Chlorine vs Winter Covers


Jun 12, 2016
West Babylon
TLDR: Should you cover your pool when there is still chlorine in it, or does chlorine degrade your winter cover? Especially if you bring to shock level.

So after reading this forum a few years ago, I started shocking my pool before closing it - just to make sure nothing is left in the pool and to have a smooth opening next year.

Last spring I opened the pool. and it was cloudy, had to SLAM for 3 days to correct. Also my water level was lower than I had made it the past winter.

This year I put my winter cover on, and within 30 mins, I noticed some water pooling on the cover. I tried patching one tiny pinhole but after it rained it was impossible to see where the other holes are. I tried running my cover pump to dry the cover, even pushed water towards it, but it was a never ending fight. I let the cover pump run all day, and now my water level is way below where I left it. Anywhere I push down on the cover, water seeps through, there must be water seeping through everywhere, even though there is no visible holes.

A friend of mine said he no longer shocks his pool and he actually removes chlorine a few days before closing, and just uses an algaecide (which I also use). He heard that trapping the chlorine gas under the cover can wear down the cover.

I purchased a very good winter cover with a 20 year warranty, and it barely lasted three seasons....I bought it with my pool just over 3 years ago.

So is there any truth to this? It makes sense because if you ever tried to remove a stain from a white shirt, and you leave your shirt to soak in bleach for a while, the shirt will just shred to pieces easily.


Jun 12, 2016
West Babylon
I did come across this, does say not to shock and then cover. I did that this year, and I think last year I poured a half gallon of liquid bleach just before covering (again got that idea on these forums)



Gold Supporter
TFP Guide
Jul 21, 2013
Northern NJ

A combination of chlorine and Polyquat 60 seems to be the best prevention against a green spring opening.


If you raise your water to SLAM FC level then let the FC drift down before adding Polyquat 60. Having a moderately higher level (say, half-SLAM level so FC/CYA ratio of 20%) is OK, but the idea is that the Polyquat should last through the winter even if the chlorine does not.

I have a 20 year old Meyco cover that still works. My pool is closed with normal chlorine level and Polyquat 60. I have patched holes in my cover with wide strips of duct tape. The holes come from mice eating the cover while stored and from areas rubbing against the coping. Once I learned where the wear spots were I put folded towels down when the cover is put on to cushion those areas.
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TFP Expert
LifeTime Supporter
In The Industry
Jun 12, 2009
NW Ohio
TL;DR is No, closing at SLAM FC level is not going to hurt your cover.

Bottom line is that the advice you are getting is from people who do not understand the relationship of FC and CYA. SLAM level is actually quite low in the active FC department, thousands of times less than your shirt example. If you close after the water temperature is 60 or less (as recommended here) then the chlorine will actually be quite non-reactive and definitely won't be forming chlorine gas as you friend seems to believe. This is doubly true if your water is clear and there is not much in it for the chlorine to be reacting to. On top of that, using minimal chlorine and algaecide opens your water up to bacteria that might feast on the CYA and leave you with a nice ammonia present to deal with upon opening. No guarantee it won't happen anyway, but chlorine helps protect against it and polyquat doesn't.

I have had my cover for 6 seasons now and there are a couple holes in it. Those holes, however, all all above the water line and could not have been caused by chlorine. Indeed the sunlight hitting the cover is far more damaging than the chlorine. Same with debris, freeze/thaw cycles, and general stress from the wind and water pulling and pushing. I close at full SLAM and open to clear water that still has an FC reading. I even pump water in from on top of my cover to decrease the amount of high-TA well water I have to add in the Spring and still manage to have clear water with only a day or two SLAM.

I haven't added polyquat to my pool once with this cover, so maybe my experience shows that polyquat kills pool covers. It doesn't, but that should be a reminder that the advice people give can be based upon misunderstood observations.
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Jun 12, 2016
West Babylon
So you all that don't have issues with your pool covers, do you drain the water off or just leave it? I have trees near by and wind always carries them into the pool. I try to remove water and leaves as much as possible up until it freezes.


Silver Supporter
Jul 8, 2013
Columbus, Ohio
I've used a similar solid cover for twenty years. I ran through two of them, each lasted about 10 years before they started to show enough wear that I wasn't sure they were safe enough. I probably could have squeezed a couple more years out of each one before replacing them. We will see how long the auto cover I have now holds up.


Bronze Supporter
LifeTime Supporter
Apr 10, 2009
When we moved into our house, the previous owners left the pool cover. It was the solid tarp kind but it was heavy and worked great. That is until one winter I watched a branch come down like a harpoon right into the center of the cover. That was the end of that cover unfortunately. Tarp covers these days are awful. You're lucky to get a couple years out of it. I found no matter what you spend, they're all like that. We use water blocks around the cover and I find those easier to use than water bags. We wait until all the leaves and junk from our monster maple tree fall and then my husband cleans leaves off the cover and we pump off the cover. In order not to suck up the water with the pump, we put the pump in a plastic basket like the kind you use at a grocery store. It keeps the pump off high enough so it does't suck up the water in the pool and only the water on the cover.

We usually close at slam or half slam levels and haven't had any problems.