Will Pool Plaster Seal Active Leaks?
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I very often will see posts online from concrete pool owners who are worried/complaining about water that they can see leaking into their (currently empty) pool. This normally happens during the initial build of the pool or when the pool has been drained for replastering of the interior surface. I can definitely imagine the worry from a home owner who is having a pool built to see water leaking in from multiple places and starting to pool in the deep end. This looks like the pool shell is faulty and is leaking...and they are half right. This question poses a somewhat complicated technical answer that is not exactly as straightforward as it may seem. Hence this article to help you (the pool owner) understand a little more about what is happening with your pool when you see water leaking in.
In particular this article will help pool owners to understand that plaster will NOT seal leaks in your concrete pool. Plaster is in fact not waterproof at all. It is simply sand and cement mixed together. It is only a half inch thick in most cases! By comparison your pool shell itself will be 8" to 12" thick of consolidated, steel reinforced, high strength concrete. It is much more likely to resist water than a half inch of smooth troweled mortar (plaster). The plaster is really more for bather comfort and aesthetic appeal than waterproofing. At absolute most you could state that pool plaster is water resistant. However the way in which concrete / mortar wicks water means that no matter what your pool is NOT waterproof. Concrete is not waterproof. Adding plaster, which is the same materials only much thinner, also does not make it waterproof.
The plaster will NOT seal leaks in your pool - When a worker tells you that the plaster will seal the leaks in your concrete pool they are either misinformed or putting you off in hopes that you just go away eventually. You see, it is actually very normal for water to leak into a pool like this. Which does in fact mean the pool is not waterproof, and yes you can expect that water will leak out in the same areas you can see it leaking in. This is actually considered (somewhat) normal for a concrete pool to do. Unless an additional step is taken to waterproof the pool structure then you can expect that your pool leaks. For most of history this is just how concrete pools were built including still to this day, concrete pools are sometimes built without any waterproofing which means that they will actively wick / leak water.
Concrete pools leak. This is just a function of the material that they are made from. Only in more recent years do we have advanced polymer modified concrete waterproofing systems, or also membrane based waterproofing systems being used in concrete pools. These materials can make a concrete pool actually waterproof. When you build a concrete pool without a waterproofing layer then you can count on water escaping the shell, likely in a few areas. So is that a problem? Well, I would definitely say it is not ideal.
If you are building a pool on top of a roof of a fancy hotel then I think it would be a big problem to not have a waterproofing system in place in the pool. However most concrete pools, at least residential ones, are typically buried which means you do not actually see if the pool has wet walls on the back side of the wall. Well, I can tell you that without a waterproofing system you can pretty much count on the water finding a path (probably a dozen paths) from the wet side to the dry side of the wall. If you build a concrete pool with exposed walls you will be able to see the water leaking through. If you dig up existing pools you will be able to see the results of water leaking through...efflorescence. This calcium-like buildup of scale actually plays a pretty big role in this whole question about concrete pool waterproofing and whether pool plaster will stop leaks.
Efflorescence in concrete pools - One of the reasons why concrete pools can have leaks but continue to run "without problems" is the process of efflorescence growth. Any place where you have water passing through concrete or mortar you will likely see efflorescence growth, which is an accumulation of minerals which calcify. If you have ever tried to clean efflorescence from pool tiles then you already know how much of a pain that scale is to remove. This scale develops in the concrete pathways where water has found a route to track through. As the water passes and efflorescence starts to grow, the rate of water leaking in that area will reduce. Eventually the water leak might stop entirely in that area as the water now finds a new path of least resistance through the porous substrate of your pool shell. This is how you can have a concrete pool that leaks constantly but the pool owner never notices or is aware. The pool leaks slowly, and then sort of self patches with efflorescence growth, only to have the water start a new leak somewhere else. It is just minor enough of a loss as to not be overly noticeable.
Changing rates of water loss in leaking pools - Concrete is porous. When you have a pool made from entirely porous materials then it will absorb a good amount of water upon initial filling of the pool. Once the substrate shell is saturated with water the rate of water loss will decrease to a somewhat tolerable level. If you have a concrete pool built without waterproofing then this is how I believe your pool will operate. If you were to dig up and expose the back sides of your pool walls you would see efflorescence staining all over the place. Concrete is like a very, very dense sponge. It will act just like a sponge, but in a very slow way. So you can operate a concrete pool like this, with water that permeates the shell, but it is definitely not ideal. The more a pool shell leaks, the faster it will degrade from cement washing away with the movement of water. It will also allow the steel within the structure to corrode due to the chlorinated water. The reason why more concrete pool owners do not notice or complain about leaks is simply that they are relatively small to begin with, more like a wicking sponge than an active gusher, and the rate of water loss slows as the shell and surrounding pool area become saturated with water over time.
Pool plaster is just a dense, smooth player of modified mortar that is designed to resist water permeation. Again this is like a dense sponge layer, not a rubber layer that fully resists water, so any place where a pool leaks will still leak once the plaster is installed. It is just that once the pool is filled with water you will not likely notice or care that these small leak points exist. And even if they do exist there is a decent chance that small leaks will self-plug with efflorescence growth over time. It is only in high risk pools like one on the roof of a hotel, or a pool with exposed backside walls where it becomes plainly evident that more waterproofing is needed if you want your concrete pool to actually be waterproof. Basecrete waterproofing is a product that I like to endorse professionally for concrete pool waterproofing. There are other options as well such as Laticrete, Redgard, and Miracote which all can be used for some specific concrete pool waterproofing applications.
So what do you do when you have a pool that is leaking and the contractor says the plaster will seal it? Well, not much. The time to know whether a pool contractor will use a waterproofing system is during the hiring and interview stage. Many concrete pool builders to this day do not use waterproofing, for better or for worse. So can you demand that they do waterproofing to your pool? Well, not really. That stuff is expensive. And you definitely need to know how to use it. I would not prefer my pool to be the first one where they learn how to work with these products. Concrete waterproofing systems are complicated technical systems with very specific steps and technical requirements that you must follow. So as far as existing leaks I would perhaps recommend that you request the worst leaks are cut out and patched with non-shrink grout or hydraulic cement. Commonly the underside of pipe penetrations will leak from water chasing along the pipe. During installation you can use water stops on the pipe. At pre-plaster stage construction you can seam around the pipes with a concrete urethane product like NP1 polyurethane or Vulkem 116 polyurethane. These will help to prevent water leaking from these known problem areas. In an active leaking pipe penetration you can chip out some material around the pipe and patch with hydraulic cement. This will stop the water enough to be able to apply the urethane, but in an ideal world this problem is solved before this stage by embedding water stops on the pipes inside the walls.
If you want more tips for things you should be asking your builder during the hiring stage then this page will be helpful: Tips for how to get the most value with a new pool builder.
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Swimming Pool Steve is an award winning, second generation swimming pool and spa specialist from Ontario Canada. With over 10,000,000 views on the Swimming Pool Steve YouTube Channel, winner of the 2018 Pool & Spa Industry Leadership award and author of hundreds of pool and spa articles both online and in print Steve is committed to helping pool and spa owners as well as pool and spa industry workers learn more about the technical side of building, renovating, repairing and maintaining all types of swimming pools and spas. Follow Swimming Pool Steve on Facebook, Twitter and YouTube.
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