Splash Block: Downspout Discharge and Foundation Protection

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Gutters collect a surprising amount of water. Every square foot of roof that drains to a downspout sends its share of a storm to one spot at the base of the wall. If that water lands against the foundation, it can soak the backfill, erode soil, pond against the basement wall and find its way through cracks. A downspout splash block is the simplest tool for preventing that. It is a shallow trough, usually concrete or plastic, that sits under the downspout outlet and guides water away from the house. This guide explains what a splash block does, where and how it should be placed, how concrete and plastic versions compare, the signs that one is not doing its job, when a longer extension or regrading is the better answer, and what Front Range clay soils mean for roof runoff.

Part of our Structure & Exterior guide — start there for the full picture →

What a Downspout Splash Block Does

A downspout splash block has one job: catch the concentrated stream leaving the downspout elbow and spread it out and away from the foundation so it does not dig a hole or soak the soil next to the wall. Most splash blocks are roughly a couple of feet long, open at the downhill end, with a raised back and sides that keep the stream inside the channel. Some have a textured or ribbed floor that slows the flow.

Without a splash block, water falling from a downspout elbow strikes bare soil or mulch with enough force to erode a small pit. Over time that pit fills with water during every storm and holds it right against the foundation. A splash block protects the soil surface at the point of impact and gives water a head start in the right direction.

It is worth being clear about what a splash block cannot do. It does not move water very far. On a lot that slopes toward the house, or where soil next to the wall has settled into a trough, water leaving a splash block can simply turn around and run back toward the foundation. In those cases a splash block is only one piece of the drainage picture.

Placement and Slope: Getting the Basics Right

Most of the value of a splash block comes from how it is set, not from the block itself.

Position Under the Outlet

The downspout elbow should discharge into the closed, higher end of the block, with the stream landing inside the channel rather than on the lip or on the ground beside it. If the elbow is too short or turned at an angle, water can overshoot or miss the block entirely in a heavy storm.

Pointed Away From the House

The open end of the block should point directly away from the foundation, not along the wall and never back toward it. That sounds obvious, but blocks get kicked, mowed around and bumped by snow shovels until they end up crooked. A block that has rotated parallel to the wall sends water along the foundation instead of away from it.

Sloped Downhill

The block should sit so that its floor slopes away from the house. A block that has settled and now tips toward the wall acts like a funnel in the wrong direction. Setting the block on a compacted, slightly sloped bed of soil or gravel helps it hold its angle. The ground beyond the block also needs to slope away, so the water keeps moving once it leaves the channel.

The EPA’s guide to mold and moisture recommends making sure the ground slopes away from the building foundation so that water does not enter or collect around it. A splash block works with that slope. It does not replace it.

Where the Water Ends Up

Water leaving the block should reach an area that can absorb or carry it, such as a lawn, a planting bed or a drainage path, without flowing onto a neighbor’s property, across a sidewalk where it can freeze, or into a window well. The EPA’s Soak Up the Rain program describes redirecting downspouts to landscaped areas where rain can soak into the ground, while noting that this may not be appropriate in every location and that homeowners should consider local rules and where the water will go.

Concrete vs Plastic Splash Blocks

Splash blocks come in a few common materials. Each has tradeoffs.

Concrete

Precast concrete blocks are heavy, which is their main advantage. They resist being knocked out of position by mowers, pets and runoff. They also tolerate sun well. The downsides are weight during installation, a tendency to crack or spall over many freeze-thaw cycles, and a plain look that some homeowners dislike. A heavy block can also settle into soft soil over time, so it may need to be reset.

Plastic and Composite

Plastic and resin blocks are light, cheap and easy to reposition. Many come in colors or textures meant to blend into landscaping. Because they are light, they move more easily. They can float or shift in a heavy storm, get pushed sideways by snow, or warp in strong sun. Some have stakes or ribs underneath to help them stay put.

Decorative and Alternative Options

Some homeowners use stone, cast decorative blocks, or a short bed of river rock in place of a manufactured splash block. Those can work if they are sized and sloped correctly, but rock beds can trap debris and allow water to soak in right next to the house if they are not graded away. Flip-up or roll-out extensions are another option where a long fixed extension would be in the way of a mower or walkway.

The material matters less than whether the block stays aimed and sloped correctly. A cheap block that is checked every spring can outperform an expensive one that has been ignored for a decade.

When a Splash Block Is Not Enough

Splash blocks are a minimum standard, not a complete drainage solution. Several situations call for more.

• Large roof areas draining to one downspout. A single outlet serving a big roof section can deliver more water than a short block can spread. Longer discharge, a second downspout or a larger downspout may be needed.
• Flat or reverse-graded lots. If the yard is level with or slopes toward the house, water leaving a splash block has nowhere to go. A drainage swale, regrading or an underground drain line may be required.
• Short distance to a basement wall or window well. Water released only a couple of feet from the house can still saturate the backfill zone.
• Walkways and driveways nearby. Runoff that crosses a walk can freeze into a slip hazard during Front Range winters.

A downspout extension carries water farther than a splash block before releasing it. The downspout extension guide compares rigid, flexible, flip-up and buried options and how far discharge is commonly carried. For drainage problems that affect the whole property, the yard drainage guide covers grading, swales and dry wells.

Signs a Splash Block Is Failing

A splash block that has stopped working usually leaves evidence around it and inside the house.

Erosion and Washouts

A trench or pit at the end of the block, exposed roots, mulch washed into the lawn, or soil scoured from around the block all suggest the flow is too strong or the block is too short. Erosion can also undermine the block so it tips.

Settled or Rotated Blocks

A block whose open end is higher than its back end, or one that has turned to point along the wall, sends water the wrong way. Soil that has settled next to the foundation often creates a low channel along the wall, and a splash block sitting in that channel may be draining into a trough rather than away from the house.

