How to Inspect Expansion Joints Around a Pool for Water Problems

The difference between good pool maintenance and expensive repairs often comes down to noticing small changes before they become bigger problems. The expansion joint around an inground pool is easy to overlook because it sits quietly between the pool coping and the surrounding deck. When that joint opens, cracks, hardens, or pulls away, however, water can begin reaching places it was never meant to go.

Expansion joints are designed to let the pool structure and surrounding deck move independently as temperatures, moisture levels, and soil conditions change. On many concrete pools, the visible joint runs around the perimeter between the coping and deck and is filled with a flexible sealant often called pool mastic or expansion-joint sealant. Inspecting it periodically can reveal water-intrusion problems, movement, and deterioration long before a dramatic crack appears.

What an Expansion Joint Is Supposed to Do

A concrete pool shell and a concrete deck are separate structures. They heat up, cool down, settle, expand, and contract differently. The expansion joint creates a flexible separation so those movements do not force the deck directly against the coping or pool bond beam.

The sealant across the top of that joint has another important job: limiting the amount of rainwater, splash-out, and cleaning water that can run down into the gap. A healthy joint should remain flexible and reasonably well bonded to both sides while leaving enough separation for movement.

Do not confuse the expansion joint with grout between individual coping stones or with control joints cut into the deck. Coping grout is usually rigid. The main coping-to-deck expansion joint is supposed to accommodate movement and normally requires a flexible material.

Start With a Slow Walk Around the Entire Pool

Inspect the joint when the deck is dry and there is enough daylight to see the sealant clearly. Walk the complete perimeter rather than checking only the area where you first noticed a problem. Joint failure is often localized, and a six-inch opening on one side of the pool can be more significant than cosmetic discoloration along several feet elsewhere.

Look closely for these conditions:

• Sealant pulling away from the coping or deck edge
• Cracks running through the sealant
• Sections where the sealant is completely missing
• Hard, brittle, or sun-baked material that no longer appears flexible
• Deep gaps that collect dirt, leaves, or standing water
• Weeds, ants, or sand appearing from inside the joint
• Uneven coping or deck sections near a failed area
• Water stains or persistent dampness along the joint after surrounding surfaces have dried

Pay Attention to Where Water Goes

An open expansion joint is not automatically a pool leak. In fact, the coping-to-deck joint usually sits outside the watertight interior of the pool. Its failure is more commonly a pathway for surface water to move beneath the deck, behind coping, or toward supporting material around the pool.

That distinction matters. If the joint becomes wet after rain, irrigation, deck washing, or heavy splash-out, the problem may be water entering from above rather than pool water escaping from inside the shell.

Watch the area during normal pool use. Does water repeatedly flow toward the joint because the deck slopes poorly? Does a sprinkler soak one section every morning? Does a roof downspout discharge nearby? Fixing failed sealant without addressing a constant source of water can leave the underlying problem unchanged.

Look for Problems in the Coping and Tile Nearby

The joint itself is only part of the inspection. Check the coping immediately beside suspicious sections. A coping stone that rocks when lightly pressed, mortar that is crumbling underneath it, or a crack that repeatedly reappears may indicate movement or water intrusion around the pool edge.

Also inspect the waterline tile below the coping. Loose tiles, hollow-sounding sections, cracks near the top of the pool wall, or recurring deterioration in the same area deserve attention. Water that repeatedly works behind coping and tile can contribute to damage, particularly where freezing weather allows trapped moisture to expand.

In warm climates, freeze-thaw damage may be less important, but heat, UV exposure, soil movement, irrigation, and heavy rain can still deteriorate the joint and surrounding materials.

Check Whether the Joint Can Still Move

A properly functioning expansion joint should not be packed solid with rigid material. Dirt, sand, old mortar, pebbles, and repeated patching can gradually fill the space. Once the joint becomes rigid, movement in the deck can transfer stress toward the coping and pool structure instead of being absorbed by the joint.

Be especially suspicious if someone previously filled the entire coping-to-deck gap with mortar, concrete patch, or another hard material. A repair can look neat while defeating the reason the expansion joint exists.

Similarly, do not assume that pouring fresh sealant over loose old material is a complete repair. Successful resealing usually depends heavily on removing failed material, cleaning the bonding surfaces, using appropriate backing where required, and installing a flexible sealant designed for that joint.

Inspect After Heavy Rain

A dry-day inspection shows physical deterioration. A post-rain inspection shows how water behaves.

After a substantial rain, look for sections of the expansion joint that remain wet much longer than the surrounding deck. Check for puddles against the coping, water disappearing quickly into an open gap, or muddy residue emerging from beneath a deck edge. These observations do not identify exactly what is happening underground, but they can help pinpoint areas worth closer investigation.

If one deck section has settled and now slopes toward the pool instead of toward a drain or yard, water may repeatedly collect along the expansion joint. In that situation, replacing sealant may be necessary, but drainage or slab movement may also need attention.

Separate Expansion-Joint Trouble From Actual Pool Water Loss

If your inspection began because the pool level itself keeps dropping, measure the water-loss issue independently instead of assuming the visible joint is responsible. A Mini Bucket Test can be used as a simple first step to help compare normal evaporation against possible leak-related water loss. It does not identify the location of a leak or prove that the expansion joint is involved, but it may help you decide whether further leak investigation is worth pursuing.

This is particularly useful when several symptoms appear at once. For example, a homeowner may find deteriorated joint sealant, damp soil beside the deck, and a pool that seems to need frequent refilling. Testing whether the pool is actually losing more than normal evaporation helps keep those observations from being incorrectly lumped together.

When the Problem Deserves Professional Attention

Minor surface deterioration may simply mean the joint needs proper cleaning and resealing. Larger movement patterns deserve a closer look. Consider having a pool, concrete, or appropriate repair professional inspect the area when you find loose coping, widening cracks, a noticeably raised or sunken deck slab, recurring tile failure, voids beneath the deck, or sealant that repeatedly separates soon after replacement.

Professional leak detection is also appropriate when testing suggests the pool is losing more water than evaporation and the source is not obvious. The leak could involve plumbing, a skimmer, a fitting, the pool shell, a light niche, or another component unrelated to the expansion joint.