Finding the pipe your builder left behind
Newer homes hide a small mystery in the basement: a vertical PVC pipe, three or four inches across, rising out of the slab and vanishing into the framing above. It isn’t plumbing; no fixtures connect to it. Sometimes a cap. Sometimes a label, if the builder was thorough: “RADON VENT.”
That’s a passive radon rough-in, and here’s the tour. Start in the basement or utility room and look for that unattached vertical run near a wall or in a closet corner. Follow it in your head: a proper rough-in starts below the slab in the gravel layer, rises through the house, typically hidden in closet chases or interior walls, and exits through the roof like a plumbing vent. In the attic, look for the same pipe passing through on its way up. On the roof, it reads as one more vent stack.
Builders install these for a plainly economic reason: a pipe route costs little while the walls are open and the slab is being poured, and a retrofit later costs multiples of that. Roughing in the path during construction is cheap insurance in a region where the geology writes the checks.
So if you find one, the builder handed you a head start. What the next section will insist on is this: a head start is all it is.
Why “passive” doesn’t mean “protected”
A rough-in without a fan works on stack effect alone: warm air drifting up the pipe pulls, weakly, on the soil below. It’s better than nothing. It is nowhere near a guarantee, and homes with passive rough-ins come back above the action level all the time.
It’s worth saying soberly: new construction tests high too. Radon isn’t a symptom of old, cracked foundations. It’s a product of the rock underneath, and a brand-new slab over gas-rich soil can post a bigger number than a century-old rowhouse two towns over. Tight modern construction can even work against you, holding whatever enters. The birth year of the house tells you nothing the soil didn’t already decide.
Which leads to the only advice this page has: test. Not because the pipe is suspect, but because the pipe is irrelevant until a number says whether it’s needed. A test is cheap, takes days, and converts the mystery into one of two comfortable outcomes: a low number you can file away, or an elevated number in the single easiest kind of house to fix. The testing guide covers how to run one properly, kit or professional.
Skip the step where you assume the builder’s pipe settled the question. It didn’t. It just made the answer cheaper.
Found the capped pipe and a high number? That's the easy case.
If the number comes back high: activation
Here’s where rough-in owners get their reward. For most homes, an elevated test means designing a mitigation system from scratch: where to core the slab, how to route pipe through finished space, where a fan can live. Your house already answered those questions during construction.
Activation, in the typical case, means adding the missing organ: an inline fan on the existing run, usually up in the attic, plus a U-tube gauge downstairs so you can see the system pulling. The passive drifter becomes an active machine, drawing soil gas continuously and venting it above the roof. Then the step no radon job should skip: a follow-up test verifying the level actually came down. The fan is hardware; the retest is the result.
The honest hedge: “typical” is doing some work in that paragraph. An assessment still has to confirm the rough-in was built right, that it reaches a proper gravel layer and holds air, before a fan gets bolted on. Passive system activation covers the whole job, including what happens in the minority of homes where the builder’s pipe turns out to be the wrong path.