Splashback Stains

Mud splashed onto siding, foundation coating or window wells near the downspout shows that water is hitting hard and bouncing back. It can also indicate a missing or displaced block.

Basement or Crawl Space Moisture Near the Downspout

Damp spots, efflorescence or water stains on the inside of a foundation wall near a downspout location are a strong clue. Water that soaks the backfill can find its way through cracks, tie holes and the joint between the wall and floor. The basement waterproofing guide explains how surface water control fits with interior and exterior waterproofing methods, and the foundation cracks guide describes how inspectors and engineers look at cracks that may be admitting water.

Ice Sheets in Winter

A patch of ice that forms below a downspout every time snow melts can signal that water is collecting rather than draining away. On walkways and driveways, that ice is a slip hazard as well as a drainage issue.

Colorado Soils and Roof Runoff

Parts of the Colorado Front Range have clay-rich soils that can swell when wet and shrink when dry. Expansive soils are a well-known concern for foundations and flatwork in many Front Range communities, though conditions vary considerably from lot to lot and even within a single yard. Where expansive clay is present, concentrated water from a downspout soaking the soil next to a foundation is generally considered undesirable. Geotechnical reports for newer subdivisions and builder drainage instructions often call for keeping roof water well away from the foundation and maintaining positive slope.

That is one reason Front Range homes frequently have long downspout extensions, buried drain lines or both, rather than a short splash block alone. Homeowners in neighborhoods with known soil movement should look for any builder or engineer recommendations that came with the house, and should be cautious about landscaping changes that raise soil against the foundation or redirect runoff toward it.

The semi-arid climate also means many yards are watered heavily in summer. Sprinkler heads that spray against the foundation or beds that are irrigated right next to the wall can add more water than the roof does, which can make downspout and splash block problems harder to diagnose.

Tricky Spots: Window Wells, Decks, Patios and Inside Corners

Some downspout locations make a simple splash block hard to use well.

Next to Window Wells

A downspout that drops beside a basement window well is a common source of water in the well and, eventually, at the window. Even a properly aimed splash block may release water close enough to soak the gravel at the bottom of the well. Rerouting the downspout or carrying its water past the well with an extension is often a better arrangement than relying on the block alone.

Under or Beside Decks

Downspouts that end under a deck can be difficult to see and maintain. Blocks there tend to get buried in debris or shifted out of position, and soil under decks often settles without anyone noticing. It is worth looking under the deck after a storm to confirm water is actually moving away from the foundation.

Onto Patios and Driveways

Discharging onto a patio or driveway that slopes away from the house can work, but the slab must actually pitch away. Slabs that have settled toward the house can carry water straight back to the foundation joint. In winter, water discharged onto concrete may refreeze into hazardous ice.

Inside Corners

Inside corners collect water from two roof planes and often from a valley above. A single short block at an inside corner may be overwhelmed during intense summer storms, so a longer extension or an additional downspout is often worth considering there.

What Home Inspectors Typically Note

Under the ASHI Standard of Practice, a home inspector inspects vegetation, grading, surface drainage and retaining walls that are likely to adversely affect the building, and inspects roof drainage systems. The same standard states that inspectors are not required to inspect geological and soil conditions or erosion control and earth stabilization measures. In practice, an inspector walking the exterior will usually look at where each downspout discharges and whether the ground near the foundation appears to slope away.

Typical report items include:

• Downspouts discharging at the foundation with no splash block or extension.
• Splash blocks missing, displaced, settled toward the house or pointed along the wall.
• Negative grading or soil settled into a trough next to the foundation.
• Erosion at discharge points.
• Evidence of past moisture inside the basement or crawl space near downspout locations.

These are usually inexpensive corrections, but they matter because they connect to more costly problems below grade. Buyers comparing inspectors can use the home inspector hiring guide to understand scope and reporting styles, and the structure and exterior topic hub gathers related guides on grading, roofs and foundations.

Downspout Splash Block Costs

A splash block is one of the least expensive drainage improvements a homeowner can make. Plastic blocks are typically sold for a modest price at home centers, and concrete blocks cost a bit more and weigh much more. Exact prices vary by region, retailer and style, so it is worth checking local stores rather than relying on a single figure.

Costs rise when the block is part of a bigger fix:

• Adding or lengthening extensions adds material and may require a different elbow.
• Regrading soil next to the foundation can range from a modest landscaping job to a larger project if soil must be brought in or patios and walks are in the way.
• Buried drain lines to daylight or a dry well involve trenching, pipe and a proper outlet, and the cost depends heavily on distance, soil and obstacles.
• Interior moisture repairs or foundation crack sealing cost far more than any exterior drainage fix and are often a sign the exterior work is overdue.

Simple Maintenance Steps

A short seasonal routine keeps splash blocks working:

1. Walk the house during or right after a steady rain and watch where each downspout’s water goes.
2. Reset any block that has rotated, sunk or tipped toward the house.
3. Fill erosion pits and rebuild soil so it slopes away from the wall.
4. Clear leaves and debris from the block and the area beyond it.
5. In fall, make sure extensions and blocks will not direct meltwater onto walks where it can freeze.
6. Check the basement or crawl space near each downspout after big storms.

Small adjustments made each spring and fall usually cost little and can prevent much larger moisture problems below grade.

References

• A Brief Guide to Mold, Moisture and Your Home — U.S. Environmental Protection Agency
• Soak Up the Rain: Disconnect / Redirect Downspouts — U.S. Environmental Protection Agency
• ASHI Standard of Practice for Home Inspections — American Society of Home Inspectors

Wondering whether the drainage around a home you are buying will keep the basement dry? Reach out to us and we can point you toward a Front Range inspector who pays close attention to grading and downspout discharge.