Pool Grout: Submerged-Rated Grouts, Water Chemistry and Cure Before Filling
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Pool tile needs a grout the manufacturer rates for continuous submersion: usually an ANSI A118.3 epoxy or an ANSI A118.7 high-performance cement grout with a submerged-use listing. Balanced water (pH about 7.2 to 7.8, with a near-zero saturation index) protects cement grout. Most TDS require 7 to 21 days or more of cure before filling.
Key takeaways
- Submerged service is its own category. A grout that is fine in a shower may not be approved for pools, spas or fountains. Look for the words "submerged" or "swimming pools" in the TDS.
- Aggressive water (low pH, low alkalinity or low calcium hardness) dissolves the cement paste in grout and leaves sandy, recessed joints. Scale-forming water (high pH or high calcium) coats joints with white deposits.
- Epoxy resists water chemistry far better than cement grout, but it needs careful installation on glass and porcelain mosaics and may chalk or shift color in strong sun.
- Cure time before filling depends on grout type and temperature. Common TDS figures run from about 7 days for some epoxies to 14 to 21 days or longer for cement grouts, and longer in cold weather.
- Expansion joints in pool tile get a sealant rated for immersion, never grout.
What submerged service does to grout
Most grout spends its life wet some of the time and dry the rest. Pool grout is wet all of the time, under pressure from the water above it, in water that is deliberately treated with oxidizers and pH adjusters. It also endures temperature swings at the waterline, sun on exposed tile bands, and, in cold climates, freezing when the pool is winterized.
Continuous immersion removes the drying cycles that let cement grout recover. Any soluble component of the grout (mainly calcium hydroxide, a byproduct of cement hydration) has years to leach out. Water chemistry decides how fast that happens. This is why grout makers test and list products specifically for submerged use, and why pool tile installers treat water balance as part of grout maintenance.
Grout types rated for pools
| Grout type | Standard | Submerged suitability | Chemistry resistance | Key cautions |
|---|---|---|---|---|
| Epoxy | ANSI A118.3; ISO 13007-3 RG | Many products approved; check TDS | High; tolerant of pH swings and chlorine | Difficult cleanup on mosaics; some products yellow or chalk in UV; cure fully before fill |
| High-performance cement | ANSI A118.7; ISO 13007-3 CG2WA | Some products approved for submerged use | Moderate; vulnerable to aggressive water | Longer cure before filling; keep water balanced |
| Standard cement | ANSI A118.6; ISO CG1 | Generally not recommended | Low | Erodes quickly in unbalanced water |
| Premixed and urethane | Varies | Most products exclude submerged use | Varies | Do not use unless the label explicitly approves pools |
| Industrial or specialty | Varies | Specified for commercial pools and water features | High | Usually specified by a design professional |
Epoxy grout is the most chemically resistant option and is common on commercial pools and glass tile waterlines. Its weaknesses in pool work are installation (it must be cleaned off glass and textured tile thoroughly before it sets) and sunlight; some epoxies shift color under strong UV, especially lighter shades, so look for a product that addresses exterior and UV exposure. See how to apply epoxy grout.
A high-performance cement grout is easier to install and is used on many residential pools. It must carry a submerged-use listing, and it depends on balanced water for its life. ISO 13007-3 CG2WA (improved, reduced water absorption, high abrasion resistance) is a useful baseline designation, though the submerged approval is what counts. See grout standards: ANSI and ISO.
The setting material matters as much as the grout. Pool tile is typically set with a polymer-modified mortar that the maker also rates for submerged use, over a substrate the pool builder has prepared. Grout cannot compensate for tile that is losing bond.
