How to Repair Cracks in a Concrete Pool Without Draining It

Inspect the crack closely before deciding whether to drain or patch

Before you grab tools or buy supplies, take a slow look at what kind of crack you have. Inground concrete pools have two distinct layers. The thick outer shell is made of gunite (a sprayed concrete pool shell that forms the structural bowl in the ground). Over that rough concrete sits the plaster (a smooth waterproof top layer spread across the shell to keep water in and create a smooth finish). Knowing which layer has cracked tells you whether you have a simple weekend patch job or a serious structural problem.

Most thin cracks are cosmetic blemishes in the plaster called crazing (fine web-like surface hairline cracks that appear as plaster cures or weathers). Crazing looks like fine spiderwebs or shattered porcelain. These tiny surface fissures do not go through to the concrete shell, and they rarely leak water. You can safely leave cosmetic crazing alone or buff it smooth during a future remodel.

A true structural crack runs completely through the plaster and penetrates deep into the concrete shell. These cracks are usually straight, wide, and continuous across large areas of the floor or wall. You may also see rust stains bleeding through the crack line, which means water has reached the steel reinforcement bars inside the concrete.

Confirm whether the crack is actively leaking water before you start any repair. Turn off your pool circulation pump so the water settles completely. Put on a dive mask, swim down to the crack with a small bottle of dark food coloring, and squirt a tiny cloud of dye right next to the crack seam. If the colored dye drifts aimlessly in the water, the crack is dormant and sealed. If the crack pulls the ribbon of dye inside like an open drain, you have an active leak that requires sealing.

Never drain your concrete pool to fix a crack unless a licensed pool builder tells you it is safe. Emptying an inground concrete pool removes thousands of pounds of water weight. If the ground around your yard is damp, underground water pressure can lift the entire concrete bowl out of the ground like a boat, cracking plumbing lines and destroying your pool deck.

Seal underwater plaster cracks with two-part epoxy putty

You do not need to lower your pool water level to seal minor cracks in the plaster. A moldable two-part waterproof repair stick (epoxy putty) cures fully submerged in pool water. It stays in place without sagging, hardens like rock, and seals small leaks directly under water. Follow these steps to apply it correctly:

1. Shut down your pool equipment. Switch off your main circulation pump and remove any cleaning equipment from the water. Keep any power supply unit at least 10 feet back from the pool edge, and unplug it from the outdoor GFCI outlet (a protected ground-fault outlet with a weatherproof cover). Never work in the pool while pool cleaners or power cords remain plugged in.
2. Clean the crack surface underwater. Put on a dive mask or goggles. Take a flat-blade putty knife or a stiff wire brush and scrape away loose plaster flakes, algae, and calcium deposits around the crack edges. The epoxy putty needs a clean, rough surface to bond properly.
3. Cut and mix the epoxy putty. Work on the pool deck with dry hands. Cut off equal portions of the resin and hardener from the two-part stick. Knead the putty firmly between your fingers for two to three minutes. Continue mixing until the two parts blend into one uniform color with no streaks remaining.
4. Shape the mixed putty into a ribbon. Roll the blended putty between your palms to form a long rope or pencil-thick snake. Match the length of the putty ribbon to the section of the crack you plan to patch on that dive.
5. Press the putty deep into the crack. Dive down to the repair spot. Push the putty ribbon firmly into the crack with your thumbs, forcing the material deep into the opening rather than just laying it across the surface. Feather the edges flat against the surrounding plaster with your fingers or a plastic putty knife so the patch stays flush.
6. Allow the putty to cure undisturbed. The underwater putty will begin firming up within minutes and cures completely over the next 24 hours. Leave your pool circulation pump turned off for at least four hours so strong water currents do not disturb the uncured compound.

Use fast-setting hydraulic cement for dry cracks above the waterline

When you have a crack located above the waterline (the tile border where water meets the pool wall), hydraulic cement is your best option. Hydraulic cement is a fast-setting waterproof powder patch that expands slightly as it cures, creating a tight mechanical seal inside concrete seams. Use these steps to patch dry cracks along the upper pool walls or skimmer throat:

