Direct Answer
Voids under concrete slabs form when the soil beneath the concrete washes out, settles, erodes, shrinks, decays, or was not compacted properly before the slab was poured. In Florida, the most common cause is water movement. Heavy rainfall, poor drainage, plumbing leaks, irrigation leaks, pool leaks, deck drain leaks, downspouts, and erosion can all remove soil or base material from beneath concrete.
Poor soil compaction during initial construction is another common cause. If the soil below the slab was loose or not compacted properly, it may settle later and leave unsupported space beneath the concrete. In some areas of Florida, sinkhole-related conditions can also contribute, but most voids beneath slabs are caused by water, poor compaction, erosion, or shallow soil movement.
A void under a concrete slab means part of the slab may no longer have full support. Over time, the concrete can sound hollow, crack, sink, rock, slope, or create a trip hazard. The right repair depends on what caused the void, whether water is still active, whether the slab is still in repairable condition, and whether the issue is isolated concrete settlement or part of a larger soil or foundation problem.
A void under concrete is missing support below the surface. The crack, hollow sound, sinking slab, or trip hazard is the visible symptom. The real question is why the soil or base material disappeared.
Homeowner Tip: A hollow sound, visible gap, or sinking slab is the symptom. The real repair question is whether the soil loss has stopped or whether water, drainage, leaks, or poor support are still active.
If the slab is still in repairable condition, concrete lifting or void filling may be an option.
Read more: Concrete Lifting
What Is a Void Under a Concrete Slab?
A void is an empty space beneath concrete where soil or base material used to provide support. The concrete may still be in place, but part of it may be spanning over missing support.
Concrete slabs are designed to rest on the soil or base below them. When that support is lost, the slab may still look intact from above for a while. But without support underneath, the concrete is more likely to crack, settle, rock, slope, or break.
Voids can form beneath many types of concrete, including:
- Pool decks
- Driveways
- Sidewalks and walkways
- Patios and lanais
- Garage slabs
- Interior floor slabs
- Slabs near foundations
- Concrete near seawalls or waterfront structures
- Concrete around stormwater structures or drains
Some voids are small and isolated. Others can grow if water keeps moving beneath the slab. The sooner the cause is identified, the easier it is to determine whether the slab can be supported, lifted, stabilized, or needs replacement.
Read more: Concrete Lifting vs Replacement: Which Is Better?
Common Causes of Voids Under Concrete in Florida
Voids beneath concrete usually form because the material below the slab has moved, washed out, settled, or was not installed properly.
| Common Cause | How It Creates a Void |
| Soil washout from drainage or water movement | Water carries soil or base material away from beneath the slab |
| Poor soil compaction before slab placement | Loose soil compresses after construction and leaves unsupported space |
| Loose sandy soils beneath the slab | Sandy soils can shift, erode, or wash out more easily |
| Plumbing leaks, irrigation leaks, or pool leaks | Active water can soften or remove soil beneath concrete |
| Erosion along slab edges | Edge support is lost and voids can spread beneath the slab |
| Poor grading or water draining beneath concrete | Water repeatedly moves through the same weak area |
| Organic material decay beneath the slab | Support changes as buried organic material decomposes |
| Settlement of fill soils | Fill soils compress and leave space beneath the concrete |
| Poor concrete thickness or construction | Thin or weak slabs may crack once support is lost |
| Sinkhole-related conditions in certain Florida regions | Deeper ground movement may contribute in some areas |
Water is the leading cause in many Florida void problems. When water moves beneath or along the edge of a slab, it can carry soil or base material away. Over time, that movement can leave open space beneath the concrete.
Poor compaction is another major cause. If the soil was not compacted properly before the slab was poured, it may compress after construction. As the soil settles downward, the slab can be left unsupported in spots.
Sinkhole-related conditions are less common than water washout and poor compaction, but they can contribute in certain parts of Florida. Not every void beneath concrete is a sinkhole. Many are shallow soil or water-related issues.