How pool chemistry affects grout
Pool water is either aggressive (it wants to dissolve calcium from surfaces it touches), balanced, or scale-forming (it wants to deposit calcium). The Langelier Saturation Index (LSI) combines pH, total alkalinity, calcium hardness, temperature and dissolved solids into one number that predicts which way water leans. Pool professionals generally aim for an LSI near zero, often between about -0.3 and +0.3.
| Parameter | Common target range | If too low | If too high |
|---|---|---|---|
| pH | 7.2 to 7.8 | Water becomes aggressive; cement grout etches and joints go sandy | Scale forms; chlorine works less effectively |
| Total alkalinity | About 80 to 120 ppm | pH swings; water tends toward aggressive | pH drifts up; scale risk rises |
| Calcium hardness | About 200 to 400 ppm (varies by surface type) | Water pulls calcium from grout and plaster | White calcium deposits on grout and tile |
| Saturation index (LSI) | About -0.3 to +0.3 | Corrosive to cement-based materials | Scale-forming |
| Free chlorine | Per local code and pool professional | Algae growth in joints | Bleaching of colored cement grout and some epoxies over time |
Low-calcium fill water is a common problem. A pool filled with soft water, or water from a source low in hardness, will pull calcium from any cement-based surface it touches until it reaches equilibrium. Cement grout, with its large exposed surface in each joint, gives it up readily. The result is grout that feels sandy, recedes below the tile edge and exposes its aggregate. Epoxy is largely immune to this mechanism because it contains no soluble calcium.
The opposite problem, scale, appears as white crust along the waterline and on grout joints. Hard water, high pH and evaporation at the waterline all concentrate calcium there. Salt chlorine generators add a further factor: salt water left to evaporate on the waterline tile leaves crystals that can build up on grout, and the generator cell tends to raise pH.
Warning: Acid washing a pool, or adding acid near the tile line without circulation, attacks cement grout directly. Protect or avoid tiled areas during acid treatments, and never pour concentrated acid along the tile band.
Cure time before filling
This is the most common installation question for pool grout, and the answer comes from the TDS of the specific grout and setting mortar, not from a general rule. Cement grout must hydrate enough to resist leaching and pressure, and epoxy must reach full chemical cure before it is submerged.
| Material | Typical range at about 70 deg F | Cold-weather effect |
|---|---|---|
| Epoxy grout | About 7 to 10 days | Longer below about 60 deg F; some products do not cure properly in the cold |
| High-performance cement grout | About 7 to 14 days, some products up to 21 days or more | Extend cure significantly below 50 deg F |
| Setting mortar under the tile | Often 14 to 28 days for submerged service | Extend in cold weather; setting mortar often controls the schedule |
In practice the setting mortar under the tile often controls the fill date, not the grout. Cold weather, high humidity and shade all slow cement curing; see grouting in hot and cold weather and how cement grout cures. The grout cure planner helps plan schedules.
When the time comes, fill steadily and balance the water promptly. Fill water should be tested and adjusted for calcium hardness and alkalinity early, because young cement grout is at its most vulnerable during the first weeks.
Movement joints and sealant in pools
Pool shells move with temperature and water load, and tile bands span long distances. Movement joints in the tile layer should line up with joints in the pool structure and follow TCNA EJ171 guidance. These joints are filled with a sealant rated for continuous immersion and pool chemicals (often a polyurethane or polysulfide type specified by the sealant maker), not with grout and not with ordinary bathroom silicone. See caulk vs grout movement joints.
The waterline tile band, coping edges and steps are also change-of-plane locations. Follow the pool builder's and tile system's details, which may call for sealant at the coping-to-tile joint.
Discoloration: causes and fixes
| Color | Likely cause | Typical response |
|---|---|---|
| White crust or haze | Calcium scale, or efflorescence from cement grout | Correct water balance; remove scale with a product suited to the grout type |
| Green or black spots | Algae in recessed or porous joints | Brush, shock per pool professional; fill recessed joints |
| Blue-green | Copper from corroding heat exchangers, ionizers or some algaecides | Find the copper source; use a metal sequestrant per pool professional |
| Brown or rust | Iron in fill water or corroding metal fittings | Sequestrant; address the source; see rust stains on grout |
| Faded or lightened color | Aggressive water leaching cement paste, chlorine bleaching | Balance water; regrout eroded joints |
| Yellowing or chalky epoxy | UV exposure at the waterline or on deck tile | Use a UV-stable product on replacement |
White deposits that come back after cleaning point to ongoing water movement through the grout or chronic scale-forming water. See efflorescence on grout and grout discoloration and shading for diagnosis.
