Leaks in the pool structure refer to leaks anywhere within the body of the pool (not in the plumbing system). Leaks within the structure of a pool are difficult to diagnose as there is not much in the way of definitive testing that you can do to find them.
If you have determined that you have a leak somewhere in the structure of your pool then the process of finding it is simply by close inspection. It is also helpful to know how swimming pools tend to leak as this can help you in focusing on the most likely areas.
Leaking pool lights - Leaks from swimming pool lights are one of the more common structural leak locations in a pool. Light canisters (niche) are attached to the pool wall to receive the light assembly. Lights can leak in a number of different ways but the most common would be through the electrical connection on the back of the niche. The electrical connection point on a pool light niche is prone to failure and not something that is easy to fix unless you are able to dig up the light from the back side of the pool wall. Light niches also can crack from age or from freeze and thaw conditions in the pool. If this is the case you should be able to see cracks in the niche upon close inspection and a dye test may confirm if the cracks are actively leaking. Inspecting a pool light is not the easiest thing to do since they are so far below the water level of the pool. Using swimming goggles to get close enough for a visual inspection is your best bet. In some cases you simply may not be able to see inside the niche to inspect it. If you suspect that your pool light is leaking though the electrical conduit then you should try to seal the conduit with this electrical conduit sealer. This product does not harden like epoxy so you can reach into the light niche and pack the electrical conduit hole without causing future problems for yourself. If the light is leaking through the electrical conduit connection then you may be able to slow or stop the leak this way. Failing this you will need to dig up and expose the back side of the light niche to properly inspect and repair it.
Leaking skimmer - Leaks in the pool skimmer would be part of the pool structure (not to be confused with leaks in the skimmer plumbing line) and are a very common cause of water loss in swimming pools. Skimmer bodies can crack with age, chemical damage or improper winterizing. If you have a crack in the body of your skimmer then this is almost certainly leaking water. You can try to dye test the skimmer cracks to see if you can get a definitive leak diagnosis or you can simply try to repair with some pool epoxy. Replacing a pool skimmer is a big job and one that often leads to needing to do other work as well so replacing a broken skimmer on a pool is not always an option. To learn more about how to replace a broken pool skimmer and why doing this often leads to other work you can read this article on pool skimmer replacement. If you have a vinyl liner pool then the skimmer can also leak through the flange and gasket that compresses it to the pool liner. If you suspect a leak in this area on a vinyl pool then you should check that each screw in the faceplate is very tight. Any loose or spinning screws are guaranteed to be losing water. On a concrete pool the skimmer is different and is usually encased in a solid cube of concrete. While this will often prevent water from escaping this area, it will not always be enough. If there is even the slightest potential for water to escape the pool system then it will find a way. On a concrete pool skimmer this will most often be through the face of the skimmer itself. Water will chase along the sides of the skimmer itself and escape the pool. If you suspect that your skimmer is leaking then try to tap on the throat lightly with a hammer. The throat of a concrete pool skimmer should be completely encased in concrete. If you tap and the skimmer sounds hollow, especially on the bottom side of the skimmer throat, then this is likely from a long term slow leak in this area of your pool.
Leaking returns - Leaks in pool return lines are one of the most common causes for a leak in swimming pools. Right now however we are looking at structure leaks so for this section we are not talking about the return lines themselves, but instead the return flange inside of the pool where the return line terminates. In a vinyl liner pool it is very common to have leaks through return gaskets and fittings if the gasket orientation was not installed correctly to begin with, or if the flange itself has become cracked or damaged over time. Similar to vinyl pool skimmers, if there are any screws not installed, not tight, or spinning in the hole, then this is a guaranteed leak location. On a concrete pool the returns are inclined to leak in a different way. In a concrete pool the water will try to escape the system by tracing along the pipe penetration through the concrete wall of your pool. To combat this many pools have "water stops" installed inside the pool wall which stop the water from being able to trace back along the pipe. Older pools especially are prone to having problems in the return locations from insufficient concrete being packed underneath of the return pipe. Shotcrete and gunite pools both involve a spray process for installing the concrete. Due to the way that pools are built there is a specific weakness for the underside of pipe penetrations to not get packed with concrete correctly. This will start as a slow leaching leak that would be imperceptible, but over time the passage of water through this location will weaken the concrete here and will eventually one day turn into a more serious, active leak location. If you have a leak under the return ports in your concrete pool then the best way to fix this would be to lower your water level below this area and use a chipping hammer to remove any loose concrete. Quite often this will end up exposing a hole under the return line which has been there since the pool was originally installed. You can pack the area with a water stop hydraulic cement product, or you can also use a urethane sealant, or both.
