Walk around the front of a house where the walkway has sunk at one corner, and you will almost always find a downspout right there. The gutter did its job and collected the roof water. The leader dropped it in one spot. Years of that washed the soil out from under the slab edge, and the concrete followed the soil down.
This is a two-trade problem. The water comes from the gutters and leaders. The damage shows up in the concrete. Fixing one without the other means doing the job twice. Here is how we look at it, from the gutter to the slab.
How a downspout undermines a slab
A typical roof section sheds a surprising amount of water into one downspout during a hard rain. All of it leaves the leader at a single point near the ground. If that point is next to a walkway, the water does three things.
It saturates the soil under the slab edge, which softens it. It carries away the fine particles in the soil and the stone base, a little each storm. And in winter, it keeps that soil wet so it freezes, expands, and pushes the slab up, then lets it drop when it thaws.
The result is a slab corner that sinks, a slab that tips toward the house, or a crack across the walkway where the supported part meets the unsupported part. On a walkway, that is a trip hazard. Next to the foundation, it is also a wet basement waiting to happen, which we cover in our post on basement slab cracks.
Diagnose it in the rain
The best time to look at this is during a steady rain. Stand back and watch the downspout. Where does the water land? Where does it go after it lands? Does it pool against the slab, run across it, or sink in right at the edge?
Then look at the slab itself. Is the low corner the one nearest the leader? Is there a gap between the slab and the soil at that edge? Is there a stain or a channel in the mulch or lawn where water runs?
If the leader lands within a couple of feet of the slab and the low spot is right there, you have your answer.
Fix the water first
There is no point lifting or replacing concrete if the same water hits the same spot the next week. Water first.
Leader extensions
The simplest fix is to extend the downspout so it discharges well away from the slab and the foundation. A rigid or flexible extension that runs across the lawn works, though it is in the way of mowing. A buried extension of solid pipe that pops up in the lawn or discharges to a lower part of the yard is cleaner.
The discharge point needs to be somewhere the water can soak in or run off harmlessly, and on your own property. A leader that dumps toward a neighbor just moves the problem.
Splash blocks
A splash block is a short concrete or plastic trough that catches the water at the leader and pushes it a couple of feet away. It is better than nothing and worse than an extension. On a walkway that is already undermined, a splash block alone is rarely enough, because two feet is often still on top of the washed-out area.
Regrading
Soil next to the house should slope away from the foundation and away from slab edges. Over the years it settles, mulch builds up and plantings change the grade. Pulling the grade back so water runs off instead of pooling at the slab edge is part of almost every one of these fixes.
Dry wells
A dry well is a buried chamber or pit filled with stone that takes the leader water and lets it soak into the ground slowly. It gets the water away from everything on the surface.
Towns in Suffolk have stormwater rules, and dry wells are sometimes required for new work. For an existing house, whether one is required or just a good idea varies by town. Check with your Town building department before digging. In Brookhaven, that is the Building Division. Even when it is optional, a dry well is the most complete answer for a leader that has nowhere else to go.
Then fix the concrete
Once the water is handled, look at what it did.
Lift
If the slab section is in one piece, not badly cracked and just sunk, it can sometimes be lifted by drilling small holes and pumping material under it to fill the void and raise it back level. This keeps the existing concrete and is quick. It only makes sense if the slab is worth keeping and the void is the whole problem.
Replace the section
If the slab is cracked into pieces, tipped hard, or the surface has scaled from years of sitting wet, we cut out that section and replace it. That means removing the concrete, digging out the washed and softened soil, placing and compacting new stone base, and pouring a new section tied into the walkway with proper joints.
This is also the moment to fix the slope of the slab so it sheds water away from the house. A lot of older walkways were poured flat or pitched toward the foundation, and once a section is out, correcting that is easy.
Replace the walkway
If the undermining is widespread, or the rest of the walkway is already at the end of its life, replacing the whole thing gives you one clean surface with the right pitch and joints. It costs more than a patch, but a patched walkway with three different pours shows every seam.
If you are deciding between resurfacing and rebuilding a front entry, our post on whether to rebuild or resurface a stoop covers the same question for the steps.
Watch the stoop too
The downspout that undermined the walkway is often right next to the front stoop. Stoops are heavier than walkways and sit on footings or a slab of their own, but the same water softens the soil around them. A stoop that has pulled away from the house or dropped at one side often has a leader to blame.
While the walkway is open, check the stoop, the siding above it and the foundation wall behind it. Fixing the water now protects all three.
What we check on a visit
We look at every downspout, not just the one near the problem. We check where each one discharges and whether it can be extended or tied to a dry well. We look at grade along the whole front of the house. Then we check every slab for voids, tipping and cracks, and tell you which ones can be lifted and which should be replaced.
The goal is one visit that handles water and concrete together, so you are not calling a gutter company and a mason separately and hoping they agree.
Ready to stop the washout?
JDG Concrete & Construction in Centereach handles gutters, leaders and the concrete they damage, under Suffolk County Home Improvement License #HI-68303. We will trace the water, fix where it goes, and lift or replace the slab so the walkway stays level. Call or text (631) 672-1618 or request a quote.
Frequently asked questions
Why is my front walkway sinking next to the downspout?
Every rain, the downspout concentrates all the water from that section of roof into one spot. That water carries away fine soil under the edge of the slab a little at a time. Eventually the slab has nothing under it at that corner and it drops or tips. The downspout is the cause and the slab is the symptom.
Is a splash block enough to protect a walkway?
Sometimes, if the ground slopes away and the block moves the water a couple of feet off the slab edge. Often it is not, because the water still lands in the same general area. A buried or surface leader extension that carries water several feet away from the slab and the foundation is more reliable.
Can a sunken walkway slab be lifted instead of replaced?
If the slab is in one piece and not badly cracked, it can sometimes be lifted by pumping material underneath it. If it is cracked into pieces, tipped at an angle or the surface is already scaling, replacing that section on a new compacted base is the better fix. Either way the water problem has to be solved first.
Do I need a dry well for my downspouts in Suffolk County?
Towns in Suffolk have rules about keeping stormwater on your own property, and some require dry wells for new construction or additions. Whether one is required for an existing home varies. Check with your Town building department. Even where it is not required, a dry well is a clean way to get roof water away from slabs and foundations.
