Look along the edge of any pool where the coping meets the deck and there is a seam, usually a strip of grey or tan flexible material. Most owners never think about it. It is one of the most important maintenance items on the whole pool, because when it fails the damage lands on the tile, the coping and the structure behind them.
What the joint does
A pool shell and a concrete deck are two separate structures. The deck expands in summer heat, contracts in winter, and moves as the clay under it swells and shrinks with moisture. The pool, anchored deep and full of water, moves much less. If the two were locked together, every expansion of the deck would push against the pool's bond beam. The expansion joint is the deliberate gap between them, filled with a flexible sealant so the deck can move and water cannot get in.
Why it fails
Mastic is a flexible material living in the worst spot in the yard: full sun, chlorinated splash, foot traffic and, in the Piedmont, a freeze-thaw cycle that runs from December into March. Over several years it hardens, shrinks, cracks, and pulls away from one side or the other. Once it opens, rain and pool splash run straight down into the gap.
What happens next
- Water under the deck. It saturates the base, softens the clay, and in winter freezes and expands. The deck heaves.
- Pressure on the coping. The deck pushes toward the pool. Coping stones lift, crack or come loose.
- Cracked tile. The waterline tile sits on the bond beam right below the coping. It is the first thing to show the pressure, with cracks and pieces popping off.
- Bond beam damage. In a bad case the top of the pool wall itself cracks, which is a structural repair.
A failed joint is, in our experience, behind a large share of the tile and coping problems we are asked to fix. Signs your pool needs renovation lists the symptoms, and a good number trace back to this seam.
Signs the joint is failing
Visible cracks or gaps in the mastic. Mastic that has sunk below the surface or bulged above it. Sections missing entirely. Grass or weeds growing in the seam. Tile cracking along the waterline with no other explanation. Coping that rocks underfoot. Deck sections that have lifted at the pool edge.
How it is replaced
- Remove the old mastic completely, down to clean concrete on both sides. Partial removal is why replacements fail early.
- Clean and dry the joint. Dust, old sealant and moisture all prevent adhesion.
- Set a backer rod in the gap to control the depth of the new sealant. A joint that is too deep tears; too shallow and it cannot stretch.
- Apply a pool-rated self-leveling sealant, sized to the joint width, and tool it flush with the deck.
- Let it cure fully before foot traffic or water exposure. Sand can be broadcast on the surface for grip and a matte look.
It is a careful job rather than a hard one. Where the deck has already heaved or the coping has lifted, the joint is replaced as part of a larger repair, and the underlying drainage problem gets fixed at the same time. Deck drainage on a sloped lot explains why water under a deck is the root of so many issues.
Other joints on the deck
Control joints cut into the deck slab, and expansion joints where the deck meets the house or a wall, do a similar job in different places. They should be sealed and inspected the same way. A deck with open joints against the house foundation is sending water where nobody wants it.
Put it on the spring list
Every spring when the pool opens, walk the edge and look at the joint. If it is sound, move on. If it is failing, fix it before the summer heat and the next winter's freeze do their work. It is a small job in spring and a tile-and-coping job by the following year. Opening your pool in spring has the rest of the checklist.
We build new decks with the joint done right and repair failed ones as part of our renovation work across the Triad, including Archdale and Welcome. Request a free consultation if the edge of your pool is showing any of the signs above.