Splash Blocks on St. Louis Clay: Where Water Should Stop

A downspout that empties at the foot of the wall is only half finished. The gutter has done its job by carrying the water to the outlet, and then the outlet decides whether that water runs away from the house or sinks back against the foundation. On St. Louis clay, that choice matters more than most homeowners expect. A splash block is the simplest fix for the last few feet, and it is cheap, but it only works when it is set and sloped correctly.

Why clay changes the splash block job

Much of the ground in St. Louis County and the city is heavy clay. Clay takes on water slowly, swells when it is wet, and shrinks as it dries. Water that sits at the foot of a wall softens the soil right where the footing bears, and repeated wet and dry cycles can open cracks in the foundation or push the soil against the basement wall. A splash block set flat on the grass does little. Water runs off the block, lands on the same clay, and soaks back in beside the house.

Freeze and thaw adds a second problem. In a St. Louis winter, water that pools near the wall can freeze into a sheet of ice, and that ice keeps the soil saturated through the next thaw. Clay that stays wet longer heaves more. The aim is to move the water far enough out that it has a chance to drain away before it reaches the footing.

What a splash block should do

A splash block is a short trough, usually molded plastic or concrete, placed under a downspout outlet to catch the falling water and spread it. It is not a drain. Its job is to take the water off the wall and onto the ground in a controlled way, so the water does not carve a channel straight down beside the foundation. Splash blocks are sized for light flow, and they are not built to carry a full downspout of water from a large roof during a hard storm.

The block should sit so that its low end points away from the house. Set it level side to side, and give it a slight fall away from the wall, enough that water does not pool in the trough. If the outlet drops onto the block at a steep angle, the splash can bounce back onto the siding. Pull the block out far enough that the water lands on the block and runs off it, not against the wall.

How far out the water should go

There is no single number that fits every yard, but the common guidance is to carry the water several feet from the foundation, and a longer run is better on clay. A block that ends a foot from the wall leaves the water right where it will soak in. A longer extension, either a corrugated extension pipe or a buried line to daylight, moves the discharge to ground that can take it. Where the lot falls toward the house, a longer run is the only reliable answer, because a splash block alone cannot reverse the grade.

Check the slope of the ground under the block as well. If the soil at the discharge point is lower than the soil at the wall, water will flow back toward the foundation no matter how far the block reaches. Fill the low spot with compacted soil so the ground falls away from the house, and then set the block on the new grade.

Common mistakes on a St. Louis lot

• Setting the block tight against the foundation, so the splash lands on the wall and the soil right beside it.
• Using a block that is too short for the outlet, so the water shoots past it onto bare clay.
• Laying the block on a patch of mulch or stone that holds water and never drains.
• Leaving a crushed or bent elbow at the outlet, which sends water sideways into the same spot every storm.
• Assuming the block handles every downspout on the house, when the back downspouts may need their own extensions.

The elbow is worth a look before the block goes in. An elbow that has been crushed, or a joint that has pulled apart, will push water out at an odd angle, and no block can make up for that. If the outlet is clogged with the same debris that sits in the gutter, the block will catch whatever gets through, but the outlet itself will keep backing up.

What it costs

Splash blocks are one of the least expensive parts of gutter work. A plastic block costs a small amount at a hardware store, and a concrete one costs more but lasts longer in the sun. The bigger cost is the labor and the extension pipe needed to move the water farther than a block can reach. A buried drain line to daylight costs much more, because it requires digging, a proper slope, and a clean outlet. Ask the installer whether the quote includes the extension, the grading, and the outlet at the far end, because those lines are easy to leave out.

What to ask

Ask where each downspout will discharge and how far from the foundation that point sits. Ask whether the quote includes a block, an extension, or a buried line for each outlet, and whether the price covers any grading needed to make the ground fall away from the house. Ask what happens at the back of the house, where the grade is often lowest and where the water tends to collect. A written answer to those questions tells you more than a promise that the water will be handled.

If you are unsure what your lot needs, walk the yard during a hard rain. Watch where the water goes from each outlet and whether it runs back toward the wall. Note the spots where it pools, and bring that list to the estimate. The job is simpler to price when the problem is described in specifics.