Farmhouse Sink Rough-In: The Factory-Side Guide

Most kitchen components forgive a planning miss. A filler panel can be scribed, a worktop joint can be moved, a cabinet face can be reordered. A fireclay farmhouse sink cannot. The body leaves our kilns as a single fired piece: the drain opening, its position, the bowl depth and the apron profile are all in the mould before the first cast is pressed, and once the glaze is fired, nothing about that geometry is negotiable on site.

That is why the rough-in — the drain, supply and framing work that happens before the sink arrives — matters more for a ceramic farmhouse body than for a steel one. A stainless fabricator can drill a new waste hole and re-cut the bowl. We cannot, and neither can the installer. The rough-in either matches the drawing, or the installation plan has to be corrected before anything proceeds.

This guide is written from the factory side, because the rough-in advice online is written almost entirely by installers working with whatever sink the client has already bought. We work the other way round: we know exactly what the body will demand, because we tooled it. What follows is what to leave in place — and what to confirm in writing — before a fired farmhouse sink is anywhere near the cabinet.

Start with the one number that is not a decision at all. Kitchen sink drains are a standardised fitting — the 3.5-inch waste outlet, a size the North American supply chain settled on decades ago and which the strainer, the waste body and the disposal collar all assume. When a listing says nothing about drain size, that is the size it means.

On our drawings the standard shows up with manufacturing honesty. The farmhouse fireclay drawings carry the tolerance on their face: the S12183 and S332025K blueprints dimension the drain opening at 92 mm ± 2 mm — printed on the drawing itself. On models whose drawings leave the drain undimensioned, the CAD geometry closes the gap: measuring the cast arc on the 27-inch and 32-inch S1915 files gives Ø90–92 mm, inside the same window. The inset CD bodies are drawn at Ø90 mm ± 2 mm, and the 1.5-bowl CD150 carries two of them, one per basin. One standard, three drawings, every one of them checkable — a buyer can verify the number instead of trusting it.

Two practical consequences. First, when you order a fireclay farmhouse sink, you should not need to specify a drain diameter — but you should confirm it once, against the drawing, because the answer tells you whether the supplier is working to the standard or improvising. Second, the tolerance is a manufacturing fact, not a concession: a fired body moves in the kiln, and a supplier who cannot quote a tolerance on the drain opening has probably never measured one.

The drain position of a fireclay sink is fixed at mould-design stage: center, rear or offset is a pre-production choice, not a site one. A steel sink can be re-punched; our bodies cannot — the bowl floor slopes toward the drain as part of the forming process, and the slope, like the opening, comes out of the kiln finished.

The three positions answer different cabinet problems. A centered drain is the default and the simplest to rough in. A rear-positioned drain pulls the waste against the back wall of the cabinet, which frees the space under the bowl — the space a drawer stack, a disposal or a water filter wants. An offset drain moves it further again, trading a slightly more complex waste assembly for a fully usable cabinet interior behind the door.

On a 1.5-bowl body the question doubles: our CD150 carries a drain per basin, and the two outlets must be served by a drainage arrangement designed for that bowl configuration and the applicable plumbing code — one trap or two, planned before the cabinet, not discovered during it.

The factory-side point is blunt: if drain position matters to the project — and in a cabinet with a disposal or a pull-out it does — it is a line on the purchase order, confirmed against the drawing, before the body is fired. Position is not a site modification on this material.

Bowl depth is where the rough-in gets physical. Our fired bodies run from 200 mm deep on the inset CD series (150 mm on the CD150 half bowl) to 254 mm — a full 10 inches — on the 30- and 33-inch farmhouse range. That depth is the selling point of the format, and it is also the number that eats the cabinet.

Under the bowl floor sits the tailpiece, then one of two things: a trap, or a disposal. A P-trap needs its drop; a bottle trap trades drop for horizontal space, which is why it is common in Europe and rare in North American runs. A disposal adds the largest single block of volume in the cabinet — typically the height of the unit plus the swing of its discharge elbow — and it hangs directly off the drain opening, which is why the drain-position decision in the last section and the disposal decision here are the same decision made twice.

The arithmetic that matters is simple and should be done against the drawing, not against a category photo: bowl depth, plus the waste assembly, plus whatever else the cabinet carries — and then confirm the cabinet actually offers that height from its floor to the underside of the mounting line. On a farmhouse body there is a second, easily missed demand: the apron. The exposed front panel hangs proud of the cabinet face, and the carrier or support frame behind it has to be planned while the cabinet bay is still empty.

