What to Do Before Your Bathroom Remodel Plumbing Starts

Bathroom remodels succeed or struggle on decisions made before demolition starts. Once the walls are open and the tile is on order, the plumbing questions stop being design choices and start being change orders.

These are the questions worth settling while the layout is still a drawing.

Are fixtures staying where they are?

This is the question that drives the budget more than any other. Keeping the toilet, sink, and shower in their existing positions means working with the drain and vent layout already in the floor. Moving any of them — particularly the toilet — means new drain runs at the correct slope, and potentially new venting.

Drain lines need a consistent fall to work, conventionally about a quarter inch per foot for the sizes used in a residential bathroom. In a rowhouse where the bathroom sits over a finished room below, there may be very little depth available between the bathroom floor and the ceiling underneath to achieve it. That constraint is invisible on a floor plan and decisive in reality, which is why a designer's layout is worth checking against the actual joist depth before it is finalized.

Moving a sink a foot along the same wall is usually straightforward. Moving a toilet to the opposite side of the room frequently is not.

Tub-to-shower conversions

Converting a tub to a walk-in shower is one of the most popular remodels we see, and the drain is the detail that catches people. A tub drain and a shower drain are different sizes — tubs are typically served by a smaller drain than a shower needs, because a shower sheds water over a much larger area with no reservoir to buffer it.

That means the conversion usually involves opening the floor to change the drain, not simply capping the tub spout and tiling the walls. Curbless and low-threshold showers add a further constraint, because the drain has to sit lower still to keep the floor flush with the room outside.

What is actually behind the walls

In a prewar building, the supply lines feeding the bathroom are frequently original galvanized steel. Eighty or a hundred years of internal corrosion narrows the bore, and a line that delivers acceptable pressure to a single old showerhead may not deliver acceptable pressure to a rain head and a body spray.

A remodel is the cheapest opportunity you will ever have to replace that piping, because the walls are already open. Doing it later means opening the new tile. This is the single most common regret we hear from homeowners a few years after a remodel — not the tile choice, the pipe they left in the wall.

The same applies to the waste and vent stack. If the cast-iron stack serving the bathroom is heavily corroded, that is worth knowing while it is accessible rather than after the room is finished.

Venting, which nobody thinks about and everyone needs

Every fixture needs a vent so that draining water does not siphon the trap dry. A dry trap is what lets sewer gas into a room, and it is why a rarely-used basement bathroom sometimes smells. Relocating a fixture can move it out of range of the existing vent, and older buildings often have venting arrangements that were acceptable when installed and would not be designed that way today.

Where a fixture cannot easily reach the main vent, there are established solutions — but which are acceptable is a code question, not a preference, and it is worth resolving before the layout is fixed.

Permits, co-op boards, and the paperwork layer

Scope drives permitting. A like-for-like fixture swap is a different regulatory question from relocating fixtures or altering waste lines, and the answer depends on the specific work. All regulated plumbing and permit-required work we are involved in is completed by appropriately licensed partner professionals in accordance with New York's licensing and permitting requirements.

If you are in a co-op or condo, the building's own process usually runs longer than the city's. Expect an alteration agreement, proof of contractor insurance naming the building, approved working hours, and a freight elevator schedule. Buildings frequently restrict work to weekday daytime hours, which lengthens the calendar even when the work itself is short. Start that paperwork before you order materials.

The checklist, condensed

• Are fixtures staying in place, or moving — and has anyone checked the available floor depth for new drain runs?
• Is a tub-to-shower conversion planned, and does the existing drain support it?
• Do you know the age and material of the supply lines behind the walls, and is this the moment to replace them?
• Is venting adequate for every fixture in the new layout?
• What does the scope require in terms of permits, and who is pulling them?
• If you are in a co-op or condo, is the alteration agreement started?
• Where will the household wash for the duration, if this is the only bathroom?

Sequence matters

Rough plumbing happens after demolition and before anything closes up, and it has to be inspected where inspection applies before walls are covered. Fixtures that arrive late are the most common cause of a stalled bathroom, because the tile layout often depends on the actual fixture dimensions rather than the catalogue ones.

The practical advice: have the fixtures on site before rough work begins. A shower valve that turns up two weeks late holds up everything behind it.