Read more: How Florida Soil Affects Foundations
Minor Hollow Sound vs Void or Settlement Concern
| What You Notice | Possible Minor Issue | Possible Void or Soil Concern |
| Concrete sounds hollow | Surface separation or small unsupported area | May indicate a void beneath the slab |
| Slab rocks or moves when stepped on | Loose slab section | Strong sign support may be missing |
| Concrete has settled lower than nearby slabs | Soil settlement or washout | May need lifting and void filling |
| Cracks plus hollow areas | Unsupported concrete | More concerning than cracking alone |
| Gap visible under slab edge | Soil washout or erosion | Should be evaluated |
| Water pooling near slab | Low spot or drainage issue | May continue washing out soil |
| Soil washing out along slab edge | Active erosion | May create or enlarge voids |
| Trip hazard between slabs | Settlement or lifting | May need slab lifting or replacement |
| Void near pool deck, drain, or filter area | Possible leak or washout | Leak source should be checked |
| Void near foundation | Soil support issue near structure | Should be evaluated for related movement |
This table is a guide, not a diagnosis. A hollow sound may be minor in some cases, but if the slab is sinking, cracking, rocking, or losing soil around the edges, the support below the slab should be evaluated.
If the void appears beneath a driveway, see our driveway settlement guide.
Read more: Why Is My Driveway Sinking in Florida?
If the void appears beneath a sidewalk or walkway, see our sidewalk trip hazard guide.
Read more: Uneven Sidewalks and Trip Hazards: What Homeowners Need to Know
If the void appears beneath a pool deck, see our pool deck settlement guide.
Read more: Pool Deck Sinking Near Coping: Causes and Repair Options
Checklist: Signs You May Have Voids Under Concrete
You may have voids beneath a concrete slab if you notice:
- The concrete sounds hollow when tapped or walked on
- One slab section rocks or moves
- A gap is visible beneath the slab edge
- Soil is missing along the edge of the concrete
- Water pools near the slab
- Water drains beneath the slab
- Soil is washing out after rain
- Cracks are widening or spreading
- One slab is sinking lower than the next
- A trip hazard has formed
- Pavers or concrete near the slab are also settling
- The same area keeps settling after repair
- The void is near a pool deck, drain, driveway, sidewalk, patio, garage slab, or foundation
- There are signs of plumbing, irrigation, pool, or drainage leaks nearby
If the issue is isolated and the slab is still stable, repair may be straightforward. If the void is growing, water is still moving, or the slab is settling, the cause should be corrected before cosmetic repairs are made.
Read more: What a Foundation Inspection Looks Like
Why Voids Are Common Beneath Florida Concrete
Florida concrete slabs are vulnerable to voids because many are built over sandy soils, fill soils, or shallow soils that can move, wash out, or settle over time.
| Florida Condition | Why It Matters for Concrete Voids |
| Heavy rainfall | Can move soil from beneath or along slab edges |
| Loose sandy soils | Can wash out or shift more easily than dense soils |
| Poorly compacted fill | Can settle after the slab is placed |
| Poor drainage | Can send water repeatedly beneath concrete |
| Plumbing leaks | Can wash out or soften soil beneath slabs |
| Irrigation leaks | Can create localized soil softening or erosion |
| Pool leaks | Can wash out soil beneath pool decks or nearby slabs |
| Pool equipment or pool filter area leaks | Can create soft, eroded, or unsupported areas nearby |
| Leaking or poorly sealed deck drains | Can discharge water into soil instead of carrying it away |
| Downspouts discharging near slabs | Can concentrate water along slab edges |
| Erosion along slab edges | Can allow support loss to spread beneath the slab |
| Soil movement near seawalls or waterfront structures | Can remove support near waterfront slabs |
| Settlement of fill soils | Can leave unsupported space beneath concrete |
| Organic material decay beneath slabs | Can change support conditions over time |
| Sinkhole-related conditions in certain Florida regions | Can contribute to settlement in some areas |
Many exterior slabs are thinner and lighter than structural foundation elements. Pool decks, driveways, sidewalks, patios, and lanais rely heavily on the soil below them. If that support disappears, the slab may begin to move.
Water is often the main driver. It does not always take a dramatic washout event to create a void. Repeated drainage, small leaks, irrigation overspray, poor grading, or water entering through open joints can slowly remove material from beneath the concrete.
