Bathroom Plumbing in a Slab Foundation House

Ask a plumber to move a toilet two feet in a house with a crawl space and it is an afternoon's work from below. Ask the same question in a house on a slab and the answer involves a concrete saw, a jackhammer, a compactor and a curing schedule.

Since around a third of Mountain View's housing was built in the 1950s and 1960s, and nearly all of it slab on grade, this is the most consequential thing to establish about a bathroom before anyone starts drawing.

Why the slab changes everything

In a raised-foundation house, the drain and vent system hangs in the crawl space under the floor. It is accessible, adjustable and reachable without touching a finished surface.

In a slab-on-grade house, the drain lines were laid in the ground before the concrete was poured and are now inside the structural floor. Supply lines may run in the slab too, or overhead, or in the walls depending on era and builder, but the drains are the constraint, because drains depend on gravity and therefore on exact positions and falls.

So the existing drain locations are effectively fixed points. A design that keeps fixtures on them is a normal bathroom project. A design that moves them is a concrete project with a bathroom on top.

What cutting a slab involves

The sequence, roughly:

1. Locate what is in there. Existing drain runs, any post-tension cables, and in a mid-century house with radiant heating, the heating tubing. Scanning and thermal imaging both have a role. This step is not optional and skipping it is how expensive things happen.
2. Score and saw. A concrete saw cuts the outline of the trench. This is wet-cut to control dust and it is loud enough that neighbours notice.
3. Break out and excavate. The concrete comes out in pieces, then soil is dug to expose the existing line and give room to work.
4. Re-plumb. New drain run set at the correct fall, connected and tested.
5. Inspection. Underground drainage is inspected before it is buried.
6. Backfill, compact, pour. Soil replaced and compacted properly, otherwise the patch settles and cracks, then new concrete poured and left to cure.

Two things in there are schedule, not labor: the inspection has to be booked, and the pour has to cure before tile can go over it. Both are why a slab cut adds days rather than hours.

What it costs and how long it adds

In Santa Clara County, budget $1,500 to $4,000 per fixture relocated, and the upper end for a toilet, because a soil pipe needs more fall and therefore a deeper, longer trench than a basin waste.

In schedule terms, expect several days per relocation once cutting, plumbing, inspection, backfill and cure are counted. Relocating three fixtures in a full remodel is not three separate exercises, since the trench work overlaps, but it is still a meaningful block of the calendar and it sits early in the project where everything else waits behind it.

When it is worth doing

Sometimes it plainly is. The situations that justify it:

• The room does not work. A door that hits a fixture, no clear space to stand, a toilet crammed against a wall with no clearance beside it.
• You are going curbless. The shower floor has to be recessed, so concrete work is happening anyway, and moving the drain within it costs relatively little extra.
• Accessibility. Clear approach and transfer space beside a toilet or shower often cannot be achieved without moving something, and this is not a decision to defer.
• Combining two rooms or borrowing space. If a wall is moving, fixtures are moving with it regardless.

And the situation that usually does not:

• Preference without a problem. "The vanity would look better on that wall" is several thousand dollars and a week for something nobody will be able to articulate a year later. That budget spent on the shower, the lighting or the cabinetry will be noticed daily.

The useful discipline is to name the specific difficulty the current layout creates. If you can name it, move the fixture. If you cannot, keep it.

The alternatives to cutting

• Work within the existing rough-in. Fixtures can usually shift a modest distance along the same drain run without a new trench. A skilled designer will find more flexibility here than a homeowner expects.
• Build the floor up rather than cutting down. Raising the floor level lets new drainage run above the slab. It works, but it costs headroom, creates a step at the doorway and rarely suits a mid-century house with low ceilings.
• Move the wall instead of the fixture. Occasionally a partition can move to give the clearance you wanted, leaving the plumbing untouched.
• Change the fixture rather than its position. A wall-hung basin, a shorter vanity or a compact toilet often solves the clearance problem that prompted the idea.

Other slab consequences

• Crack isolation. Slabs move and crack over time, and a rigid tile assembly bonded directly to one transmits that crack upward. A crack isolation membrane uncouples them for $300 to $800 and is worth it on any slab floor.
• Cold floors. Concrete sits at ground temperature. Electric radiant floor heating at $1,500 to $3,500 is a more noticeable comfort upgrade in a slab house than anywhere else.
• Radiant heating already in the slab. In Eichlers and much mid-century building, the concrete carries the heating. See the Eichler guide before anything is cut.

Not sure whether your house is on a slab? If there is no crawl space hatch and no basement, and the floor sits close to outside ground level, it almost certainly is. For the whole project in order, see the Mountain View remodel guide.