Measure/Does your yard drain toward the house
Drainage, explained
How to tell if your yard drains toward your house
You can establish the direction of drainage yourself, in an afternoon, with a string line and a hose. What you cannot easily establish from the yard is the volume — how much upstream land is being delivered to each wall — because most of it is not visible from where you are standing. This page covers both: three checks you can run, and what a terrain model adds.
Check 1 — the string line, which is the reliable one
You need two pegs, a string, a spirit level and a tape measure.
- Push one peg in at the base of the wall and run the string out three meters, perpendicular to that wall.
- Pull the string tight and level it — spirit level on the string itself, not on the ground.
- Measure the vertical gap between the string and the ground at the far peg.
About 15 cm of gap over three meters is a 5% fall, which is the usual target. A small gap means shallow fall. No gap, or the ground touching the string closer to the wall than at the end, means the ground is not falling away — that is negative grading.
Do it at every elevation. Not one reading for the house. The commonest finding is that three sides are fine and one is not.
Why the phone level app is the weaker tool
A phone measures the tilt of whatever it is resting on. On a lawn that is the tilt of the grass and the topsoil under it, which can differ from the slope over three meters by more than the thing you are trying to detect. Use it for a first impression, then use the string.
Check 2 — the hose test
Run a hose slowly at the base of one wall for several minutes and watch. Water that spreads outward and keeps moving is being carried away. Water that tracks along the wall, or pools, is not.
Two things to watch for that the string line cannot tell you. First, where the water goes after it leaves — if it runs three meters and then stops in a dip, you have moved the problem rather than solved it. Second, whether it disappears suddenly, which can indicate a settled backfill trench taking water down against the wall.
Check 3 — watch it in real rain, and look for convergence
The instinct is to look for where the rain is heaviest. The useful thing to look for is where flow converges: a shallow swale you have never noticed, the line between two gardens, the gap between a garage and a fence. Those collect from a wide area and deliver narrow.
Also note, in order:
- Where every downpipe actually discharges, and whether it is connected to anything.
- Anything that is still standing an hour after the rain stops. That is a closed depression — a dip with no downhill exit.
- Whether water arrives from a neighboring property, which is common and is invisible on a dry day.
- Whether patios, paths or driveways are pitched back toward the house. Hard surfaces shed nearly everything that lands on them.
What you cannot get from the yard
Two things, and they are the two that decide how big a job you are looking at.
Contributing area
The number of square meters of upstream ground draining to a given wall. Two walls with the same adverse slope can be entirely different situations — one collecting nine square meters from a side yard, the other collecting the back half of a hillside including your neighbor's. You cannot see it because most of it is not in view, and much of it is off your property.
Closed depressions you have not noticed
A dip with no outlet fills until it spills over its lowest lip. Millimeters matter here in a way that is genuinely counter-intuitive: on real terrain data, 282,572 m² of contributing area in Chicago and 201,312 m² in Pittsburgh drained into pits two to four millimeters deep. Real water spills over a 2 mm lip; a naive model does not, which is why depression filling is a required step rather than an optional one.
How a terrain model answers those two
From a bare-earth surface — the ground measured in 3D, with the trees and buildings stripped out — water is routed downhill and accumulated, spreading the way overland flow actually behaves: near-single-channel in a steep gully and dispersive across a lawn.
Every hollow is filled to its spill point first, and the depth it filled to is kept rather than thrown away. Buildings act as walls in the model, so water routes around structures, including your neighbor's garage.
Then the slope at each wall is measured over the first three meters, 0.75 m to 3.25 m out on that wall's own outward normal — essentially your string-line test, done on every wall at once, repeated along the wall and combined so no single reading decides it.
What neither you nor a model can settle from outside
A yard sloping toward the house is a condition that raises the risk of water reaching the foundation. That is ordinary building science. It is not a finding that your house has water, and neither a string line nor a terrain model can establish that. Soil type, infiltration rate, water table, and anything already installed underground are all outside what either can see.
Compare your string line against the survey
Type your address. We identify your building, place it on the bare-earth survey and show you the shape of what we measured — free, no card. Every wall as a number, with its direction and its range, is in the $25 report.
If we cannot tell which building your address names, we say so and charge nothing.
Can I use a phone level app?
For a rough impression. It measures the tilt of the grass under it, not the fall over three meters. The string line is both cheaper and better.
Why do all four sides behave differently?
Because drainage is a per-wall quantity. Different aspect, different ground, different upstream area. A house graded correctly overall can have one elevation that is a bowl.
What can a terrain model see that I cannot?
Mainly contributing area — the upstream land draining to each wall, most of which is out of sight and often off your property — plus closed depressions with no outlet.
Does a yard sloping toward the house mean I have a problem?
It means a condition that raises risk. It is not a finding that your house has water, and nobody measuring from the air can tell you that it does.