Read more: What Causes Foundation Settlement in Florida?
How Water Creates Voids Beneath Slabs
Water creates voids by softening, moving, or washing away the soil beneath the concrete.
This can happen quickly during heavy rain or slowly over months and years. Water may enter beneath the slab from the side, through cracks, through open joints, or from leaking pipes, drains, or pool systems.
| Water Source | How It Can Create a Void |
| Rainwater flowing along slab edges | Can erode soil and remove edge support |
| Poor grading that sends water beneath the slab | Can keep water moving through the same soil area |
| Downspouts discharging near concrete | Can concentrate water near slabs |
| Irrigation leaks | Can soften or wash out soil below concrete |
| Plumbing leaks | Can remove support beneath interior or exterior slabs |
| Pool plumbing leaks | Can wash out soils beneath pool decks or nearby patios |
| Pool shell leaks | Can contribute to soil movement around pool areas |
| Pool filter or equipment area leaks | Can soften or erode nearby soils |
| Cracked or leaking deck drains | Can release water into soil below the slab |
| Water draining from patios, lanais, or pool decks | Can move beneath connected slabs |
| Seawall soil loss near waterfront slabs | Can remove support toward the water |
| Stormwater structures or drains that leak | Can erode surrounding soils and create voids |
Once the soil begins washing out, the concrete may be left spanning over an empty space. At first, the slab may only sound hollow. Later, it may crack, drop, rock, or form a trip hazard.
Filling a void without correcting the water source may only be a temporary repair. If water continues moving beneath the slab, it can wash out soil around the new polyurethane support or create new voids nearby.
Read more: Drainage Problems and Foundation Settlement in Florida
Where Voids Commonly Form
Voids can form beneath almost any concrete slab, but Helicon commonly sees them in certain areas.
| Common Location | Why Voids May Form There |
| Pool decks | Failed mastic joints, pool leaks, deck drains, pool equipment leaks, and constant water exposure |
| Driveways | Poor compaction, water moving along slab edges, erosion, and repeated vehicle loads |
| Sidewalks and walkways | Water along edges, thin slabs, irrigation issues, and poor compaction |
| Patios and lanais | Roof runoff, downspouts, poor drainage, pool deck drains, and weak base soils |
| Near seawalls or waterfront structures | Soil loss toward the water, seawall joint leakage, failed filters, storm surge scour, or erosion |
| Garage slabs | Poor compaction, water intrusion, plumbing leaks, erosion near slab edges, or fill settlement |
| Interior floor slabs | Poor compaction, plumbing leaks, settlement, or washout beneath the home |
| Around foundations | Soil support loss near structural areas that may need closer evaluation |
| Around stormwater structures or drains | Leaking pipes, catch basins, trench drains, or eroded surrounding soils |
Pool Decks
Pool decks are one of the most common places to find voids because they are constantly exposed to water. Voids may form near coping, failed mastic joints, pool deck drains, pool equipment areas, plumbing lines, or outer deck edges.
Before pool deck lifting or soil stabilization is performed, pool leaks, pool plumbing leaks, filter area leaks, deck drain leaks, and irrigation leaks should be checked and repaired if present.
Read more: Pool Deck Sinking Near Coping: Causes and Repair Options
Driveways
Driveway voids often form from poor compaction, water moving along slab edges, driveway drainage, erosion, or repeated vehicle loads on weak soils. Homeowners may notice hollow-sounding concrete, sinking sections, trip hazards, or gaps beneath slab edges.
Read more: Why Is My Driveway Sinking in Florida?
Sidewalks and Walkways
Sidewalk voids often appear where water drains along the edge, where soil was poorly compacted, or where irrigation or drainage issues wash out the base. A small void can create a noticeable trip hazard because sidewalks are relatively thin.
Read more: Uneven Sidewalks and Trip Hazards: What Homeowners Need to Know
Patios and Lanais
Patios and lanais may develop voids due to poor drainage, downspouts, roof runoff, pool deck drains, soil washout, or poor compaction. If the slab is near the home, water movement should be evaluated carefully so it does not also affect adjacent foundation soils.