One more component that belongs in this conversation is the overflow, because it changes the waste geometry. Not every farmhouse body carries one — it is a model feature, not a category feature. Where it exists on our inset bodies — the CD86, for example, has a Ø26 mm overflow high on the back wall, set toward the drainer side — it must align with the overflow fitting the installer plans for; an overflow that is not in the drawing is an overflow that will not be in the crate.

The single most-asked rough-in question in the trade is a number: how high off the floor should the drain be stubbed out? Forum threads run for years on it, because the honest answer depends on what sits above the stub-out — and for a farmhouse body, what sits above it is a bowl you already know the depth of.

Published guidance varies: installer references put the drain centre anywhere from about 12 to 21 inches (≈305–533 mm) off the floor, moved by the cabinet and by whether a disposal hangs on the waste — a deeper sink with a disposal needs the drop, a shallower run can sit higher. These are indicative trade layouts rather than one standard, and local code wins wherever it differs. But the factory side can add the part forum threads keep arguing about: the limit is not the wall, it is the tailpiece. Between the bowl floor and the stub-out there must be enough vertical run for the trap's drop — and the bowl floor's height is set by the bowl depth, which on our bodies is a drawn dimension: 200 mm on the inset CD range, 254 mm on the 10-inch farmhouse bowls. A stub-out that works under a 200 mm bowl can be wrong under a 254 mm one, with the same cabinet. That is why the question "is 25 inches too high?" has no universal answer — but it always has an answer once the bowl depth and the trap assembly are on the table, and both of those are known before the wall is closed.

The drain gets the attention because it is large and irreversible; the supply side fails more quietly. Two angle stops, hot and cold, live on the back wall of the cabinet — positioned high enough to clear the bowl's back wall and the trap assembly, and reachable by feel, because they are the valves a homeowner will need in a hurry. Published trade layouts differ here too — figures of roughly 18 inches (≈455 mm) for a standard run, dropping toward 12–14 inches (≈305–355 mm) where a deep bowl and a disposal compress the space, appear across installer references. These are indicative layouts, not a factory figure and not a universal standard; final valve positions depend on the sink geometry, the cabinet and local plumbing requirements. The useful factory-side rule is not a number but an order of operations: rough the supplies after the body's position in the run is fixed, because the bowl depth and the apron define how far back the faucet deck — and therefore everything feeding it — can sit.

Tap holes are the other supply-side item, and on a ceramic body they carry the same irreversibility as everything else fired. For models with an integral faucet deck — our inset CD range, for example — the tap hole is drilled to the buyer's market specification before despatch: position and diameter against the order drawing, Ø35 mm on the standard single-hole layout, and once cut the body's orientation is fixed. For deckless bodies the tap hole moves to the worktop: confirm whether the model carries a deck before the countertop order is placed, and plan the worktop cutout separately. Either way the hole layout is a drawing line, not a site option — confirm count and spacing the way you confirm the drain, once, in writing, against the drawing, and the faucet order can be placed against the same document.

A farmhouse sink does not drop into a standard run the way an inset does — it becomes part of it. The apron replaces a section of cabinet face; the bowl occupies the bay; and the internal clearances have to be checked against the drawing rather than against the category. The measurement that decides most outcomes is not the cabinet's outside width — listings quote that in several conventions, and the same figure can mean different things — but the internal clearance the bowl, its waste assembly and the supplies actually need. Confirm which convention a cabinet figure uses before anything is ordered. Cabinet width alone does not establish compatibility — the internal opening, the support arrangement and the clearance around the waste assembly all have to be checked against the sink drawing.

Depth of the box matters as much as width. A deep bowl pushed against a shallow base means the trap sits forward, the disposal swings into the door, and the supplies end up under the bowl where they cannot be reached. The honest method is the one fabricators already use for stone: take the real installation drawing — which carries the drain position, the tap holes and the apron geometry — and lay the cabinet out around it. The farmhouse sink dimensions chart shows how the size ladder maps to cabinet sizes across the apron-front range, and the same confirm-it-against-the-drawing logic applies there. If the drawing is not available, that is not a detail to guess; it is a supplier to change.

The installers we work with converge on the same sequence, and it is worth writing down because its logic is the logic of an unforgiving material:

1. Get the installation drawing for the exact model — not a category photo, not a "similar to" — and confirm drain position, drain size, tap holes and overflow against it.

