How Tight Should A Pool Winter Cover Be?
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When you want to find out how tight a pool winter cover should be the first step in getting the information you need is to determine exactly what kind of pool cover you have. It would be pretty easy to misinterpret some information that you read online or hear from a helpful neighbor to the detriment of your pool. The types of problems you can have from too much tension on your pool cover range from inconvenient like a tarp cover falling into the pool which allows leaves and debris into your nice, clean pool all the way up to damage or destruction of your cover altogether!
There are three main types of winter pool covers along with a handful of variations for each of these three types. Regardless of which type of cover you have something that is universal with all covers is that you must not attempt to fight the force of gravity. There is no pool cover system that is strong enough to hold the weight of even a few inches of water in the cover. Water is immensely heavy and the force of trying to lift or hold this weight is the source of damage for a lot of pools in the winter. No matter what kind of pool cover you have you need to avoid working against gravity. A pool cover must have the water underneath the cover to support the weight on top of the cover. So basically the cover is not meant to span the pool, but instead meant to partially span the pool at most and rest either partially, or entirely, on the surface of the water in the pool.
How much does water on a pool cover weigh?
Just for fun let's consider an average size swimming pool which we will assume to be 16x32' in size for this example. So how much would even a small layer of water weigh on this cover? Let's use 4" of water on the cover as an example number which is not even all that much water, but still likely to be very heavy. First the surface area of the pool is 16x32 = 512. The calculation for gallons in one inch of water is surface area (512) x 0.0833 x 7.5 = 319.87 gallons. For four inches of water this would be 4 x 319.87 = 1279.48 gallons. To convert gallons to pounds you would multiply the gallons (1279.48) by 8.3 = 10,619.68 pounds. So yeah, over ten thousand pounds of water for just a few inches of water on your cover. This is why you need to work with gravity and not fight it when it comes to your pool cover.
Of the different kinds of pool winter covers there will be some that sit entirely on the surface of the water in the pool (such as a tarp cover) while others will span across the surface of the pool deck with plenty of space between the surface of the pool water and the cover. That is at least until the cover is exposed to loading like from a heavy snow storm. At this point the cover would stretch down until it reached the surface of the water in the pool, or it failed under the weight of the load on it, whichever came first. Again this highlights the importance of knowing what kind of cover you have and understanding the limitations of trying to hold tend of thousands of pounds of dynamic weight.
Pool tarp covers
Pool tarp covers are supposed to be held in place by water bags or sand bags that are placed around the perimeter of the pool. The tarps themselves used to be thick and durable going back to the 1970's and 1980's but pool tarp covers now are typically very thin material suitable for a few years use in a best case scenario. They are just not thick enough to withstand sticks and other random sharp debris that finds its way into the pool. Even more commonly pool tarps will be incorrectly used which results in the thin tarp material attempting to absorb forces from the weight of the water in the cover. As we established above the weight of the water on the cover is impossible to hold even for the most robust pool covers so a cheap, entry level tarp has no chance to hold this weight. When properly employed a pool tarp should bear no weight at all from water on the surface. When you lay a tarp on the pool you should cover the start with a foot or two of cover on the pool deck, and then manually force the cover to turn 90 degrees down the wall and run straight down the wall until it reaches the surface of the water in the pool. This assumes that the weight of snow and water will eventually end up on the cover, and so you preempt this by removing all of the span across the pool right away. Another tip is that pool covers are usually oversized. Instead of having ten feet of pool cover on your deck, any place there is extra cover simply tuck it under leaving only one to two feet of cover on the pool deck, and this is where you place the sand bags or water bags.
Lock in cover (polyweave / vinyl)
A lock in cover is not the same thing as a safety cover although this often confuses swimming pool owners. A lock in cover is something used exclusively on vinyl liner swimming pools which use a bead lock system to retain the pool liner. A coping track is hung around the top perimeter of the pool and this is where the bead of the pool liner hangs from. With a lock in pool cover there is a second coping track directly above the one that retains the liner. This second track is empty during the swimming season, and during the winter season the pool cover hangs from this coping track. This system can be challenging to use and install on some swimming pools, especially elaborate shaped pools or pools which have a cover that is too small requiring extensive stretching to reach the full pool perimeter. The advantage of lock in covers is that they require zero deck space for installation and use, something which is unique to only lock in pool covers. Again, as with tarps and other pool covers these winter covers lack the ability to hold the massive weight of water so they must run directly down the pool wall and sit flat against the surface of the pool water. Vinyl lock in covers specifically have a tendency to shrink over time, especially when not stored correctly in a vat of water and algicide when not in use on the pool. A shrunken vinyl lock in cover may no longer rest on the surface of the pool water, and this would be strain on the coping where the cover connects to the pool walls resulting in cracking or bending of the coping track. It is important to never fight the weight of the water, and also important to store vinyl lock in covers wet when not in use on the pool.