Read more: Concrete Patio Sinking: Causes and Repair Options
Near Seawalls or Waterfront Structures
Concrete near seawalls, docks, patios, pool decks, or waterfront slopes may develop voids if soil is being lost toward the water. This can occur when seawall joints leak, filters fail, storm surge scours soil, or waterfront erosion removes support beneath slabs.
Read more: Seawall Soil Erosion Repair
Garage Slabs
Garage slab voids may form because of poor compaction, water intrusion, plumbing leaks, erosion near the slab edge, or settlement of fill soils. If garage slab settlement appears with foundation cracks, sticking doors, or gaps, the surrounding foundation should also be evaluated.
Read more: Why Is My Garage Floor Sinking?
Interior Floor Slabs
Interior slab voids may be related to poor compaction, plumbing leaks, soil settlement, or washout beneath the home. These issues may show up as hollow-sounding floors, cracks, uneven floors, or gaps under interior walls or cabinets.
Read more: Interior Slab Settlement: Causes and Repair Options
Around Foundations
Voids near a foundation are more concerning when they appear with stair step cracks, sticking doors or windows, uneven floors, gaps, or exterior settlement. A void near the home does not automatically mean foundation failure, but the surrounding symptoms should be evaluated.
Read more: Foundation Problems in Florida: The Complete Homeowner Guide
Around Stormwater Structures or Drains
Voids may form around stormwater structures, catch basins, trench drains, yard drains, or drainage pipes when water leaks from the system or erodes the surrounding soil. In those cases, the drainage structure or pipe may need repair before void filling is performed.
Read more: Stormwater Structure Soil Stabilization
Hollow-Sounding Concrete vs Sinking Concrete
Hollow-sounding concrete and sinking concrete are related, but they are not always at the same stage of movement.
| What You Notice | What It May Mean | Why It Matters |
| Concrete sounds hollow but is still level | Separation, small unsupported area, or early void beneath the slab | May be an early warning sign |
| Concrete sounds hollow and has cracks | Unsupported concrete may be beginning to move | More concerning than a hollow sound alone |
| Concrete rocks or moves | Support is likely missing beneath part of the slab | Should be evaluated |
| Concrete has settled lower than nearby slabs | Soil or base material has compressed, washed out, or no longer supports the slab | May need lifting, void filling, or replacement |
| Concrete has a visible gap beneath the edge | Soil loss or erosion is present | Cause should be identified |
| Concrete is sinking with water nearby | Active water may still be removing soil | Water source should be corrected |
Concrete may sound hollow when there is separation, a small unsupported area, or a void beneath the slab. The slab may still be level, but support below the surface may be missing.
Sinking concrete means the slab has already moved downward. This usually indicates that the soil or base material beneath the slab has compressed, washed out, or no longer supports the concrete.
A hollow sound by itself does not always mean an emergency. But if the hollow area is growing or paired with movement, the slab should be evaluated.
Read more: Concrete Lifting
Why You Should Find the Cause Before Filling the Void
Filling a void is only part of the solution. The cause of the void matters.
If the void formed because of a one-time construction issue or poor compaction that is no longer active, void filling or slab support may be enough. But if water is still moving beneath the slab, the void may return or appear in another area.
Before filling a void, the repair plan should consider:
- Is water still draining beneath the slab?
- Is there a plumbing, irrigation, pool, or drain leak?
- Is soil still washing out along the edges?
- Is the slab still moving?
- Is the concrete strong enough to lift or support?
- Is the void near a foundation, pool shell, seawall, drain, or structural area?
- Is the problem isolated or part of a larger soil movement pattern?
This is especially important around pool decks, patios, lanais, seawalls, garage slabs, and foundations. If the visible slab issue is only a symptom of a larger water or soil support problem, the repair should address more than the empty space.
Read more: Can I Wait to Fix Foundation Issues or Will It Get Worse?
