Channel Drain (Threshold Drain): The Strip That Stops Your New Bifolds Flooding the Kitchen

You spend a serious chunk of your build budget on a set of aluminium bifold doors (a 3-panel installed set sits in the £3,150–4,670 range for standard spec, more for bespoke). The builder finishes the patio flush with the threshold so it looks magazine-ready. The first November storm rolls in. Wind drives rain horizontally against the doors, water sheets off the patio, hits the door track, and runs straight under the threshold seal into your new oak floor. The repair bill for damaged flooring, skirtings, and door reseating runs £2,000–5,000. The thing that would have prevented all of it costs £29 – £40 per metre and an afternoon's labour. That thing is a channel drain.

What it is and what it's for

A channel drain (also called a threshold drain, slot drain, linear drain, or simply "an ACO" after the dominant brand) is a long, narrow trough buried flush with the surrounding paving. Water runs off the patio, falls into the slot, travels along the channel, and exits through an outlet pipe to a drain. It looks like a thin metal grate sitting at ground level. That's the visible part. Below the grate is a polymer or PVC channel body bedded in concrete.

For a UK kitchen extension with bifold or sliding doors, channel drain isn't decorative. It's a regulatory requirement that turns up in three different places.

The first is NHBC Standard 10.2.6, which governs paving against the house. Paving must sit at least 150mm below the damp-proof course (DPC) and fall away from the building, OR a channel drain must be installed at the wall to intercept surface water. The 150mm step is the default. A flush threshold is the exception, and the channel drain is what makes the exception legal.

The second is NHBC Standard 7.1.16 on accessible thresholds. Where the threshold is flush (15mm maximum upstand, anything above 5mm chamfered), a drainage slot or channel adjacent to the door cill is mandatory. Combined with a platform falling 1:60 to 1:40 away from the door.

The third is Approved Document M (access to and use of buildings). New extensions are expected to provide a level threshold so the building is accessible to wheelchairs, prams, and anyone with limited mobility. Level threshold means flush. Flush means you've just removed the natural water barrier, and you need a drain to replace it.

So when your architect specifies "level access threshold" on the bifold doors and "flush patio at threshold" on the landscaping drawing, what they're really specifying, whether they label it or not, is a channel drain. Without one, the build doesn't comply with Part M. The first heavy rain proves the point.

Uniflow vs dualflow: the distinction nobody explains

This is the technical detail that separates a working installation from a leaking one, and it's missing from almost every consumer guide.

A uniflow channel drain has a single inlet at the top: the slot or grate that water drops into from the paving. That's all it captures. A standard ACO ThresholdDrain or any generic patio channel is uniflow.

A dualflow drain has two inlets: the top slot for surface water from paving, AND a second inlet at the back of the channel that captures water exiting from the door track weep holes. Bifold and sliding doors have small drainage holes in the bottom of their tracks. Wind-driven rain that gets past the seals collects in the track and exits through these weep holes. On a stepped threshold (where paving is 150mm below DPC), that water just drips harmlessly onto the patio. On a flush threshold, those weep holes spray straight onto the paving inside the channel's reach, and a uniflow drain often catches it. But under heavy wind-driven rain or where the door cill sits directly over the channel, weep-hole water can run behind the channel into the wall.

The purpose-designed product for this is ACO AS350. A waterproof membrane clips under the door threshold and runs down into the back of the channel body. Anything exiting the door tracks flows into the channel rather than the wall. Aqualevel makes the premium equivalent. Both cost significantly more than a standard ThresholdDrain.

For most domestic kitchen extensions with reasonably specced bifolds and a patio that falls gently away, a uniflow ACO ThresholdDrain is sufficient. For high-spec glazing systems (Schueco, Sunflex, Solarlux, Skyframe) where the architect or door installer specifies a particular detail, follow that specification. If your builder offers to install "any A15 channel from Wickes" at a flush threshold above a premium bifold package (see over £15,000 as an illustrative real-project order), that's the wrong saving.

Types and variants

The channel drain market is dominated by ACO. Almost every builders' merchant in the UK stocks the ACO range, and the brand has effectively become a category name (homeowners ask for "an ACO" the way they ask for "Hoover"). FloPlast, Clark-Drain, Brett Martin, and Marshall's Stormcheck are alternatives, mostly priced as budget options. At the premium end, Aqualevel targets architectural glazing.

The choice splits along two axes: load class (how much weight the grate takes) and product family (what the channel is designed to do).