Main drain leaks - A leak in the main drain of your pool is not a situation that you want to have. The biggest problem is simply that the main drain is located on the floor of the deep end of the pool and can be very difficult, or impossible, to access. If you can not get close enough to your main drain to inspect it or plug it off for testing then you will have a very hard time trying to determine if this is the cause of your leak. In many cases troubleshooting the main drain is simply a matter of testing all other components of your pool. By using this process of elimination you can identify if the main drain is likely the cause for your water loss. Main drains tend to leak in two ways most commonly. The main drain has a port on the bottom of it which is intended to house a hydrostatic relief valve. This valve is a one way, spring loaded valve that is designed to allow water into your pool should there be an elevated water pressure under the pool due to high water tables. This is the safety protection that all concrete and fiberglass swimming pools need to prevent from lifting however many vinyl liner pools have hydrostatic relief valves also even though they technically do not need them the same way concrete and fiberglass pools do. A hydrostatic relief valve is operated by a mechanical spring and as such the spring can deteriorate, rust and fail over time. This could easily account for a leak in the main drain of a pool. It is also fairly common for hydrostatic relief valves to get stuck open from debris obstructing the valve from closing. If this happened then it is possible for your entire pool to drain out through the bottom of the main drain.
Equalizer line leaks - Another very common way that main drains leak is through the equalizer line. The plumbing pipe on most pool main drains does not actually connect directly to the suction lines for the pool. Often the main drain has a non-pressurized equalizer line that connects from the main drain port to one of the skimmer ports on the bottom of the skimmer. In this common configuration the main drain pipe does not actually experience any suction pressure from the filtration equipment. Instead the equalizer line allows water into the bottom of the skimmer should the water level in the pool fall below the mouth of the skimmer. Without this feature it is very likely you would burn out your pool pump if the water in the pool ever dropped below the mouth of the skimmer (which is only a few inches below the normal operating level of your pool). Having an equalizer line from the main drain to the skimmer is the best way to protect yourself from this problem. The only concern is that should you ever develop a problem in this pipe then you will have little recourse to fix it. In fact the equalizer pipe that connects from the main drain to the skimmer is the single most likely pipe on your pool to fail. The reason why is simply how inaccessible this pipe is. Even during full scale pool renovation projects this equalizer line is often not replaced. Either the line continues to function without leaking or it will simply be plugged off in both the main drain as well as the skimmer. The pool will continue to operate more or less effectively, just that you must be careful to not let the water level of your pool drop below the mouth of the skimmer or you risk burning out your pump. If you have leak in this location of your pool then the repair process is the same. You must plug the port in the main drain as well as in the bottom of the skimmer to isolate this equalizer line from both ends. It is not easy to swim down and plug a main drain inside a pool full of water but it is possible. If you are not able to do this yourself then you should not have much trouble finding a pool service company or diver to plug it for you.
Structural leaks (vinyl liner pools) - Structure leaks are very common in swimming pools. In addition to the individual items such as the skimmer, returns, lights and main drains already discussed, leaks through the physical structure of the pool itself are very common. On a vinyl liner pool a structure leak means that you most likely have a hole in the liner somewhere (or multiple holes). You can lose a surprising amount of water through a small hole in your vinyl liner. The process of finding a hole in your liner involves more inspection than anything else. While there does exist electronic tools for helping to locate leaks in a vinyl liner, most holes will be found simply by looking in the most common areas. A hole in a vinyl liner will typically be crescent shaped and around 1/4" in length. To find holes like this you would simply use a systematic approach to inspecting the liner. Most commonly this would be done from inside the pool while wearing goggles to help you see clearly. Start by inspecting the walls and give specific attention to where the walls meet the floor in the shallow end (the cove) as well as in the corners of the pool. Use your hands and feet to help you feel for any imperfection in the liner and then inspect these areas closely. Holes in the vinyl liner can come from broken maintenance equipment with sharp edges, automatic vacuum cleaners, sharp toenails or swimwear that has buttons, clasps or other metal parts. Once you locate the hole (holes) in your liner then a simple vinyl liner patch kit is all you need to fix the leak. A slightly less common way that vinyl liners can develop a leak is though a separation of the welded seams. A vinyl liner is made by welding sheets of vinyl together. This used to be a process done by hand by skilled vinyl welders however most liner manufacturers have now switched to a machine controlled process. A vinyl seam is most likely to separate at the point the liner is first installed. If you are searching for a leak in a brand new liner installation then be sure to heavily scrutinize the seams in the vinyl. Run your hands over them to see if you can feel any loose sections that might indicate a seam separation. Older liners can also experience a seam separation however this would be less common since the liner tends to be under even pressure once it has the weight of water on top of it.