Repair Options for Voids Under Concrete Slabs
The right repair depends on the cause of the void, the condition of the slab, and whether water or soil movement is still active.
| If the Cause Is… | The Repair May Include… |
| Water causing soil washout | Drainage correction before or along with void filling |
| Plumbing, irrigation, pool, or drain leaks | Leak detection and repair before slab lifting or soil stabilization |
| Voids beneath repairable concrete | Structural polyurethane slab injections or void filling |
| Settled but repairable concrete slab | Expanding polyurethane foam concrete lifting |
| Shallow weak or loose soils below the slab | Polyurethane soil stabilization |
| Concrete too broken, thin, deteriorated, root-damaged, or unsuitable for lifting | Concrete replacement may be better |
| Voids near the home with foundation warning signs | Foundation evaluation may also be recommended |
If water is causing soil washout, drainage correction may be needed before or along with void filling.
If plumbing, irrigation, pool, or drain leaks are suspected, leak detection and repair should be completed before slab lifting or soil stabilization.
If voids are present beneath concrete and the slab is still in repairable condition, structural polyurethane slab injections may be used to fill the voids and help restore support below the concrete.
If the slab has settled, expanding polyurethane foam concrete lifting may be used to raise and support the slab while filling voids beneath it.
If shallow soils below the slab are weak or loose, polyurethane soil stabilization may be recommended to improve soil support and reduce continued settlement.
If the concrete is too broken, thin, deteriorated, root-damaged, or not suitable for lifting, concrete replacement may be the better option.
If voids are near the home and appear with foundation warning signs, a foundation evaluation may also be recommended.
The best repair is not always the most aggressive repair. The best repair is the one that fills the void, addresses the water or soil problem that caused it, and lifts or stabilizes the slab only when appropriate.
The right repair path depends on whether the issue is water-related, soil-related, slab-related, leak-related, or connected to nearby structural movement.
For concrete void filling and slab support, see our concrete lifting service page.
Read more: Concrete Lifting
If weak shallow soils are part of the problem, see our polyurethane soil stabilization service page.
Read more: Polyurethane Soil Stabilization
For voids near the foundation or structural warning signs, see our foundation repair service page.
Read more: Foundation Repair
Polyurethane Void Filling vs Mudjacking
Some homeowners search for mudjacking, slab jacking, concrete raising, or concrete void filling when they notice hollow or sinking concrete. Mudjacking typically uses a cement-based slurry that is pumped beneath the slab to raise it.
Helicon uses polyurethane foam because it is often a better fit for Florida concrete voids and soil conditions.
| Polyurethane Void Filling and Slab Lifting | Mudjacking |
| Lightweight | Heavier cement-based slurry |
| Fast-curing | Usually slower to cure |
| Less invasive | More invasive in many cases |
| Installed through small injection holes | Usually requires larger holes |
| Useful for filling voids beneath slabs | Primarily lifts with heavier fill material |
| Effective for lifting many settled concrete slabs | Can lift some settled slabs but adds more weight |
| Less likely to add heavy weight to already weak soils | Adds significant weight beneath the concrete |
| Suitable for many driveways, sidewalks, pool decks, patios, lanais, and slab support areas | May not be ideal when soil support is already poor |
Mudjacking can add significant weight beneath the concrete. If the soil below the slab is already weak, loose, wet, or poorly compacted, adding heavy material may not be ideal.
Polyurethane foam expands beneath the slab, fills voids, and helps support or lift the concrete without adding the same heavy load to the soil below.
Read more: Polyurethane Concrete Lifting vs Mudjacking
What Homeowners Should Not Do
The biggest mistake homeowners make is filling or patching the visible gap without fixing the water source.
A visible gap may be only part of the problem. If water is still moving beneath the slab, soil may continue washing out after the gap is patched or filled.
Homeowners should avoid:
- Filling or patching the visible gap without fixing the water source
- Resurfacing, coating, or replacing concrete before stabilizing the void
- Ignoring hollow-sounding concrete until it cracks or sinks
- Assuming voids are harmless because the slab has not collapsed
- Assuming every void means a sinkhole
- Using heavy fill material that adds weight to weak soils
- Ignoring plumbing, irrigation, pool, drain, or drainage leaks nearby
The right first step is to understand why the void formed. Once the cause is known, the repair can be matched to the actual problem.
Read more: Can I Wait to Fix Foundation Issues or Will It Get Worse?
