You go over the top, not through the floor. New water lines are run through the attic and dropped down inside the walls to each fixture, and the old lines buried in or under the slab are cut back, capped and abandoned in place. Breaking concrete to chase supply lines is almost never the right answer, because the attic route reaches the same fixtures without touching the structure.
That is the whole method in a paragraph. The rest of this page is why, and where the exceptions are.
Why go through the attic instead of the floor?
Because the concrete is the expensive part and the pipe is not.
Going through the slab means jackhammering the floor, dust through the whole house, care around the reinforcement in the concrete, and then repouring and refinishing every foot you opened — tile, wood, carpet, whatever was on top of it. Going through the attic means sheetrock repair. Same fixtures, same new pipe, one trade instead of three.
The concrete is the expensive part. The pipe is not.
What does the work actually look like?
- Access gets cut at each fixture, at the water heater, and wherever a line drops out of the ceiling.
- New PEX-a is pulled through the attic in continuous runs, with as few joints as the layout allows. A joint you cannot see is a joint you do not want.
- Drops go down inside the walls to each fixture, and the stub-outs are set.
- The system is tied in and pressure tested before a single piece of sheetrock goes back.
- Old lines are cut back and capped. They stay in the slab, dead and depressurised.
Is it a problem leaving the old pipe in the slab?
No. Once it is cut back, capped and no longer carrying water, dead copper or galvanized in a slab is inert. It is not doing anything and it is not going to. Nobody breaks up a floor to retrieve pipe that has been taken out of service.
What about the drain lines under the slab?
This is the real exception, and it is worth being clear that it is a different job from a repipe.
Drains work by gravity, which means they run under the slab by design and cannot be rerouted through an attic. When an under-slab drain has actually failed, the choice is between cutting through your floor and tunneling underneath the house from outside.
We tunnel. On a Clear Lake job the camera showed the line sitting full of water, which meant it had at minimum lost its fall. We tunneled fifteen feet under the slab and found the line had shifted underground and pulled apart at a joint. The repair happened under the house, not through the middle of it. See tunneling and under-slab drain repair.
Two different problems, two different answers. Supply lines on a slab house get rerouted overhead. Drain lines on a slab house get reached from underneath. If somebody proposes breaking your floor to replace water lines, ask why the attic will not work.
How do you get to a valve without wrecking the tile?
Around Baytown a lot of tub and shower valves back onto a closet. When that is the case we cut the sheetrock from the closet side and fit an access panel instead of patching it. It costs nothing extra now and the next time anything needs doing back there, the plumber opens a panel instead of cutting your wall again.
Where the valve allows it, we cut in from the front through the tile instead, measured so the escutcheon covers the opening. Matching fifty-year-old tile is close to impossible and a tile setter is a second trade to pay for.
What material goes in?
Uponor PEX-a inside the house. It handles the bends an attic route needs, it takes freeze better than rigid pipe, and it lets us run continuous lengths with fewer joints.
Copper stays copper anywhere it is exposed outdoors, because PEX degrades in sunlight. That is not a preference, it is what the material does. See water line repair.
How do I know it is time?
Brown flecks or grit at the tap, pressure that has dropped off, or several leaks close together. Galvanized silts up internally as it ages, which is why people replace fixtures and buy bigger water heaters chasing a weak shower when the problem is the pipe. See repiping and how long a repipe takes.
Common questions
How do you repipe a house on a slab?
You go over the top, not through the floor. New water lines are run through the attic and dropped down inside the walls to each fixture, and the old lines buried in or under the slab are abandoned in place. Breaking the slab to chase supply lines is almost never the right answer on a repipe, because the attic route reaches the same fixtures without touching the structure.
Why not just break the slab and replace what is there?
Because the concrete is the expensive part and the pipe is not. Going through the floor means jackhammering, dust through the whole house, structural care around the reinforcement, and then repouring and refinishing every foot you opened. The attic route reaches the same fixtures with sheetrock repair instead of concrete work.
What about the drain lines under the slab?
Those are the exception, and they are a different job. Drains run under the slab by design and gravity, so they cannot be rerouted through an attic. When an under-slab drain has to be replaced we tunnel underneath the slab rather than cutting through the floor, so the work happens outside the house.
Do the abandoned pipes cause a problem left in the slab?
No. Once they are cut back, capped and no longer under pressure, dead copper or galvanized in a slab is inert. It is not worth breaking concrete to retrieve it.
What material do you use?
Uponor PEX-a for the lines inside the house. Copper stays copper anywhere it is exposed outdoors, because PEX degrades in sunlight. We match the material to where the pipe actually lives rather than using whatever is on the truck.
Will you have to cut into my tile?
Only where there is no alternative. Around Baytown a lot of tub and shower valves back onto a closet, and where that is the case we cut the sheetrock from the closet side and fit an access panel instead of patching it. Where we do have to come in from the front, we measure the cut so the escutcheon covers it.
Last updated · written by a licensed master plumber, MPL 38224