short period is not an issue.
What kind of drop is on your pipework? Generally speaking 25mm/m is desirable, or above.
Another way to tackle your issue is that if you have a high flow rate, you could always restrict it slightly in various ways.

Thanks for reply, it is a 50mm waste (McAlpine ST90CPB-P-70) and it only backs up very slightly into the tray before it reaches a kind of equilibrium where that extra few mm of standing water increases the pressure on the waste so its flow rate matches the flow rate of the shower. I think I will ask McAlpine for a new one under warranty (it should carry away 34 litres of water per minute and I’m sure the shower is not supplying that much). Another option would be just drill a few holes towards the top of the cup thingy which would in effect leave me with a 40mm trap instead of 50mm. The trap is always absolutely full, the design of it is such I can’t imagine I would ever in reality get a problem with that.

The other option is to leave it as it is as it sounds like it is working absolutely fine!
The flow rate quoted will be that under ideal conditions and with no pipework connected to the trap. With pipework connected, the rate can drop dramatically.
Another thing that can restrict the flow of water is if the showerhead is directly over the drain, as the water coming down pushes away the water but he’s trying to exit the area. Obviously any changes to the trap or pipework are not going to make any difference if that is your problem!

Sorry to reopen an old post, but did you find a resolution to this? I have the same problem with the same trap and have considered the same fix, drilling holes in the bowl to help the trap drain.