Dig tolerance zones and marking symbols: the rules in each state we work in

Paint on the ground does not mean the line is under the paint. It means the line is somewhere within a defined distance of it, and that distance is written into state law. The area is called the tolerance zone, and what you may do inside it is also set by statute.

Here is the rule in each state we work in, with the citation, plus how to read the marks themselves.

Tolerance zones by state

Washington: 24 inches

RCW 19.122.020 defines reasonable accuracy as location within 24 inches of the outside dimensions of both sides of a facility. In practice that is a corridor 48 inches wide plus the width of the line itself. Inside it, RCW 19.122.020 defines safe and careful work methods to include potholing, hand digging where practical, and vacuum excavation.

Oregon: 24 inches

OAR 952-001-0010 defines the tolerance zone as the area within 24 inches surrounding the outside dimensions of all sides of a facility. OAR 952-001-0090(3)(c) requires hand tools or other non-invasive methods either to determine the exact location of the facility, or down to 24 inches beyond the depth of intended excavation, when working within that zone. Non-invasive methods are defined to include hand digging, air cutting, vacuum excavation and hydro excavation.

Oregon also has a rule worth knowing for boring work: under OAR 952-001-0090(5), an excavator using horizontal directional drilling must visually monitor the drill head and back reamer passing through the tolerance zone.

Idaho: 24 inches

Idaho Code 55-2202 defines reasonable accuracy as location within 24 inches horizontally of the outside dimensions of each side of a facility. Idaho Code 55-2207(2)(a) requires the excavator to determine the precise actual location of marked facilities by hand digging within 24 inches of them. Idaho defines hand digging broadly, to include hand shovel digging, manual posthole digging, vacuum excavation and soft digging.

Montana: 18 inches

Montana is the exception in our service area. MCA 69-4-501 defines reasonably accurate as location within 18 inches of the outside lateral dimensions of both sides of a facility. The state requires a minimum 18 inch horizontal clearance between an unexposed facility and the cutting edge of any power-operated equipment, and hand digging inside that zone. If your crews work across the Idaho line into Montana, the number changes.

Northern California: 24 inches

California Government Code 4216 defines the tolerance zone as 24 inches on each side of the operator's field marking. How it is measured depends on how the line was marked: 24 inches from each side of a single centreline mark, 24 inches plus half the stated size where the size is specified, or 24 inches from each outside mark where the width is shown.

Government Code 4216.4 requires the exact location to be determined with hand tools before any power-driven equipment is used in the zone. A hand tool is defined as human-powered, not driven by a motor, engine, hydraulic or pneumatic device. Vacuum excavation may be used in the tolerance zone only if the operator has marked the installation, has been contacted, and agrees to its use, and the excavator must tell the notification centre of their intent to use it when obtaining the ticket.

Quick comparison

• Washington: 24 inches each side. RCW 19.122.020.

• Oregon: 24 inches each side. OAR 952-001-0010 and 952-001-0090.

• Idaho: 24 inches each side. Idaho Code 55-2202 and 55-2207.

• Montana: 18 inches each side. MCA 69-4-501.

• California: 24 inches each side of the field marking. Government Code 4216 and 4216.4.

Measure from the outside of the line, not the centre of the paint, unless the mark is a single centreline mark. A single stripe over a 12 inch pipe in a 24 inch state produces a corridor four feet wide plus the pipe.

Reading the marks

The colour tells you what kind of facility it is, and we cover that separately in utility marking colors explained. The rest of what a locator writes on the ground carries meaning too.

• Arrows and direction. An arrow at a mark shows the direction the line runs, usually where it leaves the marked area or turns.

• Offset marks. Where a line cannot be marked directly, because of pavement, a structure or traffic, the locator may mark a parallel line a set distance away and write that offset distance beside it. The line is that distance from the paint, not under it.

• Width marks. Two parallel lines show the outside edges of a wide facility or duct bank. The tolerance zone runs from each outside mark.

• Numbers and counts. A number beside a mark commonly indicates the number of conduits or ducts in a bank, or the size of the facility where the operator has specified it. In California a specified size changes how the tolerance zone is measured.

• Letters. Identification letters indicate the type of facility, for example G for gas, E for electric, T or FO for telephone and fibre optic, W for water, S or SS for sewer. Montana requires marking to include identification letters and the facility width where it is greater than 6 inches.

• Unknown or unlocatable. Where an operator has identified a facility it cannot locate accurately, that is an identified but unlocatable facility, and the marks will reflect uncertainty rather than position.

What the marks do not tell you

Marks show horizontal position only. They are not a statement of depth. Depth changes with grade work, erosion, previous excavation and the original installation, and no state requires an operator to mark it.

Marks also expire. Washington marks are valid for 45 calendar days from the date notice was given. Oregon ticket life is 30 calendar days. Montana requires a re-mark if excavation has not begun within 30 days of marking. If paint has faded, washed out or been driven over, request a re-mark rather than working from what is left.

And the marks only cover public facilities. Everything past the meter is private line, which no 811 operator marks. See does 811 cost money, what's free and what isn't and what we find after 811 has been out.

If a mark looks wrong

Contact the operator that owns the line before digging. Marking concerns numbers by county: Oregon, Washington, North Idaho, Montana.

Private locating and mark verification

Advanced Underground Utility Locating has been locating private utilities since 2002. We locate the lines 811 will not mark, verify the public marks that were made, and mark public lines that were missed.

Spokane, Coeur d'Alene and the Idaho panhandle: 509-710-2621. Seattle, Tacoma and Portland: 509-954-8623. Bend, Medford and northern California: 541-678-0629. Eugene and Roseburg: 541-780-4149. Coos Bay and the Oregon Coast: 541-315-6728.

This is a general summary of statutes current at the time of writing and not legal advice. Rules change, and local operators may impose stricter requirements. Verify with your state's one-call centre before relying on any figure here.