Low-Flow Toilets and the Drain Behind the Wall: What the Research Found

What counts as a low-flow toilet, and why ask about the drain?

Every residential toilet made in or imported into the United States since 1994 is low-flow by the old measure: it may use no more than 1.6 gallons per flush on average [1], and the Florida plumbing code sets the same 1.6-gallon limit for a water closet [2]. EPA's WaterSense label goes further: 1.28 gallons or less, 20 percent below the federal standard [3].

Most testing looks at the bowl: MaP scores and the WaterSense waste-extraction test check whether a toilet clears soybean-paste test media and toilet paper from the bowl [1][3]. The drain is a separate question — once the solids leave the bowl, does the smaller amount of water carry them down a long horizontal pipe? The PERC study was set up to answer exactly that, and it says so: it was not another toilet study, and it did not rank toilet models [1].

How did researchers test whether low-flow toilets clog drains?

They left the toilets out and tested the drain itself. PERC — a coalition including IAPMO, the International Code Council, the American Society of Plumbing Engineers and the Alliance for Water Efficiency — built a 135-foot run of clear 4-inch PVC drain at a manufacturer's test lab in New Jersey, set at a 1 percent and a 2 percent slope [1]. Instead of real toilets it used purpose-built surge injectors, so each flush had a controlled volume, discharge rate and amount of trailing water.

The solids were the soybean-paste test media used in toilet testing, with toilet paper added. There were 40 test runs of 100 flushes each, at 0.8, 1.28 and 1.6 gallons, and the results were analysed statistically to separate real effects from noise [1]. PERC's second report used the same method, added runs at 1.0 gallon and repeated the work on 3-inch pipe [4].

What decides whether waste makes it down the drain?

Pipe slope, toilet paper and flush volume. Those three clearly changed how many flushes it took to carry the solids out of the 135-foot pipe; the toilet's flush rate and the share of water trailing behind the solids did not [1]. The study concluded that the toilet's design has minimal effect on how waste travels in a long building drain [1], and the second report found flush rate and trailing water non-significant again [4].

The study is careful about its own numbers. It warns against ranking slope, paper and volume against each other for design decisions — the gaps are too small for that — and says the finding is the split between the significant and the non-significant factors [1].

• Large effect: pipe slope, toilet paper, flush volume
• Little effect: flush rate, trailing water
• 4-inch drain, 135 ft, at 1% and 2% slope

Is 1.28 gallons per flush enough to carry waste away?

In the lab, yes. At 1.28 and 1.6 gallons the waste moved through the test drain in an orderly, predictable way, and the study's committee wrote that it anticipated no problems with 1.28-gallon toilets in new commercial construction [1]. Reviewing PERC's work in 2022, EPA summed it up the same way: drain blockages are not a significant concern at 1.28 gallons per flush [5].

At 0.8 gallons it was different: in 5 of the 16 runs the solids compressed into plugs that filled or nearly filled the pipe. They cleared on their own before anything overflowed, but the committee called the behaviour chaotic and unpredictable and asked for more study at that volume [1].

PERC's second report followed up: the biggest drop in drain carry came between 1.28 and 1.0 gallons. PERC does not recommend toilets of 1.0 gallon or less in commercial buildings with long horizontal drains and no other long-running flows to help — and says that advice does not apply to homes [4].

One more result is worth knowing: a 5-gallon slug of clean water at the end of a run failed to clear the line in 7 of 39 runs, so the study rejects a big flush as a reliable way to clear a building drain [1].

Can toilet paper clog a drain line?

Yes — it was one of the three big factors. Paper that stays strong when wet travelled shorter distances in the test drain, a strong inverse link [1], and in PERC's second report paper wet strength was the most significant variable at every flush volume [4]. The two papers were chosen deliberately at the extremes, so ordinary brands will differ less — but where a building's drain blocks again and again, PERC suggests testing the paper and switching to one with lower wet strength as a first step [1].

Will a low-flow toilet work with an old drain or cast iron pipe?

It depends more on the condition of the old drain than on the toilet. PERC notes that drain-carry complaints after 1.6-gallon toilets arrived were largely attributed to older or faulty drain lines, and for retrofits it suggests inspecting the existing line first for defects, root intrusion and sagging that could clog at lower flush volumes [1].

Cast iron was not tested — the test drain was smooth clear plastic — and PERC's two reports disagree about old cast iron: the first says its inside can grow much rougher with rust, biofilm and grease [1]; the second says build-up in long-installed horizontal cast iron leaves the wall smoother than when new [4]. A small follow-up is more direct about slope: with one 8-foot section of the test drain set flat, waste took more flushes to carry out than on the evenly sloped drain in 14 of 16 runs. PERC notes those 16 runs carry less statistical weight and were not peer reviewed [4].