Load classes explained

UK channel drains follow BS EN 1433:2002, which defines load classes from A15 (pedestrian) up to F900 (airport runways). Three matter for domestic work:

• A15 is rated to 1.5 tonnes. Designed for pedestrians, cyclists, and light trolley loads. Correct for a patio outside bifold doors where only people, garden furniture, and the occasional wheelie bin pass over it.
• B125 is rated to 12.5 tonnes. The right class for a driveway or a path where a car or van might roll over the grate. Polymer concrete body and cast iron grate.
• C250 and above are car parks and roads. No domestic application.

Pick A15 for a pure pedestrian patio. If there's any chance of vehicle access, even an occasional skip lorry reversing onto the patio for a future garden project, specify B125. The channel body is a different shape, so you can't upgrade the grate alone.

Grate materials

Plastic grates come fitted to the budget channels at Wickes, Screwfix, and Toolstation. They're fine functionally but they look cheap and they UV-degrade over a few years.

Galvanised steel is the default mid-range option. Strong, decent appearance, will rust at exposed cut edges over time.

Aluminium (the standard ACO ThresholdDrain grate) sits at the same price point as galvanised, looks better, won't rust, but scratches. The Heelguard pattern (narrow slots on the diagonal) prevents stiletto heels and small wheels catching.

Stainless steel and CorTen weathered steel are the premium finishes. Stainless costs roughly double aluminium. CorTen develops a deliberate rust-orange patina over a year and looks intentional with dark porcelain or grey paving. Both are aesthetic upgrades, not functional.

How to work with it

Channel drain comes in 1m sections that connect end-to-end via spring-clip joints. Most domestic threshold runs are 2m, 3m, or 4m long, matching the bifold opening width. Build the run by laying out enough channel sections to span the door opening plus an extra 100-150mm at each end so the channel extends just past the door jambs.

The full assembly is heavier than it looks once it's bedded in concrete, but a 1m polymer ACO ThresholdDrain section weighs around 4kg and a single person can carry several at a time. Cutting to length is easy: a hacksaw or angle grinder with a fine cutting disc cuts polymer or polypropylene channel cleanly. Cut square. The end caps need a square edge to seal.

End caps come in two flavours. A closing end cap seals one end of the run so water can't escape into the sub-base. An outlet end cap has a 50mm spigot for connecting downstream pipework. The 50mm outlet is the trap. UK domestic surface-water drainage is 110mm. To connect an ACO ThresholdDrain end-cap outlet to a 110mm underground drain you need a 50mm-to-110mm reducer (or a 110mm Floplast/Polypipe reducing socket plus a length of 50mm waste pipe). It's an extra fitting most homeowners and even some plumbers don't anticipate.

The cleaner solution is a bottom outlet. ACO sells a 110mm bottom outlet adaptor (£13 – £17) that drops vertically from the channel body into a 110mm pipe directly. No reducer, no 50mm waste pipe, no extra joint to fail. Use the bottom outlet wherever the channel can sit above the underground pipe run, which is normally easy at a kitchen extension threshold because the surface-water drain runs underneath the patio anyway.

Concrete bed and surround

Channel drain isn't laid on sand. It sits on C12/15 concrete, with concrete also packed around both sides of the channel body. The minimum specification:

• 100mm concrete bed below the channel base
• 100mm concrete surround on both sides, full channel height
• Cure for at least 72 hours before the surrounding paving traffics the area

Skip the concrete and the channel moves. Movement opens joints, water escapes into the sub-base, the sub-base scours out, and the patio settles around the drain creating a permanent puddle exactly where you didn't want one. Pavingexpert and the ACO installation manual both call out missing concrete surround as the most common cause of channel drain failure.

The sub-base under the concrete bed itself is a 100mm layer of clean crushed rock or hardcore, blinded with 25mm sand. NHBC 10.2.6 specifies this. Your groundworker should know.

Setting the channel height

The single number to remember: the grate sits 3-6mm below the surrounding paving. NHBC 10.2.6 specifies block pavers abutting drainage channels to be set 3-6mm above the grating. Set it level and water sheets across the grate without entering it. Set it too far below and you create a tripping hazard.

The patio falls away from the house at 1:40 to 1:60 (a fall of 25mm per metre is the easy-to-set number). Water hits the gentle fall, runs the metre or two to the channel, drops in. Done.

The channel itself runs level along its length. There's a persistent myth, repeated by some builders, that the channel needs a fall toward the outlet. Modern channel drains are designed with internal step-falls or a simply level invert, and rely on hydraulic flow to carry water to the outlet. Building it level is correct. A BuildHub thread on this exact misconception is the canonical reference if your builder argues.

