Published
What size culvert does your driveway need? The county publishes the numbers: 12 inches by 30 feet.
The two numbers, verbatim
The question every driveway owner asks first — what size pipe? — has a published answer in this county. The county’s Roadway Standards, the adopted roadway-design document the county holds its own road work to, state the driveway floor directly:
“Culverts under approach roads or driveways shall have a minimum diameter of 12" (or the size necessary), a minimum length of thirty feet (30'), and shall meet the requirements of Section G.3.”
— Fremont County Roadway Standards, section G.4, page 14. Retrieved 11 September 2026
Twelve inches of diameter, thirty feet of length — and both are floors, not answers. The parenthetical “(or the size necessary)” does real work: it is the county reserving the right to make your pipe bigger than the floor when the water demands it. More on that below, because it is the difference between buying pipe once and buying it twice.
Three county documents, the same two numbers
What makes these figures safe to plan against is that the county says the same thing in more than one place:
| County document | What it prints |
|---|---|
| The Roadway Standards, section G.4 (page 14) | Minimum diameter of 12" (or the size necessary), minimum length of 30 feet, with materials governed by section G.3 |
| The Development Code, Appendix B 1.13.040 — quoted on this site’s road-drainage page | The same 12-inch and 30-foot floors for culverts under approach roads and driveways |
| The county’s driveway application, checklist item 5b | “Minimum diameter of 12 inches, a minimum length of 30 feet… corrugated steel, aluminum or concrete… covered by at least 12 inches of material” |
Two numbers, three documents, no disagreement between them — which is rarer in county paperwork than it should be, and worth knowing before you buy pipe.
“Or the size necessary” — the floor is not the answer
The 12-inch figure is where the county’s requirement starts, not where it ends. The same sentence reserves the county’s right to size the pipe to the water your approach actually crosses:
- Diameter is a drainage question. A ditch carrying spring runoff from a long stretch of road may need more than the floor — that is what “the size necessary” means, and it is the county’s call inside the driveway permit.
- The floors run higher elsewhere. Culverts under the roadway itself carry a higher floor, and the county’s access standard sets its own figure for approaches onto county roads — both are covered where they live, on the drainage-rules page and the access-standard page.
- Your job is to ask before buying. The published floor tells you what the smallest legal pipe looks like; only the county tells you what your ditch requires. Whether you need a culvert at all is the first question; this is the second.
What section G.3 adds: the pipe you are allowed to buy
Section G.4’s cross-reference to G.3 is not decorative — it is the materials spec. G.3 limits drainage culverts to three materials and sets cover over the top of the pipe:
| G.3 requirement | What it means at the supply yard |
|---|---|
| Corrugated steel, aluminum, or concrete | The three approved materials — other types must be approved by the county’s Public Works Director first |
| 12 inches of cover, minimum | At least a foot of fill over the top of the pipe — which is why culvert depth and driveway gravel depth get planned together |
| Thickness per the county’s Table 3 | Gage minimums by material for pipes 12–36 inches — the full table is printed on the drainage page |
The driveway application’s checklist repeats the same three materials and the same 12 inches of cover, so the buyer’s short version is: steel, aluminum, or concrete, buried under at least a foot of fill.
Why thirty feet surprises people
The diameter floor gets the attention, but the length figure is the one that changes a quote. Thirty feet is the minimum length of the crossing — the pipe has to run the full footprint of your driveway fill, top width plus both sloped sides, so the water passes under you instead of through you.
Practically: a driveway pipe is not a section of pipe, it is a run. A shorter pipe leaves the face of your fill unarmored at exactly the place the ditch runs hardest, and a pipe that ends inside the fill instead of beyond it is the classic failed approach — the one that washes out in the first heavy spring and gets rebuilt at full price. When you compare excavation bids, the words to use are continuous, thirty feet minimum, to the county’s standard — and the permit conversation happens before the pipe is ordered, not after.
Buying to the published spec
- Confirm with the county whether your approach needs a culvert at all — not every access crosses a ditch, and the answer belongs in your permit file.
- Ask what size your approach requires: quote the published floor (12 inches by 30 feet) and ask whether the county wants more under the “size necessary” clause.
- Buy pipe in an approved material — corrugated steel, aluminum, or concrete — and get written approval before substituting anything else.
- Specify a continuous run of at least thirty feet, not sections that add up to it.
- Plan for at least twelve inches of cover over the top, which sets the relationship between pipe, bedding, and driveway fill.
- Put the county’s answer in writing in the scope sheet every bidder prices, so three bids cover the same pipe.
- When the numbers are settled and the pipe is on site, that is the moment to talk build: (208) 932-1140.
Common questions
What diameter culvert does a driveway need in Fremont County?
The published floor is a minimum diameter of 12 inches — or the size necessary, if the water crossing demands more. It appears in the county's Roadway Standards, section G.4, page 14, and again on the driveway application's checklist.
What length does a driveway culvert have to be?
A minimum length of thirty feet, in the same sentence of the Roadway Standards that sets the diameter floor. The length is the full footprint of the driveway crossing — top width plus both sloped sides.
What materials can the culvert be?
Corrugated steel, aluminum, or concrete. Section G.3 of the Roadway Standards limits drainage culverts to those three; any other material must be approved by the Fremont County Public Works Director.
How much cover goes over the top of the pipe?
At least 12 inches of material over the top of the pipe, per section G.3's culvert table — the same figure the driveway application's checklist prints.
Can the county require a bigger culvert than 12 inches?
Yes. The standard says 12 inches "(or the size necessary)" — the county sizes the pipe to the water the ditch actually carries, and the floors run higher still for culverts under roadways and under the county's access standard.
Where exactly are these minimums published?
Three places: the county's Roadway Standards, section G.4, page 14; the Development Code's Appendix B 1.13.040; and the driveway application's own checklist, which cites both appendices. All three carry the same 12-inch and 30-foot figures.
Is a plastic or HDPE culvert allowed?
Not by default. Section G.3 lists corrugated steel, aluminum, and concrete, and directs that other types of materials must be approved by the Public Works Director — so a plastic pipe needs that approval in writing before it goes in the ground.
Does the thirty feet mean one pipe or can sections add up?
The standard writes it as a minimum length of thirty feet for the crossing — read as one continuous run. How joints are treated in a longer assembly is a question for Public Works, not something to assume at the supply yard.
Are these the same numbers for a driveway onto a state highway?
No. An approach onto US-20 or any state highway is permitted by the Idaho Transportation Department under its own rules — these are the county's figures for county-road approaches.