Measure/Why water pools next to a foundation
Drainage, explained
Why water pools next to my foundation
Water stands against a foundation wall for four physical reasons, and only one of them is the one everybody names. Ground tipping toward the house is the famous cause. The other three — how much land drains to that spot, whether flow concentrates there, and whether the water has anywhere to go — are just as decisive and are much harder to judge by eye.
This page explains all four, how each is actually measured, and exactly what a measurement of this kind can and cannot tell you about your house.
1. The ground tips toward the wall
The usual target is a fall of about 5% over the first three meters away from the wall — roughly 15 cm of drop over 3 m. Where the ground instead rises away from the house, surface water that lands on it runs back at the wall.
The reason this is hard to eyeball is that the human eye is bad at small gradients over short distances, and gardens are full of things that fake one: a raised bed, a mulch line, a path edge, a decade of settlement over the original backfill trench.
It is also not one number for a house. Every wall faces a different way and sits on different ground. A property "graded correctly" overall can have one elevation that is a bowl, and that elevation is the one that matters.
How it is measured properly
Each wall along its own outward normal: the slope measured from 0.75 m to 3.25 m out, repeated at several stations along the wall and combined, so one bad reading — or a wheelie bin — cannot carry the wall. Nothing is sampled nearer than 0.5 m to the wall, because inside a footprint a bare-earth surface is not ground; it is interpolation across a building that was removed from the data.
2. Something upstream is draining to that spot
This is the cause people miss, and it is usually the reason one corner behaves unlike the rest of the house.
Contributing area is the number of square meters of ground uphill that drain to a given wall. Two walls with an identical 6% adverse slope can be completely different situations: one is collecting nine square meters from a side yard, the other is collecting the back half of a hillside, including your neighbor's.
You cannot see contributing area from the ground, because most of it is not visible from where you are standing. It has to be computed from a terrain model that extends well beyond your property line.
3. The flow concentrates rather than spreads
Runoff does not arrive evenly. It converges. A shallow swale you have never noticed, the line between two gardens, the gap between a garage and a fence — these collect flow from a wide area and deliver it narrow.
Modeling this properly means routing water downhill and accumulating it, not shading a map by steepness. The modeled water spreads the way overland flow actually behaves — near-single-channel in a steep gully and dispersive across a lawn.
Buildings act as walls in the model, so water goes around structures rather than through them — which is why a neighbor's garage can be the reason flow arrives at one corner of your house.
4. The water has nowhere to go
The fourth condition is the absence of an outlet. A closed depression — a dip with no downhill exit — fills until it spills over its lowest lip. If that dip is beside a wall, the water sits.
Modeling it means filling every hollow in the terrain to the point where it overflows and keeping the depth it filled to, rather than throwing it away as most hydrology pipelines do. The depth is the interesting part.
Skipping that step ruins the rest of the model, too, and by more than you would guess. On real data, 282,572 m² of contributing area in Chicago and 201,312 m² in Pittsburgh drained into interior pits two to four millimeters deep and simply stopped — against largest surviving channels of 1,034 m² and 33,931 m². Real water spills over a 2 mm lip. An unconditioned model does not.
The other suspects, before you regrade anything
Not every puddle is a terrain problem. Check the downpipes first — where they discharge, and whether they are connected. Then the backfill trench, which settles for years after construction. Then patios, paths and driveways, which are frequently pitched back toward the house. Then anything subsurface, which no remote measurement can see: an existing drain, a pump, a buried pipe run.
What a terrain measurement can and cannot tell you
| Can be measured from the air | Cannot |
|---|---|
| Ground slope at each wall, with an uncertainty range | Whether your basement has ever been wet |
| Square meters draining to each wall | Soil type, infiltration rate, water table |
| Where surface flow concentrates on the parcel | Anything below ground: drains, pumps, pipe runs |
| Depth and extent of standing water within 20 m of the house | Where the downpipes discharge |
| Which of these the model refused to answer | What a storm of a given size would do |
Said plainly
A measurement of ground slope is not a finding that your property has a drainage problem, a wet basement, or any defect. Ground falling toward a foundation raises the risk of water reaching it — that is ordinary building science and it is not in dispute. That your house has water is a different claim, and it is not one that can be made from a terrain model.
Why the number gets a range, not a decimal point
A published building outline is a trace of a roof and can sit a meter from the wall it claims to be. That matters: on a real Pittsburgh footprint, shifting it north in 0.2 m steps moved one wall from +5.4% to −23.5%, crossing zero inside 1.4 m.
So the honest way to publish a wall reading is to move the house and see whether the answer survives. Measure displaces the footprint one meter in each of sixteen directions — seventeen positions in total — and only prints a direction when all seventeen agree on the sign. Everything else is reported as too close to call, which is a less satisfying answer and a more truthful one.
See what the ground does at your address
Type an address and we find your building, put it on the survey and show you the shape of what we measured — free, no card. The numbers themselves are the product, so a full report with every wall as a figure is $25, no subscription.
If we cannot tell which building your address names, we say so and charge nothing.
How long can water safely sit next to a foundation?
There is no single safe number, and anyone offering one without seeing your soil is guessing. What is well established is the direction: repeated saturation of backfill against a wall is a condition worth removing.
Is a puddle at the wall always a grading problem?
No — see the four causes above, plus downpipes, settled backfill and hard surfaces pitched the wrong way. Slope gets blamed for a lot of things it did not do.
What slope should the ground have away from the house?
The common target is roughly a 5% fall over the first three meters — about 15 cm over 3 m. It is a target, not a code citation, and local requirements vary.
Can you tell remotely whether my basement is wet?
No, and we will not pretend otherwise. We can measure the ground outside the wall. What has happened inside it is not visible from outside.