Published
What makes a septic tank legal in Idaho — steel is out, and the lid has to reach the surface
A tank is a manufactured product with a published specification
Most conversations about a septic tank are about size. Size matters, and the tank sizing page covers how the permitted number is set. But the rule has a great deal more to say about how a tank is built than about how big it is, and none of it is common knowledge among the people buying one.
Everything below is from IDAPA 58.01.03, Section 007, Septic Tanks Design and Construction Standards. It applies to new tanks, and it is the specification an inspector works from.
Materials: one sentence rules out a whole category
New septic tanks will be constructed of concrete, or other materials approved by the Director. Steel tanks are unacceptable.
— IDAPA 58.01.03.007.01 (adminrules.idaho.gov, retrieved 2026-08-17)
Four words settle a question that comes up on every older property here: steel tanks are unacceptable. Not discouraged, not permitted with conditions. This is a rule about new tanks, so it does not by itself make an existing steel tank unlawful — but it does mean that a steel tank cannot be your replacement, and a steel tank found during a sale is a tank with no future.
The design route is equally firm. A professional engineer licensed by the state of Idaho must submit all septic tank designs to the Department for approval, and where a design does not meet every requirement of Section 007, the engineer has to demonstrate that the deviation is determined by sound engineering practice and meets the intent of the rules. That is why tanks arrive as approved products rather than as somebody's idea.
The concrete specification
For new concrete tanks the rule sets minimums that are worth knowing, because they are the difference between a tank and a box:
| Element | Requirement | Why it is there |
|---|---|---|
| Walls and floor, reinforced | At least 2½ inches thick | The reinforcement is doing part of the work |
| Walls and floor, not reinforced | At least 6 inches thick | Without steel, section thickness is the whole strength |
| Lids and covers | At least 3 inches thick and adequately reinforced | A lid is walked on, and occasionally driven over |
| Floor and lower wall | Poured at the same time (monolithic pour), for at least a 6-inch vertical portion of the wall | The floor-to-wall joint is where a cold joint would leak |
| Separately poured wall sections | Interlocking joints on the joining edge | Same reason, on the vertical seams |
| Concrete outlet baffles | Finished with asphalt or other protective coating | The outlet is the most corrosive place in the tank |
| Every tank, of any material | Water tight, and constructed of sound, durable materials | Leaking both ways is a failure: effluent out, groundwater in |
| Durability | Not subject to excessive corrosion, decay, frost damage or cracking | Frost damage is named in the rule itself — relevant on ground that freezes this hard |
The last two rows apply to every tank, not only concrete ones, and the wording is the rule's own.
The volume arithmetic almost nobody knows
Two requirements in Section 007 are the same fact seen from two directions, and together they explain why a tank is bigger than its rating:
- Total volume must be at least 115% of liquid capacity. A tank rated at 1,000 gallons of liquid is a vessel of at least 1,150 gallons.
- Scum storage: the air space above the liquid level must equal or exceed 15% of the tank's liquid capacity. That is where the extra volume goes.
The headroom is not spare capacity — it is the scum layer's designed home. An owner who reads "1,000 gallons" as the size of the hole in the ground has the wrong number, and an owner who thinks the empty space at the top means the tank is running below capacity has the wrong idea.
Other dimensional minimums: no interior horizontal dimension of a tank or compartment may be less than two feet, and liquid depth must be at least two and a half feet.
Access: the requirement most often broken on site
Manholes must extend to the finished grade. Access to each septic tank or compartment must be provided by a manhole twenty (20) inches in minimum dimension or a removable cover of equivalent size. Each manhole cover must be provided with a corrosion resistant strap or handle to facilitate removal.
— IDAPA 58.01.03.007.14 (retrieved 2026-08-17)
Read the first sentence again, because it is the one that gets ignored: manholes must extend to the finished grade. A tank whose lid sits under a foot of soil, a lawn, a deck or a parking area does not meet it. That is not a technicality — it is the difference between a routine pump-out and an excavation, and it is precisely why so many owners cannot find their tank at all. The finding-your-tank page exists because of this.
The rule also requires an inspection port measuring at least eight inches in minimum dimension. Between the manhole and the inspection port, a compliant tank is designed to be looked into without a machine.