How Helicon Evaluates Voids Under Concrete
When Helicon evaluates voids under concrete, the inspection looks beyond the visible gap, crack, or hollow sound. The goal is to determine whether the void was caused by water washout, poor compaction, loose soils, erosion, leaks, drainage problems, settlement, or nearby structural movement.
A proper evaluation may include:
- Looking at where the void or hollow area is located
- Checking whether the slab is sinking, rocking, or uneven
- Looking for gaps beneath slab edges
- Listening for hollow-sounding concrete
- Reviewing cracks, trip hazards, and settlement patterns
- Inspecting soil washout along slab edges
- Reviewing drainage and water flow around the slab
- Looking for water pooling near the slab
- Checking whether water drains beneath the slab
- Looking for nearby plumbing, irrigation, pool, or drain leak concerns
- Checking nearby slabs, pavers, patios, pool decks, driveways, or sidewalks for settlement
- Looking at whether the slab is near the home, garage, pool, seawall, stormwater structure, or foundation
- Evaluating whether the concrete is still suitable for lifting or support
- Using a soil probe near structural areas where appropriate
The inspection should answer three main questions:
- Why did the void form?
- Is the cause still active?
- Is the concrete still suitable for void filling, lifting, or stabilization?
Read more: What a Foundation Inspection Looks Like
How Much Does Void Filling or Slab Support Cost?
The cost depends on the void size, slab area, access, soil conditions, drainage issues, leak conditions, slab condition, and whether lifting, void filling, soil stabilization, or replacement is needed.
Drainage correction, leak repair, or concrete replacement may need to happen before void filling if the cause is still active or the slab is no longer suitable for support.
| Repair Type | Typical Cost Range | When It Applies |
| Void inspection | Free when part of a Helicon evaluation | Hollow, sinking, rocking, or unsupported concrete |
| Structural polyurethane void filling beneath concrete | Around $1,500 to $5,000 | Voids beneath slabs that need support restored |
| Concrete lifting or slab support | Around $1,500 to $8,000 | Settled but repairable concrete slabs |
| Polyurethane soil stabilization | Around $2,000 to $6,000 for small exterior slab areas, larger if deeper or broader treatment is needed | Weak shallow soils beneath or near concrete |
| Leak detection, plumbing repair, irrigation repair, or pool repair | Contractor-specific | Water source may still be active |
| Drainage correction | Contractor-specific | Water is contributing to washout or erosion |
| Concrete replacement | Contractor-specific | Slab is too broken, thin, deteriorated, root-damaged, or unsuitable for lifting |
| Foundation repairs if related | Project-specific | Foundation warning signs are present near the void area |
*These are typical Florida pricing examples based on published industry resources and contractor pricing guides. They are intended for educational purposes only and are not Helicon’s pricing. Every home is different, and a professional inspection is the only way to determine the appropriate repair and provide an accurate quote.
That does not mean void filling is always the right answer by itself. If the slab is too broken, thin, deteriorated, root-damaged, or unsupported, replacement may be needed. If water is still washing out the soils, the water source should be corrected. If foundation warning signs are present, a foundation evaluation may be needed.
The cost should be based on the cause, not just the empty space under the slab.
Read more: How Much Does Concrete Lifting Cost in Florida?
When to Schedule an Inspection
You should schedule an inspection if your concrete sounds hollow, is sinking, cracking, rocking, forming a trip hazard, or showing signs of soil washout.
Call an expert if you notice:
- Hollow-sounding concrete
- A visible gap beneath the slab
- Soil missing along the slab edge
- One slab section sinking lower than another
- A slab that rocks or moves under foot traffic
- Cracks widening or spreading
- Water pooling near the slab
- Water draining beneath the slab
- Soil washing out after rain
- Trip hazards between slabs
- Pavers or nearby concrete settling
- Voids near a pool deck, driveway, sidewalk, patio, lanai, garage slab, foundation, seawall, or drain
- Signs of plumbing, irrigation, pool, or drainage leaks nearby
- Foundation warning signs near the void area
Not sure if your concrete needs void filling, lifting, stabilization, or replacement? Helicon can evaluate the slab, voids, soil support, drainage, leak concerns, and nearby settlement conditions when needed.
