Pictures?

You should have made sure the deck did not leak onto the lower patio.

water hitting the same spot over and over is going to wash out even poly sand. With a deck, you have a bunch of concentrated 1/8" slits dumping water on the patio with every rain, so it's not a shocking result. You could use regular fine sand in the joints, just know that you'll be topping it off pretty regularly.

What were you using to clean off the poly sand?

SO it seems that the landscaper should have warned us that a patio under a dedicated water flow will not hold its sand. I was using a mild cleaning solution but on the pavers to get out bird droppings!

It depends. Some decks have a ton of water flowing through, others don't. We've done dry-laid flagstone (so 1/2" joints) under decks with zero issue. But clearly, for whatever reason, you're getting a lot of water coming through.

As far as cleaning, never use anything not recommended by the paver manufacturer. Pavers are bulletproof right up until they're not. We followed manufacturer's instructions to remove efflorescence from some pavers and even using that product it stripped the color right out. But since we used what they had said to, they were covered under warranty.

That is very helpful. Thank you. Our deck also has some problems but there is a lot of water. I never received any instructions from anyone about cleaning the efflorescence but will approach Belgrade.

Usually when installing any kind of patio below a deck, a "ceiling" for the lower patio should be installed so that water doesn't comedown onto the level below. The ceiling could be just some metal or acrylic roofing attached to the underside of the deck. If you use acrylic, the light would still come through.

Thank you everyone. Very helpful. My disappointment is that the people who install these- aka "Landscapers" in business should , in my opinion, have the knowledge to inform their clients of the pitfalls of what they have chosen to do. I have learned the hard way and would now advise anyone putting a patio under a deck to test how much water comes through, and then make sure their deck is in good shape, and if needed install a ceiling system with drainage before installing the nation.

I cannot imagine a more impossible maintenance chore than an acrylic ceiling below a deck ... where you can look up and see every leaf bit and mildew crust, yet have no way to clean it!

Germaintoo, on the flip side, you'd probably be amazed how impossible some clients are to reverse on a bad idea after being informed of it by the contractor or designer. Many of them want what they want and sometimes refuse to acknowledge the reality of what comes later.

Below slatted decks is not a great place for living, or even storage in many cases.

"Below slatted decks is not a great place for living, or even storage in many cases."

And yet, that is sometimes what we are forced to deal with :-) My entire entry and patio area is underneath a slatted deck. However the ground surface is a concrete pad so not the same issues with the water falling below.

And unless we are experiencing a heavy downpour, the water penetration through the overhead deck is not as bad as one would think.........it does provide a degree of protection (some......just not too much :-))

Reality ... so often getting in the way. :-)

If there were some slight spacing between the decking and the acrylic ceiling you would be able to wash it with a hose to get rid of any debris than might collect there. It would also have to be installed with a slight downslope as well.

I kind of disagree about the landscapers having to tell you that water will fall through your deck....I mean, you know this, right?

Are your gutters clogged? (just a thought to try to ameliorate the amount of water...) You really shouldn't get too much more than "normal" the amount of rainwater...obviously you'll get more than elsewhere because the rain has to go somewhere off the deck.

We found that we did have some sand loss on our patio, but as time went by it compacted more and settled enough that I didn't have to sweep it after every time it rained...

I don't need a contractor to tell me water seeps through the deck that is a given but I need a

There are under deck roof systems they sell that are slanted and have guttering so water runs out and doesn't stay. I think it might end up being a debris catcher . I think we will just live with the patio until it dissolves. Some of the tiles appear also to be lifting. I think our original idea of real stone and butted up to each other close with no sand was the better way to go in this situation but we couldn't find anyone in the area who would do it. And for the record, we would have backed away from sand filled cracks and cement pavers had we been told about the efflorescence and sand melting away. Certainly would have had a discussion about alternatives. Thank you everyone again for your input. Much appreciated.

too bad you are in Virginia!!!

We have this too. Even in areas that are not under our two decks. And it never rains here. Just condensation from fog or dew drips in certain places from the deck.

So is it the nature of the pavers? Do you think real stone would be a better bet?

We had real stone over cement for a courtyard at our last house - zero problems. But here there is movement so the cement cracked and pulled away from the house and we had to go for pavers. We only have polymer sand up against the foundation. You have to add more polymer and apply mass quantities of sealer to turn it hard, but it has to be dry to do that.

What kind of sealer did you use? We were not told about any sealer except for the pavers. Is that the same? Or is there one for the sand only?

Yes it was the same sealer as the pavers. You can tape it off and literally pour the sealer from a water bottle into the repacked joints. Spraying doesn't get it wet enough to turn almost like cement. Also don't want sealer on wet pavers, you'll get more effervesence.