What a downspout at the wall does over ten years

Take a rough number for the sake of the argument. An inch of rain on a thousand square feet of roof is somewhere over six hundred gallons of water. That is arithmetic, not a claim: an inch deep across that area is a defined volume, and it works out around that.

Now divide that roof between four downspouts. Each one is delivering roughly a hundred and fifty gallons, in perhaps twenty minutes, in a single stream, at one point. If that point is a foot from your foundation wall, you have just poured a hundred and fifty gallons into the soil against your footing. Then it rains again next week.

Why the soil there is the worst place for it

When a house is built, ground is excavated wider than the foundation so people can work. Afterwards it is backfilled. That backfilled ring is never as dense as the undisturbed ground beyond it, and it never becomes as dense. Water takes the easier path, which is straight down the backfill to the footing.

Two things follow. First, water reaches the foundation wall and, in a basement, presses against it. Second, the soil in that ring goes through repeated wet and dry cycles that the surrounding ground does not. Much of the clay-heavy soil in this region swells noticeably when it takes on water and shrinks when it dries out, so the zone that is being soaked and dried is also the zone that is moving, right where the house is bearing.

The sequence, roughly in order of appearance

• Year one. A scoured pit under the elbow. Dead grass. Splash marks up the brick or siding to knee height.
• A few years. Mulch washing out of the bed on that side. Mortar looking tired at the base of the wall. Efflorescence, the white chalky bloom, on the inside of the basement wall in that bay.
• Longer. A musty smell that concentrates in one corner. Paint failing on the inside face. On a block wall, a horizontal crack or a step crack in the same area. On a slab, a settled corner and a door that stops latching.
• Longer still. A finished basement that has to be opened up, or a corner that needs underpinning, at a cost that dwarfs everything else on this website by a wide margin.

Not every damp basement in St. Louis is caused by a downspout. Some are groundwater, some are a failed footing drain, some are the yard itself sloping in. But when the damp is concentrated, and the concentration lines up with a downspout on the outside, that is not usually a coincidence and it is by a very large margin the cheapest of those causes to correct.

What to look for after the next storm

Go out while it is raining, not after. The information is only available while the system is running. Stand back and watch each downspout. You are looking for three things: whether the water leaves or pools, whether the ground it lands on falls away from the house or back towards it, and whether the gutter above is actually delivering water to that downspout or throwing most of it over the front lip halfway along the run.

Then go inside and look at the wall behind each of those points. Damp is directional. It has a source, and the source is usually outdoors and about ten feet away.

The order to fix things in

Cheapest and most effective first, which is a rare situation and worth taking advantage of.

1. Get the discharge away from the wall. Extensions today, a buried line to daylight when you can. How those are built.
2. Make sure the gutter is actually collecting and delivering, rather than overflowing at the point of maximum inconvenience. Sizing and pitch.
3. Fix grade that falls towards the house. Not roofing work, but it is often the biggest single lever.
4. Only then consider the expensive interventions, with a specialist, having removed the variables that would otherwise make their work look like a failure.

That order costs you the least and tells you the most, and it is the order we would follow on our own houses.