Drainage in subfloor or under screed?

Paul Kiernan

My new extension will have wastewater drainage for sink, dishwasher and washing machine. The finished floor will be 50mm screed on 200mm EPS.

Do I need to put drains into my subfloor (at foundation stage) or can I just run them through the insulation and out through the wall into a gully trap (prior to pouring of screed)?

Also can that wastewater be disposed of on site (e.g. by percolation) or does it need to go to the foul sewer (which will involve pumping)? The planning application showed it being pumped.

Gumbo

Paul Kiernan wrote: »

My new extension will have wastewater drainage for sink, dishwasher and washing machine. The finished floor will be 50mm screed on 200mm EPS.

Do I need to put drains into my subfloor (at foundation stage) or can I just run them through the insulation and out through the wall into a gully trap (prior to pouring of screed)?

Also can that wastewater be disposed of on site (e.g. by percolation) or does it need to go to the foul sewer (which will involve pumping)? The planning application showed it being pumped.

Sub Floor depending on pipe diameter.
Foul water discharge should go nowhere near your percolation area and should be entering the existing foul system on site.

Paul Kiernan

Thanks for the reply.

kceire wrote: »

Sub Floor depending on pipe diameter.

By this do you mean if it's a 4" drain it goes into subfloor and if it's just a 2" white it can run under screed? There will be two locations involved, one for washing machine on inside of external wall and one for sink/dishwasher in kitchen island about 3m from external wall.

kceire wrote: »

Foul water discharge should go nowhere near your percolation area and should be entering the existing foul system on site.

Does the fact that it's just greywater (no WC) change anything?

Metric Tensor

Although not aesthetically pleasing the best way of doing sink, dishwasher, etc outlets is a penetration through the wall and dropped into a back inlet gully.

It's easier to clean, prevents smells from getting in and allows easier modifications in future.

The only downside is that it doesn't look great to see a drop pipe on the outside of the wall.

To agree with kceire - it has to go into the sewer and should also have a grease trap!

Metric Tensor

Your response came at the same time as mine - obviously through the wall penetrations won't work for the island! Should still be possible to go in the subfloor and have a drop into a gully outside.

N.B. 50mm of screed is on the thin side - have you check this out with your screed supplier/installer? Is it a specialist screed?

Paul Kiernan

Metric Tensor wrote: »

To agree with kceire - it has to go into the sewer and should also have a grease trap!

If I fit a grease and food trap under the kitchen sink then the waste would be fairly clean! Is there no approved and inexpensive way of disposing it on site?

Metric Tensor

A grease trap is a much bigger device and located under a manhole in your garden. It is normally installed to protect septic tanks / effluent treatment systems. It may or may not be necessary depending on what sewer you are pumping to and what the local authority/irish water say.

No you can't dispose of it on site unless you obtained planning permission to install a septic tank and/or treatment system on your site. If you live in a town or city you won't have a site big enough for this and even if you did you probably wouldn't get permission from it. For the purposes of this you have to treat everything that comes out of your sink or washing machine the same as anything that comes out of your toilet.

Paul Kiernan

Thanks for the help.

The outside wall is a side passage so appearance won't be an issue so I'll probably just go with the pipe into gully option.

As I'll have to pump the water out it might be worth fitting some sort of trap just so the pump doesn't get clogged up with grease.

I presume then I'll drop by gravity from my gully into a sump of some sort and then pump from there back to my foul sewer. Any idea what sort of sump and pump I'd use? I guess I'd need a float switch to trigger the pump?

Metric Tensor

Yup - from gully, through grease-trap (which will require regular cleaning), into pump sump.

Precast concrete would be best for the sump and as you said a float switch and submersible pump. You need to protect that pump with a wire mesh or similar filter and even still be prepared for it to get clogged up on occasion.

You also need some sort of alarm so that you know when the pump is not working or your garden will be under water! Make sure the sump is big enough to cater for a break-down. Although because it's only catering for a specific sink and the kitchen appliances you can just stop using them if you find out the pump has failed.

All commercial installations like this have a duty and standby pump in case one fails.

Mascerators are also common to stop waste from clogging the pump.