Pool Liner Coving
I was recently contacted by a pool owner who had a new liner installed. They are unhappy with how the liner fits in the pool and contacted me to learn more about how a liner is supposed to fit in a pool - specifically the area where the shallow end walls meet the shallow end floor. What can happen, and did happen in the case of this customer, is that there was a noticeable cove in the liner in this specific area. Which is to say that the pool liner does not follow the 90 degree contour from the floor to the wall, but instead transitions with a slope. So is this normal? Is this a problem? Did the liner installer make a mistake, or perhaps was the liner not manufactured properly? The answer, as usual, is maybe.
Nothing is ever cut and dry in the pool industry. It was very clear that this customer wanted me to tell them that this is no good, and you can not have coving in a vinyl pool, and that the original installer should come out with a new liner a make it perfect. Unfortunately I did not agree. The customer was most concerned because the liner did not have a sharp 90 degree profile in these corners...and they almost never do. So why would a liner not fit perfectly tight into these corners? There are a few reasons to consider:
Cold weather installation - Installing a liner is a warm weather operation. Sure, you might be able to throw a liner in during cold weather, but I would not let someone do that to my pool. Liners need the heat from the sun to become supple, and so that they will stretch evenly throughout the pool when you apply the vacuum. The difference between cold vinyl and warm vinyl is literally night and day when it comes to how hard (or easy) it is to fit the liner in the best possible location. A liner that fits a pool nicely, but that is installed on a cold day, will ultimately fit the pool poorly. Yes, it is possible that the liner could still fit nicely on a cold day, however the vast majority of liners need to be stretched and manipulated to achieve the best possible fit. On a cold day your ability to stretch and manipulate the liner is near zero. If the liner was cold, then the cove in the pool would be much more pronounced than the same liner left to bake in the mid day summer sun for 10 minutes. When you turn on the vacuums to pull the liner into place a cold liner will not stretch under the strength from a liner vac, but a hot liner will suck back to the wall so tightly that it will look like it is painted on.
Varying shallow end wall depth - When you measure the height of the shallow end wall, you measure in multiple places and then the liner is made to fit the shortest wall height. In a perfectly built pool these numbers should all be the same, however in the average backyard pool the floor will not be perfectly flat and will have as much as a few inches of height difference between different areas. The reason why the liner is made to the shortest wall height is simply that if it was made longer to suit the longer areas, the shortest area of wall would have too much liner material and you would have a wrinkle in the liner at this location. Wrinkles in a pool liner are unacceptable and this is why liners are made small and stretched into place. It is important to understand that a very small amount of stretch for a liner is no problem at all, and with heat and sun the liner will pretty much stretch just under the weight of gravity itself. A small amount of stretch, especially where the cove of the liner is (where the wall meets the floor) is normal, however if the liner was simply measured (or made) too small then it can become too tight in this area. This is why it is not exactly cut and dry with coving. The liner might need to be slightly undersized in some places, but ultimately, some areas with coving are better than ending up with wrinkles because there is just too much liner in some places.
Orientation of the liner - If you simply hung the liner from the coping track and turned on the vacuums it is very likely that you would have too much coving all around the shallow end of the pool. Often, the deep end of the pool will steal material from the shallow end and this results in extra coving problems. Fitting the liner to the best orientation can reduce some of the coving in the shallow end and most of the time this attention to detail when installing the shallow end is enough to get rid of (enough) coving for a quality installation. This is accomplished by putting the shallow end where it needs to be, and then weighing down the liner with salt bags (or a few helpers standing on the liner at key locations) to keep it in place while you turn on the vacuums. Once the air has been removed from behind the liner, it will be much less inclined to shift, and you will find that the shallow end is able to resist the deep end from pulling it away from the walls. If a shallow end had too much coving, in many cases simply turning off the vacuums, resetting the liner closer to the shallow end walls, and the turning on the vacuums again will resolve the problem. The deep end of the pool has so much vinyl in it that it will have no problem getting enough stretch from all directions to stretch into place. The shallow end, by comparison, has much less vinyl to work with so you need to put the shallow end right where you need it to be in order to get the best fitting liner and least coving at the foot of the walls.