Safety winter cover (mesh / solid)
Winter safety covers for swimming pools have been taking over the winter cover market for pools steadily for 30 years now. These simply are a better option for covering your pool for the winter versus other methods like tarps and lock in covers. Swimming pools are a huge liability from a safety standpoint and a big part of that is the winter season when pools can be especially dangerous. Tarp covers offer zero protection against animals or people falling into the pool. Even worse the tarp then becomes a liability as anyone falling in will surely become enveloped within the tarp...dangerous stuff. Safety covers are installed into the deck of the pool and connected with heavy duty stainless steel springs which absorb the stresses of weight that is periodically added to the cover in the form of snow, rain and the occasional animal or person who ends up on the cover. With the mesh versions of winter safety covers the water is intended to pass through the cover into the pool however snow and ice will still load the cover appreciably. This is why even with mesh covers the water level in the pool must always be high enough to support the deflection of the cover without allowing damage to the cover straps or cover spring hardware. Solid safety covers have two styles in that some are completely solid, where others are solid with the exception of a single open square in the center of the cover which allows water to drain through a mesh panel and into the pool. In effect this makes these covers the same as fully mesh winter safety covers. Full solid covers however require a cover pump in order to evacuate water that accumulates on top of the cover. Usually this pump will live on top of the cover and any time water is detected the pump will activate and pump the water off of the cover. Again, it is important that solid covers have a suitable water level within the pool to support the cover and any weight that ends up on top of the cover.
The springs used for installing pool safety covers are a heavy duty stainless steel spring which feel extremely strong. By hand you can not even budge the spring coil but under tension with the pool cover these springs can become stretched and even distended. Under too much tension the springs and hardware can actually become damaged so pool owners might wonder how much is too much tension. As pictured here the springs for your pool safety cover should never be more than half way compressed. Anything more than this leaves no further room for deflection from additional loads...and this is when damage can happen.
You can loosen the cover straps slightly to relieve some of the tension on the spring hardware. Alternatively if you have a heavy cover then you might want to upgrade from the standard single springs to the heavy duty dual spring option as pictured here. No matter which springs you use none are strong enough to hold tens of thousands of pounds of water. This is why maintaining the water level in your pool during the winter is so important.
Pool water level - One of the most important things you need to avoid in the winter as a pool owner is having a leak develop in your pool that goes undiagnosed since you have a cover on the pool. With the pool covered it is possible to not notice receding water levels in the pool. Eventually the weight of the water on the cover will pull the cover into the pool and it will break anything it needs to in order for this to happen. When water reaches a foot or so deep on top of the cover you could have over 30,000 lbs of water or more on top, and no cover system can hold this. As a pool owner you need to note the water level in your pool over the winter season to be sure that you notice any water loss. If you discover your pool is leaking then you should consult a local expert, and likely you will need to refill your pool regardless of the outside temperature as both the cover and the pool need water in the pool at all times. For more information you can read this article about water level in pools during the winter.
In addition to these types of pool winter covers some pool owners with automatic pool covers elect to use the automatic cover as the winter cover. While this is something that you can do depending on where you live and how harsh your winter conditions are, I tend to encourage pool owners to have a dedicated winter cover. Automatic pool covers are an amazing addition to a pool providing both form and function in the way of enhanced safety and security of your pool area combined with dramatic thermal efficiency and evaporation improvements.
The only catch is that automatic covers are a little on the expensive side and as such repairs or replacement costs can also be appreciable. With winter conditions being hard on pools I am inclined to say that a dedicated winter cover like a safety cover are best for the winter season, and you can save the automatic cover use for the swimming season. This should definitely reduce the wear and tear on your automatic cover and extend the service life you can expect to get from your expensive cover investment.
Once you realize that it is hopeless to try to lift thousands of pounds of water with your cover you are way ahead. Accept that the weight on the cover will eventually steal any material on the pool deck and you should not try to cove or span across pools with a regular cover. With a safety cover that is made to span across the pool you still need to be aware of weight on the cover which could potentially cause damage. Most importantly monitor the water level in your pool as lowering water levels in the pool from a leak can put undue strain on any cover system. Keep the water level in your pool up, and keep the load on your cover minimal for best results with your pool in the winter.
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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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