Repairing pool grout
Pool grout repair is constrained by the water. Small areas can sometimes be patched using products designed for underwater application, typically two-part epoxy putties sold for pool repair. These are spot fixes; they do not match standard grout perfectly and are not a substitute for regrouting.
Proper regrouting means lowering the water below the work area, letting the joints dry, removing loose and eroded grout to sound material (often at least two-thirds of the tile depth, or as the grout maker requires), and regrouting with a submerged-rated product. Then the new grout needs its full cure before the water returns. Draining a pool fully carries its own risks (some in-ground pools can lift with high groundwater), so major work should involve a pool professional. The general method is covered in how to regrout tile and how to remove grout.
- Recessed, sandy joints across large areas usually mean an aggressive water history. Fix the chemistry before regrouting.
- Loose or hollow-sounding tile means the bond has failed. Regrouting will not hold it; reset the tile.
- Cracks that run along a straight line through the tile band usually follow a structural joint or crack and need a movement joint, not grout.
Bottom line: Use an epoxy or high-performance cement grout that is explicitly approved for submerged use, set the tile with a submerged-rated mortar, wait the full TDS cure before filling, and keep the water balanced. Most pool grout failures are chemistry failures that look like grout failures.
For decks, coping and patios around the pool, freeze-thaw and sun matter more than immersion; see outdoor grout. Glass tile waterlines have their own grout considerations in grout for glass tile.
Frequently asked questions
What kind of grout is used in swimming pools?
Epoxy grout (ANSI A118.3) or a high-performance cement grout (ANSI A118.7) whose TDS specifically approves submerged use. Standard cement grout and most premixed grouts are not suitable.
How long should pool grout cure before filling?
Follow the grout and mortar TDS. Typical figures are about 7 to 10 days for epoxy and 7 to 21 days or longer for cement grouts at about 70 deg F, with longer times in cold weather.
Why is my pool grout turning sandy and washing out?
The usual cause is aggressive water: low pH, low alkalinity or low calcium hardness that dissolves the cement paste. Balance the water before regrouting, or the new grout will erode the same way.
Can you regrout a pool underwater?
Small spot repairs can be made with underwater epoxy putties made for pools. Proper regrouting requires lowering the water, drying the joints and allowing full cure before refilling.
Does pool grout need to be sealed?
Grout under water is generally not sealed, because sealers are not designed for continuous immersion. Choose a low-absorption, submerged-rated grout instead and follow the maker's guidance.
Is epoxy grout better for a saltwater pool?
Epoxy resists salt water and chlorine well and is a strong choice for saltwater pools. Salt generators tend to raise pH, so water balance still matters for any cement-based materials in the pool.
Sources and standards
- ANSI A118.3, chemical-resistant, water-cleanable epoxy grout
- ANSI A118.7, high-performance cement grouts
- ISO 13007-3, grout classifications (CG2WA, RG)
- TCNA Handbook for Ceramic, Glass, and Stone Tile Installation, swimming pool methods and EJ171 movement joints
- Langelier Saturation Index, as used in pool and spa water chemistry guidance from the Pool and Hot Tub Alliance (formerly APSP)
- Grout, setting mortar and sealant manufacturer technical data sheets for submerged applications
Grout Atlas guidance is checked against ANSI A108/A118 and ISO 13007 requirements, the TCNA Handbook and manufacturer technical data sheets. Product formulations change, so confirm details on the current data sheet for the product you buy. Spotted an error? Report a correction.