Sizing Sanitary Pipework: Discharge Units, Branches and the Stack

Six flats over three storeys share one stack. The architect has drawn a 100 mm stack. Is that right, or lucky?

The short answer

You can prove it in three steps, and the formula is short:

Q_ww = K × √∑DU

Add up the discharge units, take the square root, multiply by the frequency factor. K is 0.5 for a dwelling, 0.7 for a hospital or school, 1.0 for public toilets, 1.2 for special use.

Branches are sized differently — not by calculation but by a table where diameter, length and gradient trade against each other.

The four systems, and which one you use

BS EN 12056-2 recognises four sanitary pipework systems, I to IV, and the one used in the United Kingdom is System III: a single discharge stack with each appliance connected by its own separately ventilated or unventilated branch, each branch running full bore only briefly. Every table you use for branches and stacks in this country is the System III column.

Primary ventilated stack. The standard domestic arrangement and the cheapest, because there are no separate ventilating pipes at all — the discharge stack itself acts as the vent. It works only if the appliances are closely grouped around the stack. To keep it working: P traps with the discharge pipe the same diameter as the trap; bends and branches avoided; and the stack as straight as possible with a large radius bend, or two 45° bends, at the base.

Ventilated branch. Where a branch cannot meet the unventilated limits, that branch gets its own branch ventilating pipe, connected close to the trap so air enters behind the discharging water. Only the branches that need it are ventilated; the stack itself is still a single discharge stack.

Secondary ventilated stack. A second vertical pipe alongside the discharge stack, cross-connected to it. Only the main discharge stack is ventilated, and that is the whole point: it acts as a safeguard against positive and negative air pressure fluctuations, so trap seal loss is reduced. Water falling down a busy stack drags air with it, creating suction above and back pressure below; the parallel vent gives that air an easier route. It is only likely to be preferred where the number of appliances and their distance from the stack are large. On a house it is expense for nothing.

Stub stack. A short unventilated stack serving ground floor appliances only, closed at the top with an access cover or AAV. Allowed where all of the following are true: it connects into a ventilated discharge stack or a ventilated drain not subject to surcharging; no connected WC has a floor level more than 1.3 m above the invert; and no other branch enters with its centre line more than 2 m above that invert.

Branch discharge pipes

A basin moved to the far end of a bathroom, now 2.4 m from the stack, still in 32 mm. Every time it empties the trap gurgles and the seal drops. Nothing is blocked — the branch is simply too long for its diameter.

Make the pipe steeper and the water runs faster, filling the bore and pulling air behind it; the permitted length then falls. Make the pipe bigger and it never runs full, so it can be longer.

The basin in the story needed a 40 mm pipe from the start. And for a WC branch, take 6 m at 18 mm/m or more as the working limit — so a WC 8 m from the stack needs a branch ventilating pipe.

Gradient is quoted in millimetres of fall per metre, so the total drop is gradient × length. A 6 m soil pipe at 18 mm/m drops 108 mm over its full length. Set your levels from that figure: 1080 mm would be a ski slope and 36 mm would not shift the solids. Run it the other way too — a 3 m, 40 mm sink waste with 60 mm of fall available is 20 mm/m, inside the 18 to 90 band, so the branch is fine.

Length is not the only limit. A 32 mm basin branch is allowed no bends other than the connection bend at the trap, and no vertical drop. A 40 mm basin branch may have two bends. Bath, shower, sink and WC branches allow more bends and a vertical drop of up to 1.5 m.

Connections: junctions on branches of about the same diameter get a 25 mm radius sweep or 45°; branches of 75 mm or more joining an equal stack get 50 mm minimum radius or 45°; branches up to 40 mm joining branches of 100 mm or more connect to the upper part of the pipe wall; and no branch may discharge so as to cause crossflow into another opposite it.

When a branch is too long

Every trap holds a plug of water — a minimum of 25 mm of seal under working and test conditions — and that plug is all that stands between the room and the drain. Pressure is what takes it away:

• Self siphonage — the appliance's own discharge fills the branch, and the moving plug creates a partial vacuum behind it. Basins are the classic victims.
• Induced siphonage — another appliance discharges and empties the seal of a trap that was not even being used.
• Back pressure — air trapped ahead of falling water at the base of the stack pushes the seal back up and out.
• Evaporation, and capillary attraction where a thread or rag hangs over the trap weir.

A branch that has broken the limits has three remedies, and any one of them can be the right answer:

1. Increase the pipe diameter to the next size up, so the branch no longer runs full.
2. Fit an anti-vacuum or resealing trap.
3. Ventilate the branch.

So a 40 mm sink waste that has crept past 3 m can be upsized to 50 mm, given an anti-vacuum trap, or ventilated. The 8 m WC branch, already at 100 mm, gets a branch ventilating pipe.

That pipe: connected within 750 mm of the trap; connected at the other end above the highest spillover level of the appliances served; given a continuous incline so condensation drains back; 25 mm minimum, rising to 32 mm where the branch is longer than 15 m or has more than five bends; and if it runs direct to outside air, finishing at least 900 mm above any opening within 3 m.

An air admittance valve to BS EN 12380 lets air in but seals against foul air coming out, so it can terminate a stack or branch vent inside. It must be above the spillover level of the highest appliance served, accessible and removable.

