Belfast Sink Waste: Slotted or Not, and the 75mm Trap Rule

Choosing the sink waste and trap for a Belfast or butler sink is where a lot of otherwise careful kitchen projects come unstuck, because the fitting that works on every inset stainless steel bowl in Britain is the wrong fitting here. There are only three decisions to get right: whether the waste is slotted, how deep the trap seal has to be, and whether you have the height to fit the trap you legally need. Get the first one wrong and you get a slow leak inside the cabinet that nobody sees for months.

Belfast sinks have an overflow, butler sinks do not

The two names are used interchangeably in showrooms and they should not be. The traditional Belfast sink has a weir overflow, a slot cut into the back wall of the bowl leading to a channel that runs down inside the fireclay to the waste hole. The butler sink has no overflow at all. Fill it past the top and it goes on the floor.

That single difference decides the waste fitting:

• Belfast, with a weir overflow: slotted waste. The waste body has openings cut in its side. Water arriving down the overflow channel enters the waste through those slots and drains away.
• Butler, no overflow: unslotted waste. A solid body with no openings.

Fit an unslotted waste to a Belfast sink and the overflow channel has nowhere to discharge. Fit a slotted waste to a butler sink and every time you use the bowl, water leaves through the slots into the gap between the waste body and the fireclay, then finds its way out under the sink. The second mistake is the common one and the harder one to spot, because from above the sink drains perfectly.

Check before you buy rather than after. Look at the inside back wall of the bowl: if there is a slot or a rectangular opening near the top, you have an overflow and you need a slotted waste.

What size waste a fireclay sink needs

Two things are different about a fireclay sink compared with a pressed steel one.

The first is thickness. The base of a stainless steel bowl is under a millimetre thick. The base of a fireclay Belfast sink is thick glazed clay, and a waste designed for a steel sink often does not have enough thread below the flange to reach through it and take the backnut. This is why manufacturers list a separate item for the job. McAlpine, for instance, catalogues a Centre Pin Belfast Sink Waste as its own product rather than folding it in with the standard sink wastes.

The second is the seal. On a steel sink the waste seals against a smooth pressed surface. On fireclay it seals against a glazed radius, which is why these wastes come with a thick rubber or fibre washer above and below, and why overtightening is the classic way to crack a sink. Tighten until the washers just start to squash, then stop.

Everything downstream is standard: the outlet is 1½ inch, which connects to a 40mm trap and 40mm waste pipe, exactly as any other kitchen sink does.

The trap seal: 75mm, unless you discharge to a gully

This is the part almost no consumer guide gets right, and it is written down plainly. Table 1 of Approved Document H gives the minimum trap size and seal depth for each appliance. For a sink it is a 40mm trap with a 75mm depth of water seal.

Then there is footnote 3 to that table, which is the bit worth knowing: “Traps used on appliances with flat bottom (trailing waste discharge) and discharging to a gully with a grating may have a reduced water seal of not less than 38mm.”

A Belfast sink is a flat bottomed appliance. So if, as in a lot of older British houses, your kitchen sink discharges through the wall to an open gully outside rather than into an internal soil stack, a shallower trap is allowed, down to 38mm of seal. If it discharges into a stack inside the building, it is 75mm and there is no argument.

That matters commercially as well as legally, because the shallow traps sold to solve the height problem are shallow precisely because the seal is short. McAlpine’s shallow bottle trap for a 1½ inch waste is named for its seal: 40mm. That is fine over a gully. It is not fine on a stack.

The same document also sets two rules people forget. Paragraph 1.6: a trap “should be fitted directly after the appliance and should be removable or be fitted with a cleaning eye”, so do not glue yourself into a corner you cannot rod. And paragraph 1.13: a branch pipe discharging to a gully “should terminate between the grating or sealing plate and the top of the water seal”, which means the pipe has to go down below the gully grating, not simply spit onto it.

Check you have the height before you order

Here is the practical trap that catches people replacing an existing sink. A Belfast sink is deeper than the bowl it is replacing, so its waste outlet sits lower, so there is less height left between the outlet and wherever the waste pipe currently leaves the wall.