1. Chisel the crack into a backward groove. Use a small cold chisel and hammer to widen the crack opening to at least a quarter-inch wide and a quarter-inch deep. Cut the edges straight down or undercut them into a backward-angled dovetail shape. Never leave the crack as a shallow V-shape, because expanding cement will simply slide out of an angled groove.
2. Clear out all dust and dampen the concrete. Use a stiff nylon brush to sweep away loose concrete crumbs, sand, and dust. Splash clean water onto the opened groove to dampen the concrete. Wetting the concrete prevents dry masonry from pulling water out of your cement mix too quickly.
3. Mix a small amount of hydraulic cement. Pour a small portion of hydraulic cement powder into a clean plastic container and stir in clean water according to the package directions. Mix only as much compound as you can apply in three minutes, because hydraulic cement sets in three to five minutes.
4. Pack the cement into the groove. Scoop the mixed cement with a margin trowel or gloved hands. Push the cement into the crack opening with firm pressure, packing it tightly from the back of the groove out to the surface.
5. Hold continuous pressure and shave the patch flush. Hold your trowel or gloved palm against the fresh patch for two to three minutes until the cement turns hard. Once the patch is firm, use the edge of your trowel to shave away any extra material until the patch sits level with your wall.

Clean and widen the crack groove so your patch stays locked in place

Most DIY pool patches fail within a few months because of surface prep mistakes rather than bad materials. Check these points before pressing any compound into place:

• Do not skim over a V-shaped crack: A crack that narrows to a point at the surface has no mechanical grip. Always chisel or file the crack edges so the sides are square or slightly undercut.
• Scrape away all algae and slick biofilm: Underwater epoxy cannot stick to greasy algae or mineral buildup. Scrub the surrounding plaster with a rough scouring pad until you feel the bare plaster texture under your fingertips.
• Knead your two-part epoxy completely: If you stop kneading while you still see distinct swirls of color, the epoxy will not cure to full hardness. It will remain gummy and wash away under pool water currents.
• Avoid turning strong water jets back on immediately: High-pressure water from pool return jets can lift the edges of fresh putty before it cures. Keep your pump speed low or aim return eyeballs away from the fresh patch for the first 24 hours.
• Balance your pool chemistry after the repair: Highly acidic water can dissolve fresh cement patches and chew through pool plaster. Check your water chemistry regularly, and follow safe guidelines to raise the pH of your pool if your readings drop below the recommended range.

Call a licensed pool contractor if you spot signs of structural movement

DIY putties and hydraulic cements are temporary surface seals designed for minor cracks. They cannot hold a shifting pool shell together. If your pool exhibits any of the following warning signs, stop patching and contact a licensed pool contractor or structural engineer:

• The crack is wider than a quarter inch: Gaps wider than a pencil indicate major ground settlement or soil movement behind the concrete wall.
• One side of the crack is higher than the other: Run your hand across the crack line. If one side sticks out past the other, the concrete shell has sheared. Putting putty over a sheared wall will fail as the soil continues to push.
• The crack runs continuously from pool deck to floor: Cracks that slice straight through the pool deck, tear through the tile line, and continue down across the floor point to foundation failure.
• Rust stains continue spreading along the seam: Rust indicates that water has corroded the steel rebar cage inside the shell. Corroded steel expands, blowing chunks of concrete off from the inside out.
• Your pool loses more than an inch of water daily: Rapid water loss washes away supporting soil beneath the floor, creating hollow pockets that can cause the entire pool bowl to crack in half.

A professional contractor uses heavy-duty structural repair methods that DIY products cannot match. These methods include drilling carbon fiber or high-tensile steel staples across the crack every 12 inches to lock the concrete halves together, followed by injecting high-pressure polyurethane or structural epoxy deep into the foundation shell.

Choose a durable underwater patching compound suited to your pool surface

Selecting the right repair material depends on whether your crack sits above or below the waterline, along with your pool's specific plaster color. For submerged cracks on the pool floor or walls, look for a specialized two-part underwater epoxy putty stick. Check that the packaging states it cures underwater without shrinking or sagging. Most underwater putties come in standard white to blend with white pool plaster, but gray versions are available for gunite, dark quartz, or exposed aggregate finishes.

Take extra care with your pool maintenance routine after applying any repair material. If you use a robotic cleaner (a self-powered unit with its own internal debris basket) or a suction-side cleaner (which attaches to your pool skimmer or dedicated vacuum port and uses your pool pump's suction), remove it from the pool while your patch cures. Heavy rubber tracks or drive wheels can roll across the repair seam, grab the edge of uncured epoxy, and peel the putty right out of the crack groove.

