The essentials in 1 minute
- A sink in a kitchen island only works with waste drainage in the floor at a fall, in a new build planned before the screed.
- The planning value for the fall is 2%, that is 2 cm per metre.
- In an old building an island sink usually needs a lifting station if the outlet is below the backflow level.
- A lifting station is a stopgap: it needs regular maintenance and does not pump in a power cut.
- Where the building offers no free fall, the services-free island is usually the more honest route.
Rusi KolevArchitect & Founder
Real Estate Economist (ebs)
Questions in this guide 13
- What can go in the island and what it requires
- The waste pipe: DN 50 with a fall
- Backflow level and lifting station
- Pipe venting and drinking water
- The most common mistake with an island sink
- New build or existing building: the timing decides
- How we do it at Kolev
- Frequently asked questions
- Next step: the first consultation
What can go in the island and what it requires
A kitchen island can take a sink, a hob with a downdraught extractor (a hob extractor) or no services at all, and each version places its own demands on the floor. The sink is the most demanding, because it needs a supply, waste drainage at a fall and venting. The hob with a downdraught extractor needs power and, in a free-standing island, either a flat duct before the screed or recirculation with a plasma filter, which is the common choice in an island with us. The services-free island needs none of it and stays retrofittable in use at any time.
| Requirement | Sink in the island | Hob with a downdraught extractor | Island without services |
|---|---|---|---|
| Drinking-water supply | yes (DVGW, in the screed) | no | no |
| Waste DN 50, 2% fall | yes, in the floor duct | no | no |
| Venting (DIN EN 12380) | yes | no | no |
| Lifting station, if below the backflow level | usually yes in old buildings | no | no |
| Power or exhaust duct | socket only with a lifting station | 400 V own circuit, flat duct in the screed or recirculation with a plasma filter | socket optional |
| to be planned before the screed | mandatory | mandatory | only socket and light |
| retrofittable in an existing building | barely, open the screed | barely | any time |
The waste pipe: DN 50 with a fall
For a kitchen, the waste pipe of an island sink needs a DN 50 cross-section. DN 40 is only the normative minimum cross-section for a single hand basin, and for a kitchen with grease and food scraps, and for the long run right across the room, DN 50 is the robust choice. The pipe runs at a fall as a pipe in the floor duct: the standard minimum is 1% unvented, and an unvented pipe is only allowed over a short length; the planning value is 2%, that is 2 cm per metre, ideally 2 to 3%; above 5% is poor, because the water runs away from the solids and they are left behind. This floor duct has to be in place before the screed goes in, because after the screed the position of the island is fixed, and moving it means breaking open the floor and levelling it again.
Backflow level and lifting station
Under DIN 1986-100, the backflow level is as a rule the top of the road, and it decides whether a simple pipe at a fall is enough or a lifting station is needed. If the outlet lies above the backflow level, the water runs to the sewer under free fall, which is the normal case in a new build. If the outlet lies below it, typical in the ground floor or basement of an old building, a lifting station with a backflow loop above street level is mandatory: a simple backflow valve is not enough for a kitchen, because a kitchen is not a room of subordinate use and cannot do without drainage. For an island sink the correct standard is DIN EN 12050-2 for faeces-free grey water. A small lifting station such as the SFA Sanivite runs at 400 W, moves around 5,400 litres per hour and tolerates 40 °C continuously and around 70 °C briefly. In everyday use a lifting station means a quiet running noise after every drain, regular maintenance and a dependence on power, because in a power cut it does not pump. For us, then, a lifting station is not an item we actively offer, but a stopgap we tend to warn against. Where the existing building offers no free fall, the services-free island is usually the more honest route.
Pipe venting and drinking water
The venting of the waste pipe stops the trap being siphoned dry and drain odour being drawn into the kitchen. A gurgle at the basin is the forewarning. In the island this is handled by an air admittance valve to DIN EN 12380, which sits vertically and only opens under negative pressure, where no conventional roof vent can be reached. The drinking-water supply follows DIN EN 1717 with an angle valve and a DVGW-tested connection. Kolev prepares the supply line in the floor, and the final connection is made by the DVGW trade contractor or plumber. All together, supply, waste, venting and power, belongs as preparation in the plumbing and electrical plan before the screed goes down.
