Scale forming ABOVE water line??


Aug 9, 2019
Bethlehem, pa
I’ve noticed a weird problem and hoping someone can shed some light on it.

Previous week was warm in our area (uppers 80s to lower 90s, humid sometimes). This week was much cooler, starting on Sunday. I noticed on Sunday that we were getting scale forming on the tile line right around the grout lines...but the scale was forming near the top of the tile line (a good 2 inches above the water line). Water is balanced and I don’t see scale forming anywhere else near the water line. The only difference from weeks prior and when the scale formed was the air temperature, which dropped to mid 60s overnight on Sunday. Water temp was in the mid 70s that morning. The scale was brushed off easily.

I noticed the same issue when we opened our pool. It looked like scale formed on the tiles around the grout lines over the winter and a bunch of tiles also flaked/cracked right around where heavy scale formed...scale formed all around the pool but a few localized areas had flaking tiles. The water line was well below the tile line throughout winter so I”m confident it isn’t ice damage (we have a mesh cover, but I drained the pool a few times to keep it below the tile line). Ph and alkalinity was within range when we closed, but not sure about CH. This was much harder to clean up and didn’t simply wipe away, so I resorted to brushing with diluted muriatic acid.

I’m led to believe that it has something to do with the water and air temperature difference...the evaporating vapor from the warmer water condenses on the tile line and “leaches” calcium or whatever from the grout between the tiles...anyone have any other ideas or seen the same? And anyway to prevent this from happening, since it seems to cause damage to the tiles over the winter when the pool is closed (the edges of the tiles flake off)

Specs below if needed:

Pool size: 20x50ft (approx. 45k gallons)
Type: Plaster
Age: 15yrs
Sanitizer: SWG - T-cell-15,
Supplementing with liquid chlorine (12.5%) when needed for shocking (haven’t needed since SWG was turned on this season)

Current balance (as of this morning)
FC: 3.6
CC: < 0.2
Current pH: 7.4-7.8 (I’ll add Muriatic acid at 7.7-7.8 to bring it down to around 7.4 in 2 separate doses)
TA: 100
CH: 310
CYA: 70


TFP Expert
Mar 2, 2011
Probably efflorescence. Water migrating from behind the tile weakens the mortar and pulls minerals from the mortar.

The minerals are deposited as the water evaporates.

If you have a freeze with water behind the tile, the ice can cause cracks and delamination.
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