help appreciated on best design option for kitchen sink/appliance waste trap

[growfins]

Hi all

I'm building a new kitchen and I'm at the stage of planning out changes to the waste connections for a new sink. I'd appreciate some advice on best practice for the sink trap connections to the current 40mm waste supply.

The new sink is deeper than the old one, but I reckon a p trap would connect at the same vertical level to the current waste pipe.

However I also need to add in waste for a dishwasher. If I use a p trap with a built in appliance spigot tee, I reckon the increased depth on the inlet side will position the appliance tee lower than the waste by about 50mm.

Any advice on the best thing to do here ?

This is what I'm thinking is possible:

a. Ignore a trap with a built in appliance spigot and go for a separate trap for the DW, positioned further up the waste pipe.

b. Use a trap with a built in spigot as it may not be a problem - as long as the sink bottom is higher than the waste supply and the outlet pipe from the trap doesn't end up going up hill towards the waste.

c. Try and lower the waste pipe whilst maintaining a good fall to the stack. The length is around 1.3m and I think I currently have about 30-40 mm drop over the length.

I've attached a pic to hopefully help make clearer. Any advice is appreciated, as is also any wisdom on the magic workings of good trap planning in general.

 

[colshaws]

Do you have access to the soil stack, if so you could cut in a new position for the waste pipe and fit it at a height that suits the sink.

 

[growfins]

Thank you for the reply.

I can access the stack, but it's an old cast iron one in an internal cavity inside the house. As I only might need to drop the waste pipe level by a matter of 50mm or so I was trying to avoid that.

Excuse the ignorance at this end, but am i correct that anything connected on the inlet side of the trap needs to be at a higher level than the outlet and waste pipe side or is there a little tolerance here ?

And should I also aim for a slight gradient in the pipe between the trap and the waste pipe ?

 

[Howsie]

You should have a self cleaning gradient but not too steep to siphon the trap.

 

[growfins]

Thanks. Any recommendations for the gradient required on a 40mm waste pipe for a length of 1.3m ?

 

[colshaws]

Basic guide to calculating falls and gradients for drainage | Basic guide to calculating falls and gradients for drainage | Wyre Council

 

[SimonG]

Cut a new hole.

 

[growfins]

Thank you. How would you recommend on a cast iron stack ?

 

[Howsie]

A metal hole saw and then one of these: 100mm Cast Iron Push Fit Soil Pipe Strap on Boss TX4092 | Cast Iron Push Fit Soil Pipe and Fittings | Drainage Online

 

[SimonG]

And dont cut at too high a speed.