The most common mistake with an island sink
The most common mistake with an island sink is the wish to lay it in later in an old building without floor drainage: without a pipe at a fall, only the lifting station is left, and anyone who has not planned it in is facing a broken-open screed. The second mistake is too little fall, which the screed swallows, so deposits are left behind and it smells. Just as bad is too steep a fall above 5%, where the water runs away from the solids. The third mistake is missing venting, which siphons the trap dry and lets drain odour into the room.
New build or existing building: the timing decides
New builds and full renovations decide the island sink before the screed goes down, because it is in exactly this window that waste, supply, venting and sockets can be laid in the floor build-up. In a new build the DN 50 pipe at 2% fall in free drainage is usually enough. In an existing building everything hangs on the backflow level and on whether the screed is being opened up anyway. With the old floor in place, an island sink is almost only feasible with a lifting station, and often the more honest recommendation is a services-free island. In the renovation in Frankfurt the sink stayed on the run, and the hob with the downdraught extractor went into the island, an example of the deliberately chosen services-free island: renovation in Frankfurt.
How we do it at Kolev
At Kolev the island’s connections are in the plumbing and electrical plan before the screed goes in: waste DN 50 at 2% fall, the drinking-water supply, the venting and, if a hob is also to go in the island, either the exhaust duct of the downdraught extractor before the screed or recirculation with a plasma filter, the common choice in an island with us, all coordinated with the underfloor heating. We prepare these points, and the final connection is made by the DVGW and electrical trade contractor. We do not plan in a lifting station: if one becomes necessary, that is a planning mistake as far as we are concerned. In an old building without new screed we therefore do not put the sink in the island. We say so beforehand and plan the island services-free. Whether the hob also moves into the island is set out under downdraught extractor or cooker hood. Our kitchens start from €14,999 incl. 19% VAT, including planning, delivery and installation.
Frequently asked questions
Can I fit a sink in any kitchen island?
No, an island sink always needs waste drainage in the floor at a fall. In a new build this works as a DN 50 pipe at 2% fall, laid before the screed. In an old building it is usually only possible with a lifting station, when the outlet lies below the backflow level. Without this preparation the island is better kept services-free.
How much fall does the waste pipe of an island sink need?
The planning value is 2% fall, that is 2 cm per metre, ideally 2 to 3%. The standard minimum is 1% for a short, unvented pipe. More than 5% is poor, because the water runs away from the solids and they are left behind in the pipe.
When do I need a lifting station for an island sink?
You need a lifting station when the outlet lies below the backflow level, which under DIN 1986-100 is as a rule the top of the road, typical in the ground floor or basement of an old building. For an island sink the correct type is a grey-water lifting station to DIN EN 12050-2, such as the SFA Sanivite. Above the backflow level a pipe at free fall is enough.
Is a lifting station loud in everyday use?
A modern small lifting station such as the SFA Sanivite makes a quiet running noise after every drain. Check the sound level in dB(A) in the data sheet of the specific unit. It runs at around 400 W, tolerates 40 °C continuously and around 70 °C briefly, and needs regular maintenance. In a power cut it does not pump, which is part of the honest trade-off.
Why must the island sink be planned before the screed?
Because waste, supply, venting and sockets lie in the screed as a floor duct, and after the screed the position of the island is fixed. Moving it later means breaking open the floor and levelling it again. That is why the island sink belongs in the plumbing and electrical plan before the screed goes in.
Next step: the first consultation
If you would like to know whether your island can have a sink or is better kept services-free, let’s talk. In the first consultation we look at your floor plan, your wishes and your budget, directly with the architect, 45 to 60 minutes, for Frankfurt and the Rhine-Main region.
And a piece of advice no kitchen studio likes to give: get three offers and compare them. We are one of them.
More on the island: planning a kitchen island. Whether the hob should also go in the island: downdraught extractor or cooker hood. Appliances without our own markup: kitchen appliances.