Why Florida Homeowners Trust Helicon
Helicon has been helping Florida homeowners solve concrete lifting, soil stabilization, foundation repair, seawall erosion repair, sinkhole repair, pool deck settlement, sidewalk settlement, driveway settlement, void filling, and slab support problems since 2004.
As a family-owned Florida company, Helicon understands the soil conditions, water movement, drainage issues, construction practices, and settlement patterns that affect slabs, driveways, sidewalks, pool decks, patios, lanais, garage floors, seawalls, foundations, and waterfront properties across Tampa Bay, Orlando, Southwest Florida, and surrounding areas.
We are not there to tell every homeowner that every void means a sinkhole or serious foundation problem. We are there to determine why the void formed, whether the cause is still active, and whether the issue is isolated or connected to a larger water, soil, or structural support problem.
Sometimes the issue is poor compaction. Sometimes it is water washout. Sometimes it is a leak. Sometimes it is drainage. Sometimes the slab needs lifting. Sometimes the void needs filling. Sometimes the surrounding soils need stabilization. Sometimes the concrete needs replacement.
The goal is always the same: identify the cause, protect the property, and recommend the least invasive repair that properly addresses the movement.
Read more: About Helicon
Schedule a Free Concrete Void Inspection
If your concrete sounds hollow, is sinking, cracking, rocking, or showing signs of soil washout, Helicon can help.
Our team can evaluate the slab, visible voids, hollow areas, drainage, soil washout, nearby concrete settlement, leak concerns, and structural warning signs when needed to help determine whether the problem can be filled, lifted, stabilized, or needs another repair approach.
Schedule a free concrete void inspection with Helicon to find out what is causing the issue and what should be done next.
Read more: Concrete Lifting
FAQ
What causes voids under concrete slabs?
Voids under concrete slabs are usually caused by soil washout, poor compaction, loose sandy soils, leaks, erosion, poor drainage, settlement of fill soils, organic material decay, or water moving beneath the slab. In some regions, sinkhole-related conditions may also contribute.
Are voids under concrete dangerous?
They can be. A void means part of the slab may no longer have full support. Over time, the concrete can crack, sink, rock, slope, or create a trip hazard. Voids near foundations, pool decks, seawalls, or garage slabs should be evaluated carefully.
How do I know if there is a void under my concrete?
Common signs include hollow-sounding concrete, visible gaps beneath slab edges, soil missing along the concrete, sinking slabs, rocking concrete, widening cracks, water pooling nearby, soil washing out after rain, and trip hazards between slabs.
Can voids under concrete be filled?
Yes, many voids can be filled with structural polyurethane slab injections if the concrete is still in repairable condition. The material can fill empty spaces below the slab and help restore support.
Can polyurethane foam lift a slab with voids underneath?
Yes, if the slab has settled and is still in good enough condition, expanding polyurethane foam concrete lifting may be used to raise the slab, fill voids, and help restore support below the concrete.
Should I fix drainage before filling a void?
Yes, if drainage or water movement caused the void. Filling a void without correcting the water source may only be temporary because water can continue washing soil from beneath the slab.
Are voids under concrete a sign of a sinkhole?
Not always. Most voids under concrete are caused by shallow soil washout, poor compaction, drainage problems, erosion, or leaks. Sinkhole-related conditions can contribute in certain areas of Florida, but every void should not be assumed to be a sinkhole.
Is polyurethane better than mudjacking for void filling?
Polyurethane foam is often preferred because it is lightweight, fast-curing, less invasive, and expands to fill voids without adding heavy slurry to already weak or wet soils. Mudjacking uses a heavier cement-based material.
How much does concrete void filling cost?
Structural polyurethane void filling beneath concrete typically costs $1,500 to $5,000 or more depending on void size, slab area, access, and injection needs. Concrete lifting or slab support may range from $1,500 to $8,000 or more depending on the slab type and repair area.
When should I call Helicon about voids under concrete?
Call Helicon if your concrete sounds hollow, is sinking, rocking, cracking, forming a trip hazard, showing visible gaps, washing out after rain, or appears near a pool deck, driveway, sidewalk, patio, lanai, garage slab, foundation, seawall, or drain.