2. Fix the sink's position in the run: apron line against the cabinet face, bowl footprint inside the bay.

3. Mark and rough the drain: waste location per the drawing, and select a waste assembly compatible with the model's drain opening per the fitting manufacturer's specifications — the 3.5-inch opening is a fitting size, not the wall pipe's diameter.

4. Rough the supplies after the bowl position is fixed, not before.

5. Dry-fit the body. On a fired sink, the dry fit is the last cheap chance to find a conflict between the drawing and the cabinet.

6. Set the support or carrier, finish the faces, then mount.

The sequence has one principle behind it: everything adjustable happens before the fired body arrives, because after it arrives, nothing is adjustable. It is the sink-side twin of our toilet rough-in guide, which applies the same discipline on the fixture side — and an installer who follows it will never need to know how a fireclay sink behaves under a grinder. That is exactly the point.

When the crate is opened, five minutes with a tape closes the loop the rough-in opened. Work down the list and record each figure — it is the same list the installation drawing answers, and it is short enough that any receiving team can run it against a phone.

The habit matters more than the numbers. A dimension recorded on delivery is a claim you can make; a dimension nobody recorded is a dispute you cannot. It is the same principle behind our fireclay sink durability testing guide — measurement is how a fired product stays honest through a supply chain.

Is the drain size the same on every farmhouse sink? Yes — the 3.5-inch waste opening is the industry standard on kitchen sinks, and strainers, waste bodies and disposal collars are built to it. Our farmhouse range is drawn at 92 mm ± 2 mm and our inset CD bodies at 90 mm ± 2 mm — both inside the same standard fitting's window. Confirm once against the drawing, then order standard parts.

Can the drain position be changed on a fireclay sink? No. On a fired ceramic body the drain position is formed in the mould and finished in the kiln; it cannot be moved, re-punched or re-cut on site. If the project needs a rear or offset drain, it is specified before production.

How much space does a garbage disposal need under a farmhouse sink? Plan for the disposal's body height plus its discharge elbow swing, added to the bowl depth, against the cabinet's internal floor-to-mounting height. The numbers that matter — bowl depth and drain position — are on the installation drawing; the disposal's are on its spec sheet. If the sum is close, an offset or rear drain is usually what buys the room back.

Do fireclay farmhouse sinks have an overflow? It is a model feature, not a category feature — some do, some do not, and the listing rarely says. Ask for the drawing; where an overflow exists, its position and diameter are drawn. Our CD86 inset body, for example, carries a Ø26 mm overflow set high on the back wall.

Where should the supply valves be roughed in? Inside the cabinet, on the back wall, clear of the bowl's back wall and the trap assembly, reachable by feel. The customary trade height is around 18 inches from the floor, lower for a deep bowl with a disposal — local code governs. Rough them after the sink's position in the run is fixed, because the apron and bowl depth define how far back the faucet deck sits.

How high should the drain be stubbed out for a farmhouse sink? Published guidance varies — installer references put it anywhere from about 12 to 21 inches from floor to drain centre, adjusted for the cabinet and for a disposal. Trade practice, not a factory figure, and local code wins. What decides "too high" for a specific body is the tailpiece drop: bowl depth (ours run 200–254 mm) plus the trap must fit between the bowl floor and the stub-out. Check the bowl depth on the drawing and the number answers itself.

Can a farmhouse sink be installed into an existing cabinet and plumbing layout? Sometimes — but compatibility has to be checked first. The existing cabinet opening, support structure, drain position, waste assembly and available clearance all need to match the selected sink's installation drawing; a cabinet or plumbing modification is often the honest answer. Check the drawing before promising the retrofit.

What should I confirm before the cabinet is cut? Drain position, drain size, tap hole count and spacing, overflow present or absent, bowl depth and apron geometry — all against the model's installation drawing, in writing. It is the same list we put on a purchase order at the factory end.

Every clearance in this guide comes from a document we can send. For the exact model, request the installation drawing pack: overall dimensions, bowl depth and drain position, drain opening and tolerance, faucet-hole configuration, overflow configuration where applicable, apron profile, and mounting and support requirements. Before the cabinet, the countertop or the plumbing layout is finalised, that drawing is the reference that connects our production floor to your installation. And if you are building a range rather than a single kitchen, start from our fireclay sink collection or talk to us about OEM manufacturing — the drawings come the same way.