But note the limit: an AAV relieves suction and does nothing about positive pressure. Where a system has no open ventilation at all, alternative arrangements to relieve positive pressures have to be considered, and stacks connected to drains liable to surcharging need a ventilating pipe of not less than 50 mm connected to the base of the stack above the likely flood level.

Discharge units and the frequency factor

A single house rarely needs sizing: one WC puts you on a 100 mm stack whatever else is connected. Sizing earns its keep on flats, halls of residence, offices and public buildings.

Appliances do not all discharge alike, and they never all discharge at once. So each appliance is given a discharge unit (DU), a loading value in litres per second used only for calculation. BS EN 12056 is the document that lists them — not BS 6465, not BS 8000-13, not Approved Document G.

The WC values are a range because they depend on the pan type. Take the higher figure unless you have manufacturer's data.

The same appliances behave differently in different buildings — a basin in a house is used a few times an hour; a basin in a stadium concourse is used continuously at half time. That is what the frequency factor accounts for, and it is why how frequently the appliances are used affects the stack size — not the water temperature, the height above sea level or the incoming main pressure.

Two rules go with the formula. Continuous flows and pumped discharges are added on without any reduction. And the pipe capacity chosen must be at least the larger of the calculated flow rate or the flow rate of the single appliance with the largest discharge unit — a stack serving one WC cannot be smaller than that WC needs.

A flat with one 6 litre WC (1.7), a bath (1.3), a basin (0.3), a sink (1.3) and a 6 kg washing machine (0.6):
∑DU = 5.2; √5.2 = 2.28; K = 0.5; Q_ww = 1.14 l/s.

Sizing the stack: the worked example

Step 1. Six flats at 5.2 each gives ∑DU = 31.2.

Step 2. Flats are intermittent use, K = 0.5.
Q_ww = 0.5 × √31.2 = 0.5 × 5.59 = 2.79 l/s

Step 3. Read the stack table. Swept entries are used in the United Kingdom, so read that column and take the first capacity equal to or above your flow rate.

Going down the swept column, 2.6 l/s at DN 80 is too small and 3.5 l/s at DN 90 is the first value above 2.79. But there is a footnote: 100 mm is the minimum size where WCs are connected in System III. The answer is a 100 mm stack, with 5.2 l/s of capacity against 2.79 of demand. The architect was right, and now you can prove it.

Where the stack is secondary ventilated the capacities rise: a 100 mm stack with a 50 mm secondary vent carries 7.3 l/s with swept entries.

Approved Document H Table 3 gives the same answer in simpler form, and it is the Building Regulations figure to quote:

Alongside it, fixed minimums: urinals not less than 50 mm; closets with outlets under 80 mm not less than 75 mm; closets with outlets over 80 mm not less than 100 mm. A stack must never reduce in the direction of flow, and its internal diameter must be at least that of the largest trap or branch connected.

A correctly sized stack still fails if it is badly arranged. The bend at the foot should have as large a radius as possible and at least 200 mm at the centre line; no branch may connect lower than 450 mm above the invert of the tail of that bend in single dwellings up to three storeys. Offsets in the wet part are avoided; where unavoidable in a building of not more than three storeys, no branch connection within 750 mm of the offset. The dry stack vent may be reduced in one and two storey houses, but never below 75 mm.

The finished design is proved by the stack performance test, and it does not discharge everything on the stack. For domestic use with 1 to 9 appliances of each kind: one WC, one wash basin and one sink, rising to two of each on a stack serving 36 or more. Those appliances should be close to the top of the stack and on adjacent floors, because that gives the worst pressure conditions. Baths, showers and spray-tap basins need not normally be discharged. Afterwards every trap must retain at least 25 mm of seal, and the test is repeated three times with the traps recharged.

Materials, drawings and quotations

Material choice is a design decision, not a habit. Plastics dominate because they are light, cheap and easy to join, but they expand and contract, so pipes must be firmly supported without restricting thermal movement. Cast iron is chosen where noise matters, in flats and where a stack runs past a bedroom. Where different metals meet, separate them with a non-metallic material to prevent electrolytic corrosion, and take care that any earth bonding remains continuous.

One easily forgotten rule: sanitary pipework connected to WCs should not allow light to be visible through the pipe wall, because a translucent pipe is believed to encourage rodents to gnaw at it. Workmanship follows BS 8000-13.

To show a customer how the pipework will be laid out, the right document is a line drawing that is not to scale. A full set of CAD drawings tells them nothing, and the manufacturer's manual and the bill of quantities tell them less.

A good one shows each appliance in position and labelled with the stack as a single vertical line; writes the diameter and the length against every branch, and the gradient where it matters; marks traps, seal depths, ventilating pipes and any AAV; marks the access points so the customer knows what must stay reachable; and carries a title, a date and a revision. Because it is not to scale, nobody can measure off it — which is exactly the point: the drawing communicates the arrangement, and the written dimensions carry the design.

A quotation is a fixed price for defined work; an estimate is a considered guess. For a small dwelling it shows: the customer's name and address and your own business details; the make and model of every appliance and fitting included; the work covered and, clearly, the work not covered — making good, tiling, decoration, electrical connection; start and end dates and any disruption to expect; the price, how VAT is treated and how long the quotation stands; and scheme membership or a note that Building Control will be notified.

Attach the line drawing to the quotation. Together they answer the two questions a customer actually has: what am I getting, and what will it cost.