Inlets, outlets and markings
| Feature | What the rule requires | What it tells you on site |
|---|---|---|
| Inlet diameter | At least four inches | A smaller inlet is not a compliant tank connection |
| Inlet height | Enters the tank three inches above the liquid level | The drop is deliberate; a tank plumbed level does not work |
| Tees and baffles | Must not extend horizontally into the tank beyond twice the diameter of the outlet | Over-long baffles interfere with the settling the tank exists to do |
| Manufacturer marking | Legible and indelible name or trademark | Identifies who built it, decades later |
| Capacity marking | Total liquid capacity, marked on the tank | The rated size can be read off the tank, not guessed |
| Flow marking | An indication of the inlet and the outlet | Settles which end is which without tracing pipe |
That last one is the most useful thing on this page for anyone buying a property. The tank is required to have its own capacity written on it, permanently. If the manhole reaches grade as it should, the rated capacity can be read off the tank itself rather than taken from a seller's recollection.
How to check a tank against the rule
- Open the manhole, or find out why you cannot. If the lid does not reach the surface, that is itself a departure from the specification and the first thing to price.
- Read the markings for the manufacturer and the total liquid capacity, and compare that number against what the permit file says the property is approved for.
- Confirm the material. Concrete, or a material approved by the Director. A steel tank cannot lawfully be installed new, whatever is currently in the ground.
- Look for the inspection port — at least eight inches, and a compliant tank has one.
- Get the permit file from Eastern Idaho Public Health and check the tank on the ground against the tank on the permit. They are not always the same tank.
- Do this before closing. Everything above is visible in an afternoon if the access is where the rule says it should be.
Re-verify before relying on any figure here. This page was written from the current text of IDAPA 58.01.03 retrieved on 2026-08-17; subsections carry effective markers of 3-31-22 and 7-1-25. The rule is free at adminrules.idaho.gov, and Eastern Idaho Public Health permits septic systems in Fremont County.
Common questions
Are steel septic tanks legal in Idaho?
The rule says new septic tanks will be constructed of concrete or other materials approved by the Director, and that steel tanks are unacceptable. It governs new tanks, so it does not by itself outlaw one already in the ground — but a steel tank cannot be your replacement.
How thick does a concrete septic tank have to be?
Walls and floor must be at least two and a half inches thick if adequately reinforced, and at least six inches if not. Lids and covers must be at least three inches thick and adequately reinforced.
What is a monolithic pour and why does the rule require it?
The floor and at least a six-inch vertical portion of the walls of a poured tank must be poured at the same time. It removes the cold joint at the floor-to-wall junction, which is where a tank would otherwise leak.
Why is a tank's total volume bigger than its rated capacity?
Because the rule requires total volume to be at least 115% of liquid capacity, and separately requires air space above the liquid level of at least 15% of liquid capacity. The headroom is the designed space for the scum layer.
Does the septic tank lid have to reach the surface?
Yes. The rule says manholes must extend to the finished grade. A lid buried under soil, lawn, a deck or paving does not meet that requirement, and it turns every future pump-out into an excavation.
How big does the manhole have to be?
Access to each tank or compartment must be by a manhole of at least twenty inches minimum dimension, or a removable cover of equivalent size, and each cover must have a corrosion-resistant strap or handle to help remove it.
Is there meant to be an inspection port?
Yes — the rule requires an inspection port measuring at least eight inches in minimum dimension, separately from the manhole.
Can I tell the capacity of a tank by looking at it?
On a manufactured tank, yes. The rule requires tanks manufactured to an approved design to be legibly and indelibly marked with the manufacturer's name or trademark, the total liquid capacity, and an indication of the inlet and outlet.
Who designs a septic tank?
A professional engineer licensed by the state of Idaho must submit all septic tank designs to the Department for approval. Where a design does not meet every requirement, the engineer must show the deviation is sound engineering practice and meets the intent of the rules.
What are the minimum internal dimensions?
No interior horizontal dimension of a tank or compartment may be less than two feet, and the liquid depth must be at least two and a half feet.
How high above the liquid does the inlet sit?
The inlet must be at least four inches in diameter and enter the tank three inches above the liquid level. Tees and baffles must not extend horizontally into the tank beyond twice the diameter of the outlet.
Does the rule mention frost?
Yes. Tanks must be water tight and constructed of sound, durable materials not subject to excessive corrosion, decay, frost damage or cracking — frost damage is named expressly, which matters on ground that freezes as hard as this does.