Washington Plumber Supervision Rules — The 75% Rule, Remote Supervision, and Trainee Ratios

Washington's supervision rules for plumbing trainees are among the strictest in the country. Getting them wrong does not just risk an infraction — it risks having your trainee's documented hours permanently disqualified by L&I.

The 75% On-Site Rule

Under RCW 18.106.070, "direct supervision" means the supervising plumber must be physically present on the job site for at least 75% of the working day. Not available by phone. Not in the truck outside. Physically on-site, within sight and sound of the trainee, for three-quarters of every shift.

For an eight-hour day, that means the supervisor must be on-site for at least six hours. The remaining two hours allow for supply runs, inspections at other sites, or administrative tasks — but the trainee cannot continue performing plumbing installation work during that absence unless another qualified supervisor takes over.

Ratios by License Type

The number of trainees one supervisor can oversee depends on the certification type and the job site:

• Commercial or industrial sites (PL01 work): 1:1 — one journey-level plumber for every one trainee. No exceptions.
• Residential construction sites (PL02 work): 1:3 through December 31, 2028 — one PL01 or PL02 supervisor for every three trainees.
• Residential service (PL04 work): Normally 1:1. A temporary 1:3 ratio applies to like-in-kind residential service through December 31, 2028, after which it reverts to 1:1.
• Backflow (PL30) and medical gas (MG01): 1:1, with the supervisor present for 100% of on-site time. These are the only categories where even the 75% rule does not apply — it is effectively 100%.

Exceeding the ratio on a job site means every trainee hour logged that day is at risk. If an L&I inspector finds three trainees under one supervisor on a commercial site, the hours can be disqualified for all three trainees, not just the "extra" one.

Remote Supervision — The 40-Mile Exception

Washington allows limited remote supervision, but only under a narrow set of conditions — all of which must be met simultaneously:

1. The trainee has completed more than 2,000 documented hours.
2. The work is strictly residential service plumbing (PL04 scope — single-family or duplex, existing systems only).
3. The supervising plumber is located no more than 40 miles from the job site.
4. The supervisor is available by phone or audio-visual technology throughout the work period.

If any one of these conditions is not met, remote supervision is not legal. A trainee with 2,000 or fewer hours cannot work remotely supervised. A trainee doing new residential construction (PL02 scope) cannot work remotely supervised regardless of hours. And a supervisor 41 miles away does not qualify.

Can a Trainee Work Alone?

The short answer is no — not in the way most people mean. A trainee cannot independently bid, contract, or perform unsupervised plumbing installation work under any circumstances.

The closest Washington gets to "working alone" is the remote supervision exception described above: a PL04 trainee with more than 2,000 hours can perform residential service work while the supervisor monitors from up to 40 miles away. But even then, the supervisor is legally responsible for the work and must be reachable throughout.

The Downstream Risk

Ratio and supervision violations do not just create infractions for the contractor. Under WAC 296-400A, L&I can disqualify the trainee's hours retroactively. If an audit reveals that a trainee was working under improper supervision for six months, those six months of hours are permanently erased — pushing the trainee's journey-level qualification date back by half a year or more.

For a detailed matrix of supervision ratios, remote supervision eligibility, and hour documentation requirements, the Washington Plumber License Guide covers every scenario with the relevant WAC citations.

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