Finally, inspect every patched area when you get ready to close an inground pool for winter. Freezing groundwater can push against the outside of your pool shell while surface ice expands inside, putting extra stress on existing seams. Addressing minor hairline cracks with durable epoxy before the cold season arrives prevents small fissures from expanding into costly structural breaks by spring.

1

Epoxybond Pool Putty 2-Part Set | Swimming Pool & Spa

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  9.2
• Brand
  Epoxybond Pool Putty

Key specs3

Submerged Application

Cures fully underwater

Surface Compatibility

• Concrete and plaster
• Fiberglass shells
• Pool and spa tile

Mixing Method

Hand-mix resin and hardener at a 1-to-1 ratio until uniform before pressing into cracks.

Pros & cons43

Pros

• Cures directly underwater without requiring you to drain the pool
• Hand-moldable two-part compound mixes at a simple 1-to-1 ratio
• Formulated not to sag, shrink, or yellow after hardening
• Bonds securely to concrete, fiberglass, and broken pool tile

Cons

• Requires manual hand kneading to mix the two parts before application
• Supplied 14 oz volume is limited to spot patching rather than large repairs
• Available in white or black which might not match colored plaster finishes

2

Sikaflex Crack Flex Sealant, Gray, High performance, textured polyurethane sealant,

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  9.2
• Brand
  SIKA

Key specs2

Crack Width Limit

Up to 1 in (38.1 mm)

Joint Orientation

Designed for horizontal slabs such as pool decks and walkways without vertical slumping.

Pros & cons43

Pros

• Elastic polyurethane formula accommodates natural concrete crack movement
• Self-leveling consistency flows smoothly into horizontal joints up to 1 in wide
• Dispenses directly from a standard 10 oz caulk gun cartridge
• Textured gray finish blends with cured concrete and accepts paint

Cons

• Self-leveling fluid formula is limited strictly to horizontal surfaces
• Not rated for active underwater application or wet pool interiors
• Listing does not specify cure time before water exposure

3

DRYLOK 00917 Cement Hydraulic WTRPRF, 4-Pound, Gray

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  9.0
• Brand
  Drylok

Key specs2

Cure Speed

Sets in 3 to 5 minutes

Pressure Sealing

Expands slightly as it cures to block active water flow through masonry fractures.

Pros & cons43

Pros

• Fast-setting hydraulic powder hardens completely in 3 to 5 minutes
• Stops active water leaks even when under hydrostatic pressure
• Creates a permanent structural seal without needing a top coat
• Strong enough to anchor metal hardware into concrete and masonry

Cons

• Extremely rapid 3 to 5 minute set time requires working in small batches
• Dry powder formula requires measuring and mixing with clean water
• Rigid cured cement does not flex if concrete joints continue to shift

4

Sikacryl Ready-Mix Concrete Patch, Gray. A ready to use, textured

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  8.8
• Brand
  SIKA

Key specs2

Application Orientation

Vertical and horizontal

Deck Spall Repair

Pre-mixed textured acrylic fills shallow spalls and surface cracks across concrete surrounds.

Pros & cons43

Pros

• Premixed acrylic formula requires no water mixing or measuring
• Textured gray paste adheres to both vertical walls and horizontal flats
• Resists shrinking and cracking as it cures to a durable finish
• Cleans up quickly from tools and hands using plain water

Cons

• Not formulated for submerged repairs inside filled pool shells
• One quart tub requires a putty knife or trowel for proper application
• Acrylic formulation requires dry conditions to cure completely

5

Red Devil 0645 Pre-Mixed Concrete Patch, 5.5 Oz Squeeze Tube,

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  8.8
• Brand
  Red Devil

Key specs2

Dispenser Type

5.5 oz squeeze tube

Maintenance Cleanup

Washes off tools and hands with basic soap and water prior to setting.

Pros & cons43

Pros

• Handy 5.5 oz squeeze tube allows direct application without tools
• Premixed paste eliminates mess, measuring, and water mixing
• Flexible cured compound resists shrinking and cracking
• Cleans up easily with basic soap and water before drying

Cons

• Small 5.5 oz volume is practical only for minor hairline touchups
• Light to medium gray color may clash with bright white plaster
• Requires a completely dry surface and is not rated for submerged use

6

DAP 37584 Liquid Cement Crack Filler, 1 quart, Gray

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  8.6
• Brand
  DAP

Key specs2

Traffic Readiness

30 to 45 minutes

Pourable Deck Repair

Liquid cement self-levels across horizontal pool surround slabs and walkways.

