Water draining from the neighbor's yard

What it is not is a verdict. This page is careful about that line, because the distance between "most of the upstream land is not yours" and "your neighbor is liable" is enormous, and only the first half is something anyone can measure.

The quantity you are actually asking about

It is called the contributing area: the land whose surface runoff arrives at one chosen point. Pick the low corner at the back of the house and the contributing area is every square meter that drains, by some path over the ground, to that corner.

It very often extends well past the parcel. Water divides follow ridges. Property lines follow whatever a surveyor and a developer agreed on decades ago, and the two have no reason to coincide.

How the area is computed

From a bare-earth elevation surface — the ground with the trees and the buildings stripped out — with three steps that most quick analyzes skip.

Fill every hollow to its spill point first

Otherwise water routes into millimeter-deep artefacts and stops dead. Measured on real ground, 282,572 m² of contributing area in Chicago and 201,312 m² in Pittsburgh drained into interior pits two to four millimeters deep and terminated, against largest surviving channels of 1,034 m² and 33,931 m². Every closed depression is raised until it overflows, and the depth it was raised is kept rather than discarded.

Divide the water, do not hand it over

Modeled flow spreads the way overland flow actually behaves — near-single-channel in a gully, dispersive across a lawn. Forcing all the water into a single channel manufactures a hard line across ground where flow really spreads, which in a boundary dispute is exactly the kind of artefact that gets waved around.

Put the houses back in

A bare-earth surface has had every building removed, so unmodified it routes water straight through the house next door. Buildings are put back in as walls before anything is routed, so flow goes around structures — which matters enormously when the uphill lot is mostly roof.

And then admit the uncertainty

Accumulation values change sharply across a flow boundary, so the value at a wall is sampled at nine points and published as a range rather than a decimal. Anyone quoting you a single exact square-meter figure for an upstream area is quoting one sample of nine.

What else comes with it

• Per-wall ground slope — measured along each wall's own outward normal, 0.75 m to 3.25 m out, repeated and combined so no single reading decides it.
• Modeled flow paths — where surface water concentrates as it crosses the ground, including where it crosses a boundary.
• Standing water — depth and extent of any hollow within 20 m of the building, outside every structure, with a 3 cm floor because below that is survey noise.

And a discipline that matters here more than anywhere: a wall direction is printed only when it survives the footprint being displaced one meter in each of sixteen directions and re-measured — seventeen positions, all sixteen agreeing on the sign. Walls that fail come back undecided with their lean and their agreement count. In a dispute, a number that moves when the outline moves is worse than no number.

Where the measurement stops

Three specific things we will not tell you:

1. Whether anyone is liable. Drainage law varies by state and often turns on whether the uphill owner altered the flow. That is an attorney's question and we are a measurement company.
2. Whether the uphill lot was regraded. Establishing a change needs surveys from before and after. The underlying data is collected on a cycle, and one surface describes one date.
3. Where your boundary legally runs. We measure the ground. Published parcel geometry carries its own error and is not a survey.

What to do with the answer

The productive use is usually not confrontation. It is scope. If most of the water arriving at a wall comes from off the parcel, then work confined to the parcel is being asked to intercept somebody else's catchment, and it should be sized for that — which is a conversation with a contractor, informed by a number rather than by a guess.

If most of it is your own ground, that is worth knowing too, and it is cheaper to find out before the trench is dug.

Common questions