Click to enlarge

Outlet routing: where the water actually goes

The channel collects water. It then needs to go somewhere. Approved Document H sets a four-step priority order for surface-water discharge:

1. A soakaway sited at least 5m from any building. The first-choice option in Part H. A soakaway is essentially a rubble-filled or crate-filled pit that lets water disperse into the surrounding ground. Note the 5m minimum: a soakaway tucked next to the new extension wall is not legal and will cause subsidence under the new foundations. This 5m rule is the single most-missed detail on the whole topic, so don't let your builder shortcut it.
2. A watercourse. If your property is adjacent to a ditch, stream, brook, or land-drainage channel, discharge to it. Requires permission from the riparian owner and may require an environmental permit for non-trivial flows. Common on rural and edge-of-village plots; rare in tight urban gardens.
3. A surface-water sewer. If your road has a dedicated surface-water sewer (separate from the foul sewer), connect the channel outlet to it. Most newer UK estates have these. Older Victorian streets often don't.
4. A combined sewer. Last resort, requires water company permission. A combined sewer carries both surface and foul water; surface water adds to overload risk during storms, so water companies discourage new connections.

What is never allowed is connecting the channel outlet to the foul drain. The foul drain runs under positive pressure during use, your channel outlet is at ground level, and during a storm the foul drain backflows into your channel and the channel backs up into the threshold. It's also a public health offence (covered briefly in the Warning at the top of the page).

The ACO ThresholdDrain product page itself says "connect to soakaway." In a typical extension scenario, where the channel sits along the wall of the house, a soakaway right behind the channel is the illegal option, not the legal one. Route the underground 110mm pipe from the channel's bottom outlet to one of: an existing rear-garden surface-water drain, an existing soakaway 5m+ from the house, an adjacent watercourse with permission, or a new soakaway you're installing at the bottom of the garden as part of the extension drainage scope.

Click to enlarge

Building control: what the inspector checks

Drainage falls under Approved Document H, and the BCO (building control officer) inspects the underground drainage layout before it's covered up. Channel drain is part of that inspection. Key points:

• The channel and outlet pipe must be exposed for inspection. Don't backfill or pave over the run before sign-off. The BCO needs to see the bedding, gradient on the outlet pipe, alignment, end caps, and connection to the surface-water system.
• An air test (or water test on shorter runs) on the underground section is normally required. The same test kits used for soil pipe testing apply, see 110mm soil pipe for the test method.
• The BCO will verify the outlet connects to the correct system: surface-water drain, not foul.

The pattern that catches homeowners out: BCOs inspect the underground run thoroughly but do not always notice surface-level threshold drainage problems. A BCO sign-off on the underground drainage is not confirmation that your threshold drainage detail will work in heavy rain. Multiple BuildHub threads document cases where BCOs passed installations that subsequently flooded. Use the BCO to confirm the underground side is correct, but rely on the architect's specification and your own checks to confirm the surface-level detail (channel position, height relative to paving, connection to weep holes if dualflow).

How much do you need

A typical kitchen extension with a 3m bifold opening needs approximately 3.2-3.5m of channel (the door width plus 100-150mm overrun at each end), one closing end cap, one outlet end cap or bottom outlet adaptor, and 1-2 metres of 110mm underground pipe to connect the channel outlet to the existing or new drainage run.

Worked example for a 3m bifold opening connecting to an existing surface-water drain 2m away:

Item	Quantity	Approx unit cost (inc VAT)
ACO ThresholdDrain 1m channel (aluminium grate)	4	£29 – £40
Bottom outlet 110mm	1	£13 – £17
Closing end cap	2	£11 – £14
110mm underground pipe (3m length)	1	included in standard groundworks
C12/15 concrete (approx 0.4m³)	-	included in standard groundworks

Multiply each unit cost by quantity to derive your supply-only subtotal. For installed pricing, add labour and ground preparation: budget £40 – £100 per metre as the all-in installed rate. Where the existing surface-water drain is far from the patio or absent altogether, costs scale with the additional pipe run and any new soakaway.

Cost and where to buy

For aluminium-grated ACO ThresholdDrain, the cheapest reliable source is Castings Drainage or Drainage Superstore online. Price per 1m channel runs £29 – £40 inc VAT. A national builders' merchant (Travis Perkins, Jewson, MKM, Buildbase) will be similar pricing if it's in stock, but stock varies and you may face a 1-2 week delivery wait on the threshold-specific products.