The research was built around long commercial drains, and PERC's second report explains why homes differ: house drains are typically much shorter, and clothes washers and showers send long-running flows down the same pipe that help carry solids along [4]. As of 2016, it knew of no confirmed reports of drain blockages from home toilets using as little as 0.8 gallons, already installed in significant numbers [4]. So the lab results do not transfer one-for-one to a house — that is our reading.

When a new low-flow toilet keeps backing up, the bowl is rarely the first place we look: in our experience a sag or flat spot in the line, a slope below code, roots or very strong paper are more likely. That is our trade experience, not a study finding.

Is my Florida drain sloped enough for a low-flow toilet?

If it was built to code and has not sagged, it has at least the gentler of the two slopes the research tested. The Florida plumbing code requires horizontal drains to be laid in uniform alignment at a uniform slope of at least 1/8 inch per foot for 3 to 6 inches and 1/4 inch per foot for 2 1/2 inches or less [2] — about 1 and 2 percent, the same two slopes the study tested. That conversion is our arithmetic.

Many homes we work on around Tampa Bay sit on a slab, where a sag or flat spot in the building drain is out of sight. If a low-flow toilet keeps clogging, a camera inspection of the line tells you more than replacing the toilet again.

FAQ

Sources

We review and cite. We do not reproduce a paper or a manual — follow the links for the originals.

1. [1]The Drainline Transport of Solid Waste in Buildings (Phase 1) — Plumbing Efficiency Research Coalition (PERC) Technical Committee · PERC — AWE, ASPE, IAPMO, ICC, PHCC, PMI · 2012Laboratory study · laboratory study, read in full 2026-10-05, re-read 2026-10-11: 135 ft of clear 4-inch PVC at 1% and 2%; 40 runs × 100 flushes at 0.8/1.28/1.6 gal; response = average flushes to carry media out; slope, paper and volume significant, flush rate and trailing water not; 0.8 gal plugs in 5 of 16 runs; 5-gal clearing flush failed in 7 of 39; no problems anticipated at 1.28 gal in new commercial construction; retrofit: inspect drain first; complaints largely attributed to older or faulty drains; cast iron not tested; Energy Policy Act 1.6 gpf from 1994 (residential)
2. [2]Florida Building Code, Plumbing, 8th Edition (2023) — §704.1 and Table 704.1; §604.4 and Table 604.4 — Florida Building Commission / International Code Council · 2023Law / code · read 2026-10-05, re-read 2026-10-11: §704.1 uniform alignment at uniform slopes, minimum 1/4 in/ft for 2 1/2 in or less, 1/8 in/ft for 3–6 in, 1/16 in/ft for 8 in and larger; water closet maximum 1.6 gallons per flushing cycle (chapter 6, Table 604.4)
3. [3]Residential Toilets — U.S. EPA WaterSenseGovernment or standards body · read 2026-10-05, re-read 2026-10-11 (page last updated 2026-05-29): WaterSense toilets use 1.28 gallons per flush or less, 20 percent below the 1.6 gpf federal standard; label testing includes waste extraction (soybean paste and toilet paper cleared from the bowl) and drainline transport
4. [4]The Drainline Transport of Solid Waste in Buildings – Phase 2.0, with Supplemental Report on Phase 2.1 — Plumbing Efficiency Research Coalition (PERC) Technical Committee · PERC — AWE, ASPE, IAPMO, ICC, PHCC, PMI · 2016Laboratory study · laboratory study, revised March 2016, read 2026-10-11: same method as Phase 1, adds 1.0 gpf runs and 3-inch pipe; largest drop in transport between 1.28 and 1.0 gpf; 1.0 gpf or less not recommended for long commercial drains without other long-duration flows, not applicable to residential; paper wet strength most significant variable at all volumes; Phase 2.1 appendix (16 runs, not peer reviewed): one flat 8-ft section worsened transport in 14 of 16 runs; the report says it completes PERC's research on drainline transport
5. [5]WaterSense Performance Overview: Flushometer-Valve Water Closets — U.S. EPA WaterSense · 2022Government or standards body · April 2022, read 2026-10-11: EPA reviewed PERC Phase 1, 2.0 and 2.1; found drainline blockages not of significant concern at 1.28 gpf; chaotic performance below 1.0 gpf; set a 1.0 gpf minimum for WaterSense flushometer-valve toilets