Work it through on your own kitchen with three measurements:

1. Worktop height. Usually 900mm, but measure, because a fitted kitchen on an uneven floor rarely is.
2. Bowl depth. A common inset bowl is around 180mm. A classic Belfast is 255mm. Subtract from the worktop height to find where the outlet will sit.
3. The height of your existing waste pipe where it leaves the wall or meets the stack.

In the worked example above, the outlet drops from 720mm to 645mm while the pipe stays at 560mm, so the room available for the trap and the fall goes from 160mm to 85mm. A 40mm bottle trap with a compliant 75mm seal, plus the waste tail above it, will not always fit in 85mm. If it does not, you have three honest options: raise the sink, lower the pipe, or move to a shallow trap and confirm you are discharging to a gully.

Do this arithmetic before the sink arrives. A 595mm fireclay Belfast weighs a substantial amount and is not a thing you return casually.

The rest of the run

Once past the trap, a sink uses 40mm waste pipe. Approved Document H caps both the length and the fall of an unventilated branch pipe: its Table 2 allows a 40mm pipe to run 3.0m at a fall of 18 to 44mm per metre for a washbasin or bidet, and the limits for sinks, baths and showers are set out in its Diagram 3. The principle is the one to hold on to. A branch that is too long, too shallow or too steep siphons the trap dry, and if you exceed the tables the branch has to be ventilated, either to outside air or with an air admittance valve.

Two more things that are specific to this sink rather than to plumbing in general:

• Support the sink, not the plumbing. A Belfast sink full of water is heavy and it must be carried by the cabinet or a purpose built frame. Never let the waste pipe or the trap take any of the load. Our guide to kitchen base units covers the carcass side of this.
• Leave the trap reachable. A Belfast sits low in the run and the cupboard beneath it is often the one with a false front. If you fit a solid panel you cannot remove, you have made the trap unserviceable, which is the thing Approved Document H paragraph 1.6 is trying to prevent.

If you are still choosing the sink itself, our Belfast sink buying guide covers sizes and materials, and kitchen sink types compared sets it against the alternatives.

Frequently asked questions

Do I need a slotted or unslotted sink waste for a Belfast sink? Slotted, if the sink has a weir overflow, which the traditional Belfast does. Look inside the bowl at the back wall: an opening near the top means there is an overflow channel, and it drains through the slots in the waste body. A butler sink has no overflow and needs an unslotted waste. Fitting the wrong one either blocks the overflow or leaks into the cupboard.

What size is the waste hole in a Belfast sink? The outlet is 1½ inch, the same nominal size as any UK kitchen sink, so it takes a 40mm trap and 40mm waste pipe. What differs is the thickness of the fireclay the waste has to pass through, which is why Belfast specific wastes have a longer thread below the flange than a waste made for a pressed steel bowl.

How deep does the trap seal need to be on a kitchen sink? Approved Document H, Table 1, sets 75mm of water seal on a 40mm trap for a sink. There is one relaxation, in footnote 3: an appliance with a flat bottom discharging to a gully with a grating may go down to no less than 38mm. A Belfast sink is flat bottomed, so if your waste runs out through the wall to an external gully, a shallow trap is permitted. Into an internal stack it is not.

Can I use a shallow trap under a Belfast sink? Sometimes, and the deciding factor is where the waste goes, not how tight the cupboard is. Shallow bottle traps are sold with seals around 40mm, which meets the 38mm floor for a flat bottomed appliance over a gully but falls well short of the 75mm required otherwise. If you are on an internal stack and cannot fit a 75mm seal trap, the answer is to change the height of the sink or the pipe, not the trap.

Why is water leaking under my new butler sink? The most likely cause is a slotted waste in a sink with no overflow. Water leaves the bowl through the slots, into the void between the waste body and the fireclay, and runs out under the backnut. It looks like a failed washer and it is not. Take the waste out, check whether the body has openings in its side, and if it does, replace it with an unslotted one.

Does a Belfast sink need a different trap from a normal sink? Not a different type, just one that physically fits the height you have. It is still a 40mm trap. What changes is that the deeper bowl puts the outlet lower, so measure the gap between the outlet and your existing waste pipe before you buy anything. The seal depth requirement does not change to suit the space available.