Structural leaks (concrete pools) - If you have a concrete pool with a suspected structure leak then this is more of a concern, and a harder leak to find than with a vinyl liner leak. In a concrete pool the first thing that you should look for is the presence of any cracks in the interior finish. Cracks in a concrete pool are bad. If you have a crack then it is either just a crack in the 1/2" thick plaster layer, which is not as big of a problem, or it is a structural crack that penetrates through the concrete shell of the pool. If you have cracks in the interior surface of your pool then you can attempt to dye test them to see if they are actively leaking. Another reason why concrete pools lose water is because concrete is a porous material. Even very dense mortar finishes like pool plaster are only water resistant. All concrete pools leak - or more accurately all concrete pools leach water. Water absorbs into the concrete as part of the normal operation of a concrete pool. As the pool ages and the interior surface becomes less and less water resistant then water will leech into the concrete shell faster and faster. Quite often with concrete pools, especially older concrete pools with interior surfaces past their service life, the water loss will be the sum total of water leaching through the interior surface and into the shell over the entire surface area of the pool. If you have a concrete pool with a persistent leak problem and the interior surface of the pool is over 10 years old then there is a very decent chance that your water loss could be explained by water leaching through the finish of your pool. A new plaster interior surface is quite expensive and many pool owners will try to put off a new plaster as long as possible. Painting of concrete pools is something that is often done as a low cost alternative to new plaster but is inferior to plaster in every way. If you choose an epoxy paint or rubber based paint then you will no longer be able to plaster the pool without sandblasting first. Most pool experts agree that painting a pool with epoxy paint (or rubber based paint) is not worth the investment and you would be further ahead to simply replaster the pool given the shorter service life of paint. If you are interested in plastering your pool but would like to delay the renovation for a few years then you might want to consider painting with a low cost water based pool paint. Less durable, less expensive and easier to apply (and remove) than epoxy or rubber paints, water based pool paints are a decent option if you need to get a few more seasons out of your pool. Since the paint is lighter duty and water based it can be removed with a 3500+ PSI pressure washer as opposed to other forms of pool paint which require sandblasting to remove. If you would like more information on this painting process you can read this article on how to paint a concrete pool. A short part 2 to this concrete pool structural leak video can be found here.
In wall steps - Vinyl liner pools that have a set of in wall steps that is attached with a flange and gasket system are one of the most likely sources of a structural leak in a vinyl pool. Where a return fitting has 4 compression screws, and a skimmer can have between 8 to 16 screws, a set of in wall steps will normally have more than 50 screws in the flange. A really wide set of steps could have even twice that many screws and this increases the chances that you will develop a leak in your pool considerably. As with other compression flanges and gaskets it is critically important that each and every screw is installed and is tight. Any one screw that is stripped, spinning or missing, will almost certainly be causing a leak. For novice liner installers a set of in wall steps can be challenging to install correctly. This increases the chances of a set of steps developing a leak more so than other flanges in your pool and if you suspect that your steps might be a problem then you should try to dye test them as slowly and carefully as possible. Pay extra attention to the bottom two corners of the flange as this is an area that is likely to become damaged from the liner stretching. Remove the beauty caps so you can see the installation of the screws to see if there are any missing, or if there are mismatching screws. Ideally you want to see everything looking uniform but often you will find different screws, oversized screws or new holes that have been added to deal with previous problems. If you see evidence that your steps have been altered at some point then you should scrutinize the step flange even closer.
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