A new building needs a new sewer line, and that line only works if the trench under it has the right fall from start to finish. We dig sewer trenches for new homes, ADUs and shops, work to the grade your plumber sets, and keep the trench open and safe for the inspection.
New Sewer Line Trenching in Boise
This page is about new sewer lines: a house on an empty lot, a backyard ADU, a shop with a bathroom. If an existing line is cracked, backed up or full of roots, that's a repair job, and our sewer line excavation page covers it.
We trench new sewer services for builders, plumbers and owners in Boise, Meridian, Nampa, Eagle, Kuna and Caldwell. On new homes, the call usually comes once the house drain location is set. Owners adding a backyard ADU or turning a detached garage into a shop with a bathroom tend to call earlier, while they're still working out the route.
Where the New Line Connects
City sewer
In newer subdivisions, the developer usually left a sewer stub at the property line, and the trench runs from the house to that stub. Infill lots in older parts of Boise can be different. The connection may be at a main in the street, which brings in city requirements and often a separate permit for work in the right of way. Your city's public works department sets those rules.
A septic tank
On rural lots around Kuna, Star, Middleton and much of Canyon County, the building sewer runs to a septic tank instead. The tank location and the system design go through Central District Health in Ada County or Southwest District Health in Canyon and Owyhee counties. See our septic system installation page.
ADUs and shops
A detached ADU or a shop bathroom may tie into the main house's line or need its own connection. Cities handle this differently, so the plumber and the city answer that one before we dig. The answer changes the trench. A shared line is often a shorter run, but the tie-in point on the existing pipe has to be deep enough for the new line to fall into it. A separate connection can mean a longer trench out to the stub or the street, past patios, fences and the gas and sprinkler lines already in the yard.
Fall: The Number That Runs the Job
A sewer line works by gravity, so the trench bottom has to drop steadily the whole way. For 4 inch pipe, the plumbing code commonly calls for about 1/4 inch of fall per foot, and your plumber and inspector confirm the exact figure.
That adds up. A 120 foot run at 1/4 inch per foot drops 30 inches from end to end. So the depth where the pipe leaves the house and the depth of the stub or tank inlet have to work together. We check both ends before we start. If the numbers don't work, it's much better to find out before the foundation is poured than after. That's one reason builders loop us in early, often during foundation excavation.
In the trench, we grade the bottom as we go and check it along the run. A low spot, or belly, holds water and solids, and it's the kind of mistake that shows up as a clog a year later.
Depth, Frost and Cleanouts
Most new sewer lines end up below frost for much of their length simply because of the fall. The shallow part is usually near the house, and the plumber decides how to handle that section. That might mean more cover, a different route, or changing where the line leaves the building, which are easy calls on paper and hard ones once the slab is poured.
The code requires cleanouts at certain points, such as near where the line leaves the building and at some changes in direction, and long runs may need more. The plumber lays them out. We dig so each one can come up to grade where it's supposed to.
Deep runs need protection. Any trench 5 feet or deeper gets sloped, benched or shored under OSHA rules, and connections to a deep street main can easily reach that. Sloped or benched walls take more room at the top than a narrow cut, so on a tight infill lot we plan where the trench, the spoil pile and the machine will sit before we start.
What to Have Ready Before We Dig
Good information up front makes the estimate more accurate and dig day smoother. It helps to have:
- A site plan showing the house, the sewer stub or main, and any septic tank location
- Elevations for where the house drain leaves the building and for the stub, main or tank inlet
- Permit status from your plumber, plus any city rules for work in the street
- Access details, like a clear path for a machine and a spot to pile dirt
Working With Your Plumber and the Inspector
The usual order: we dig to grade, the plumber lays and tests the pipe, the inspector looks at it in the open trench, and then we backfill. We hand-place the bedding around the pipe so it isn't knocked off grade, then backfill and compact in layers. Under a future driveway or slab, that compaction matters most. See compaction.
Our reviews describe us as "professional, efficient and highly skilled," and on a sewer job that mostly means showing up when the plumber is ready and not leaving an open trench waiting on us.
What Sets the Price
Length and depth lead the list. After that it's soil and rock, whether the connection is at a property-line stub or out in the street, trench protection on deep sections, and any concrete or landscaping in the way. A street connection can also come with city requirements, like pavement patching, that a stub connection doesn't. We check the stub or main elevation and walk the route before pricing it.
Why Hire Ultimate Excavation LLC
We're an Idaho registered contractor (RCE-65836) with a 5.0 rating on Google. Reviewers mention Tanner's communication again and again, which is what a new sewer line needs when the builder, plumber and inspector all want the trench on the same day. Call (208) 697-2429 or request a free estimate.
