Kitchen sink - shallow trap suggestions?

[tartanterror]

Hi

Moved to a new place and been having a bit of bother with the sink. It seems like the drain point is a bit high so there’s not enough fall.

Im planning a new kitchen so want to try to fix this problem with the new layout. I can’t move the sink from its current position so how do I lift it up?

I think the current sink is 170-175 deep so I can choose a shallower version. Regular small sink options are 150. But I could choose 120 deep - would that be odd/too small in a one bed flat?

Another option is to get a compact trap which will save some space compared to now - but it seems like the overflow connection pushes the level down. Any way of making that as tight a junction as possible? Seems like the current arrangement loses 30-35mm of height. Anyone know of a particularly small overflow connector?

or last - but hoping to avoid if it’s likely to block/go wrong - a really shallow trap which would still allow one dishwasher connector?

any suggestions thanks?

Can you lower the drain point when the kitchen gets redone?

How does it leave the kitchen and where does it go?

Post some pics please. Show the entire route of the waste pipe all the way to where it joins a vertical pipe or goes through a wall. And what is on the other side of that wall.

It’s not easy to photograph as the drain is hidden away. The kitchen sink is on an external wall but the sink waste runs along that wall behind kitchen units. The goes through a wall to the neighbouring room, where it joins into a cast iron internal drain stack. I’d rather not touch the stack so am looking to improve the fall by lifting the sink/reducing the drain heights at the kitchen end.

I think the underside of the drain pipe at the stack drain point is 57cm above the floor. The kitchen sink is 3m away, so I believe I need a fall of about 7.5cm?

Counting backwards if I have a worktop at 91cm high and a sink of 15cm, and a fall of 7.5cm, then in theory that leaves 11.5cm to fit the waste, overflow and trap…. ?

Any chance of adding a separate trap for the dishwasher further along so it doesn't have to share the sink waste?

I've made my own P-traps from parts before now when tight for space, and some you can adjust anyway. Look at the waste fitting - any thread showing? A nuclear option might be to cut it down to the minimum or source one that's shorter. Plan very carefully to avoid flow problems.

Could you move the sink when you redo the kitchen so it is nearer the waste point?

So please start by posting what pics you can.

I suspect it would be a couple of hundred ponds to sort out the external drainage issue so it's worth bearing that in mind when you think about what compromises you might have to make internally if you don't just lower the external drainage.

Im away for a couple of days so I’ll get a pic by the stack when I’m home. The current sink trap isn’t relevant because im re-doing the kitchen and can buy any sink/trap arrangement that i like. At the moment the only constraint is that I need to fit within a 500 base unit and raise the sink outlet as high as possible.

I can’t adjust the internal stack so I have to work this internally

Thanks - this is what I’m thinking I’ll have to do. My wims say I should be able to get enough fall if I can get a compact trap/waste arrangement

ETA I’d be interested to hear more about your inventions?!

Good idea… this might be the answer as it would allow me to buy an off the shelf compact trap - if they are any good?

It's really just remodelling using compression (or occasionally push-fit) joints to get around awkward spaces. I have used flexible pipe but try and avoid it. It's crucial to get the water level in the trap right. For example, I've seen people fit the outflow higher than the trap exit, so (some of) the horizontal pipework stays full. Also the fall needs to be just right to work properly. It gets too complicated for me when you get issues with venting, which depends on what else is going on in the same run. Keeping it simple and horizontal should help avoid that. Dishwasher hoses need to be routed so they have a high loop. The manual will give the measurements required. A separate trap might help with that as you can enter a vertical pipe (can't think of the word for it - upstand?)

I like to make it dismantle-able for cleaning out every few years. The gunk after a decade (because that's how long I leave it) can be considerable.

Getting a shallow kitchen sink seems to be a good place to start as the current sink seems to be 170 deep and therefore a decent fall could be achieved quite simply.

Options below 150 are limited. Which would you go for?

The 120 deep sink (designed for wheelchair users!) which would provide all of the missing fall but might look slightly odd/small
https://www.kitchenfittingsdirect.com/gb/stainless-steel-sinks/rangemaster-shallow-accessible-sink.html

Or the (uglier IMO) 140 deep which would provide about half of the missing fall so I would also need to adjust the worktop height and maybe get a specialist trap, so still a lot to work out:
https://www.kitchenfair.co.uk/rangemaster-euroline-brushed-stainless-steel-376-mm-single-bowl-surface-mount-kitchen-sink-with-drainer-el860.html

Can you not move the sink nearer the outlet pipe?

A weird sink might cause issues when you sell the flat.

The ideal solution (as you already know) is to reduce the height of the waste into the stack. That solves all problems with the sink, dishwasher and everything. If that really is out of the question, what about an under-sink macerator? You can pump uphill with these, and they aren't complicated. If not, you are into all the issues with the compromises identified above.

Thinking aloud, could you run a long horizontal pipe straight from the sink waste, then drop it into a trap as close to the stack as you can? Not in line with practice or regs, but could work? As @Geneticsbunny and others say, the sink waste needs to be as close to the lowest point as possible. If you can't move the sink (or as well as), sourcing one with the outlet as close to that edge as possible would help.

Are you back yet?

Thanks for the suggestions and further thoughts.

I think I can re-use the existing drain point as long as I select the new kitchen parts carefully.

Can anyone sense-check my sums please?

• worktop at 91cm above the floor
• Height loss across trap (see pic below 18.5 - 13 + 4.3/2 =7.65cm)
• minimum fall of 7.5cm from sink to drain point
• drain point 57cm above the floor

So 91 - 7.65 - 7.5 - 57 =18.85cm height left for the sink and it’s overflow connection.

If I go for a 15cm deep sink that would leave 3.85cm for the overflow connection - does that sound likely? I haven’t been able to find waste dimensions online. My current sink measures 3.5cm so im assuming thats average?

Or should I go for a shallower sink and increase the drain fall?

Excuse my wonky diagram - that dotted line should be level! - but you may not need to worry so much about the depth of the waste fitting itself (the plughole or basket thing). You can build a trap from components (or alter an adjustable one) and bring the other end of it as high as you like under the sink. Depends if you can live with the first joint possibly being underwater. After all, the other joints in the trap are anyway. The standing water level in the trap is the lower edge of the last joint in the trap (ie onto the outlet pipe). You could probably set the last elbow in the trap with its joint almost touching the underside of the sink to give you the maximum height. This won't let you have a dishwasher spigot - that will need a second trap going into the horizontal pipe further along.

Here's how not to do it - obviously the sink would never drain. However the objective is to get point A low enough to drain the sink but high enough to give enough fall to the stack.

Thanks!
I shall give the DIY trap some thought…

In the meantime it seems like the option using conventional parts means a 140 deep sink bowl means I can get slightly more than 25mm/m fall from sink to drain.

The 3m pipe should be well supported to prevent sagging. This will help the flow. Maybe even support it with a batten instead of just clips. Smooth out the joints: clean off/deburr any cut ends; use silicone grease (sparingly) on seals, threads and joint surfaces; use slow bends where possible. Make sure it is supported and not under strain.

Here’s a pic of the drain point with boiler condensate connection. Difficult to get a good picture as it’s all tucked away behind boxings