Wickes, Screwfix, and Toolstation all stock generic A15 patio channel drain in the £23 – £26 range. These are not purpose-designed for flush threshold installation. For a patio that runs away from the house with a normal stepped threshold, they're fine and they save money. For a flush bifold installation, spend the extra and get the ACO ThresholdDrain or BrickSlot. The product is engineered for the application; the generic isn't.

For premium glazing systems where the architect specifies Aqualevel or a particular dualflow detail, follow the specification. The cost difference (£240 – £340 per metre vs £29 – £40) is real but it's a fraction of the door package.

Alternatives

There are three real alternatives to a channel drain at a flush threshold, and they all change something else about the design.

Step the threshold down by 150mm so the patio sits below the DPC and the natural water barrier is restored. This eliminates the channel drain requirement under NHBC 10.2.6 but loses the level access required by Approved Document M. For wheelchair-accessible properties or new-build housing, this isn't a route. For a private retrofit extension with no Part M trigger, it's a valid choice.

Permeable paving (gravel, permeable block paving, decking with gaps) absorbs water at the surface rather than running it off, so there's no surface flow to capture. This works at lower rainfall intensities. Heavy storms can overwhelm permeable paving and the runoff has to go somewhere. The DIYnot consensus is permeable decking against a flush threshold is acceptable; permeable block paving against a flush threshold is borderline and depends on the sub-base design.

Internal floor drainage (a wet-floor design where water that crosses the threshold is captured by a drain inside the room) is used in commercial walk-in shower-type contexts, never for a domestic kitchen extension. Don't go there.

For most extensions, the channel drain is the correct answer because the design constraints (flush threshold, solid paving, modest rear garden) lead to it.

Where you'll need this

Channel drain belongs to the groundwork phase, installed when the underground drainage is being laid and before the patio is finished:

• Drainage - the channel outlet ties into the surface-water drainage run that's installed during groundworks. Specify channel and outlet position with the groundworker before pipework is buried.
• Foundations and footings - the channel sits in concrete bedded against the new extension foundation. Coordinate so the foundation profile leaves the right space for the channel, concrete bed, and surround.
• Building control inspection - foundations - the BCO inspects the drainage layout including the channel run and outlet connection before paving covers it.

Threshold detailing also touches the structure phase (where bifold or sliding door installation must coordinate with the channel position) and second-fix landscaping (where the patio is laid to falls and the grate is set 3-6mm below the finished surface).

Common mistakes

No channel installed at all. The most common scenario reported on UK building forums: homeowner discovers water ingress at flush bifold doors after the first heavy storm, investigation reveals there was never a channel drain, the builder treated the flush threshold as a finish detail rather than a drainage detail. Once paving is laid, retrofit means lifting the entire patio. The fix costs £2,000–5,000 in repair work plus the patio rework. Specify the channel on the architect's landscaping drawing and check it's on site before paving day.

Outlet routed to a soakaway against the wall. ACO's own marketing says "connect to soakaway", but in a flush-threshold-against-the-wall context that breaches AD H3's 5m minimum. See Outlet routing above for the correct destinations.

Outlet routed to the foul drain. Never route to the foul drain. See the Warning at the top of the page and the Outlet routing section for the legal alternatives.

50mm outlet end cap connected directly to a 110mm pipe. The standard ACO ThresholdDrain outlet end cap has a 50mm spigot. UK underground drainage is 110mm. You need a 50mm-to-110mm reducer, or use the bottom outlet (110mm direct) if the geometry allows. Discovering this on site with a hole already excavated is the wrong moment.

Channel laid on sand instead of concrete. Polymer concrete or PVC channel bodies need a 100mm C12/15 concrete bed and 100mm surround on each side. Sand-bedded channels move under load, joints open, water enters the sub-base, and the patio settles around the drain. The concrete spec is in the ACO installation manual and NHBC 10.2.6.

Grate set level with the paving. Water runs across a grate set level rather than into it. The 3-6mm grate-below-paving offset is what makes the drain work as a drain. Set it right with a spirit level during installation; check it with a straight-edge before signing off.

Skipping the closing end cap. Open at one end, the channel pours water out of the unsealed end into the sub-base. Pavingexpert calls this out as a recurring failure mode. End caps cost £11 – £14 and take five minutes to fit.

No maintenance plan. Even a correctly-installed channel can fail under wind-driven rain if the grate is silted up. Grit, leaves, moss, and pollen accumulate at ground level rapidly. Inspect monthly; pressure-wash the grate every spring and autumn. If grit cements the grate down, lift it carefully (plastic spudger or paint scraper, not a screwdriver) before grit hardens further. A blocked threshold drain is a flooded kitchen waiting for the next storm.