Pros & cons43

Pros

• Ready-to-use pourable liquid requires no mixing or specialized tools
• Self-leveling consistency fills flat horizontal concrete cracks evenly
• Ready to handle foot and surface traffic within 30 to 45 minutes
• Dries to a standard gray tone that blends with pool decks

Cons

• Liquid consistency runs out of vertical pool walls and steep slopes
• Not designed for submerged pool interiors or constant water immersion
• Unopened or partially used quart jug requires careful sealing to prevent drying

7

PentaUSA Tile Grout - Grey Grout Filler Tube, 13.4 Oz

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  8.0
• Brand
  PENTA USA KITCHEN & BATH ALL FAUCETS-ACCESSORIES

Key specs2

Included Accessories

Spatula grout kit

Waterline Tile Care

Waterproof premix formula restores eroded grout joints along decorative pool tile borders.

Pros & cons43

Pros

• Premixed 13.4 oz squeeze tube includes an application spatula
• Waterproof and water-resistant formula suitable for wet tile areas
• Non-toxic and odorless water-based recipe safe for closed areas
• Triple-action compound repairs, renews, and fills damaged grout lines

Cons

• Designed for cosmetic waterline tile grout rather than structural concrete
• Supplied gray tone may not match white pool grout installations
• Not formulated for underwater curing inside filled pool basins

8

3M High Strength Crack Repair, Squeeze Tube, 7 oz

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  8.0
• Brand
  3M

Key specs2

Applicator Design

Built-in precision nozzle tip

Exterior Requirement

Requires an exterior paint topcoat to maintain weather resistance on exposed outdoor surfaces.

Pros & cons43

Pros

• Precision applicator tip directs compound deep into narrow fractures
• Premixed paste sets up to 3 times faster than standard vinyl spackling
• Versatile bond anchors into concrete, plaster, stucco, and masonry
• Dries white and accepts exterior paint for seamless cosmetic blending

Cons

• Requires sealing with exterior paint for outdoor weather resistance
• Not rated for submerged hydrostatic pressure or pool water chemistry
• Small 7 oz tube is insufficient for wide structural foundation cracks

9

Hairline Crack Sealer by GetSet2Save

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  6.8
• Brand
  GetSet2Save

Key specs2

Cured Appearance

Dries clear and flexible

Penetration Action

Low-viscosity formula wicks into hairline crevices by natural capillary draw to seal fine fractures.

Pros & cons43

Pros

• Ultra-thin liquid penetrates tight hairline cracks through capillary action
• Dries completely clear to preserve the look of decorative concrete
• Sun-resistant water-based formula withstands outdoor UV exposure
• Non-toxic recipe contains no harsh chemical solvents

Cons

• Too thin to bridge wide gaps, deep voids, or shifting structural breaks
• Not formulated to cure underwater inside a filled pool basin
• Small 4 oz bottle covers only fine cosmetic surface fractures

Last update on 2026-10-08 / Affiliate links / Images, Product Titles, and Product Highlights from Amazon Product Advertising API

FAQs

Can I repair a concrete pool crack without lowering the water level?

Yes, you can seal minor hairline plaster cracks without lowering the water by using a specialized two-part underwater epoxy putty. These moldable putties are formulated to mix on your pool deck, apply underwater, and cure fully submerged without shrinking or washing away. However, wide structural cracks through the concrete shell require professional staples and high-pressure injections that often require draining the pool under controlled conditions.

Will silicone caulk hold up against pool water inside a crack?

Standard household silicone caulk should not be used to fix pool cracks below the waterline. Pool water contains chlorine and sanitizing chemicals that break down standard silicone, causing it to loosen and peel away within weeks. Always use a moldable two-part epoxy putty designed for underwater pool service or a hydraulic cement designed for masonry water containment.

How long does an underwater epoxy pool patch usually last?

A properly applied underwater epoxy patch in a dormant plaster crack can last several years without leaking. Its lifespan depends entirely on whether the crack remains stable and how well you cleaned the groove before applying the putty. If the pool shell shifts due to soil movement or expansive clay, the crack will continue to open and break the epoxy seal.

Can an automatic pool cleaner pull off an underwater patch?

An automatic pool cleaner can peel off a fresh patch if you run the cleaner before the putty has completely hardened. The scrubbing brushes, rubber tracks, or drive wheels on robotic and suction cleaners create strong friction as they climb walls and roll across floors. Keep all automatic pool cleaners out of the water for at least 24 hours after applying any underwater repair putty.