How Much Cove Is Acceptable In A Pool Liner?
Not all liners, most in fact, will have some coving in the shallow end. Thinner 20 mil liners will be much more inclined to follow the exact contours of the pool walls and floor, and the thicker 27 mil and 30 mil liners will be more inclined to cove (especially if installed cold and without sun). It is normal to have some coving where the wall meets the floor, but how much is too much?
Certainly if there is too much coving this can put undue stress on the liner seams, and can also allow the liner to take damage from sharp objects (and vacuum cleaners) easier than a liner that has less coving. When you have way too much coving what will happen is that the cove becomes a tripping hazard and when you approach the edge of the pool your foot slides off the cove and you fall forwards. This is extremely dangerous and totally unacceptable, however a few inches of coving will not cause this problem.
The profile of the cove in your shallow end should not exceed about 2" and, in theory, is optimal at zero coving altogether. Which is to say that if your liner sits more than two inches off the floor and wall this would be considered fine, or "good enough" by most real world liner installers. Even three inches might be acceptable depending on the difficulty of the installation as well as how detail oriented the installer is. However, if you have eight inches or a foot of coving where your floor meets your wall then this is not acceptable and needs to either be reset with a better fit, or reinstalled with a new, better fitting liner.
There are a lot of variables involved with this question of floor and wall coving and just how much cove is considered acceptable. In theory, zero coving is the best and this means that the liner sits tights against the floor and walls with zero free space underneath. This is certainly possible to accomplish with careful measuring and manufacturing, and by installing the liner during ideal weather conditions. If you tried to have a liner made exactly to fit all coves in a pool without any stretch at all, the chances to have a wrinkle somewhere would be very high. Much higher than if the liner is made just a little bit small and allowed to stretch into place.
When it comes time to install, if you only need to install one liner then I suppose you can pick the best day to get it done. If you install liners all day, every day, you will experience all kinds of weather conditions so in the real world some pool liners get installed when it is cold, or overcast, or raining, all of which can compromise the quality of a liner installation. Also the pool is often in less than ideal condition as well. Some pools are built poorly, and some pools are really old, some are both. So what are ideal conditions?
Ideal liner installation conditions: First, the liner would need to be installed on a hot and sunny day to ensure equal and even stretch in the liner. Next, the pool floor would need to be flat to within 1/4" inch (most are not) or else the liner will need to be too short in some areas to avoid being too long in any areas on the wall. Too much liner anywhere equals a wrinkle so the goal is to fit the liner as best as possible, without being too large in any one place. Finally the liner needs to be oriented correctly. As a person who has installed pool liners their entire adult life I can tell you that in a lot of cases the liner can fit better, but this will involve turning off the vacuums and manually adjusting the liner. This might also include redoing some tape around the vacuums and in total could take an hour or more to reset and then try to vacuum into place again on some difficult pools.
In many cases where there is excessive coving, the installer might have been able to improve the situation if they spent the extra time and effort to do so. It is easy to take a "good enough" approach when you do the same job day in and day out, but it is important to try to see things from the perspective of the customer...they are going to have to live with this liner every day and if there is something that you can do to improve the fit of the liner, extra effort should be taken to provide the highest quality liner installation possible. This also might include waiting until a better day for more heat and sun to install the liner. This can be a bitter pill to swallow if you are a busy liner installation technician, however these are the chances you take when you attempt to install liners in marginal weather. If you end up with a tight fitting liner, no overhead sun, and a picky customer, then you might find yourself installing the liner twice. Better off to just wait for the right day to do the job the first time in my opinion. At 100 degrees Fahrenheit, with overhead sun, a vinyl liner practically has the consistency of soup and you should have no problem eliminating all coving from the pool completely - but if the pool needs a liner now, and colder weather is coming, then some coving of your liner might be the lesser of two evils at this point.
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