Gravel under the deck?

Our home got a tall deck at back of the house, the backyard has some slope. During rain, it became muddy on the ground (under the deck), also due to the drainage pipe was not smooth (might be blocked/narrowed somewhere under ground), big rain will cause water to build up underneath the deck around the foundation..
I ordered 8 yards of 1 inch gravel and put them under the deck, the deck size is about 370 sq feet. I think I ordered a lot more than what actually needed, the thickness is roughly above 4 inches and even about 7 inches at one side.
To stop the water to sink into the ground, I put a layer of plastic underneath the gravels.

Now someone told me this is a wrong way, and this will cause damage to the foundation, since water will build up there under the gravels and will not be dried by sun.

I want to know whether what I did was really wrong or not; if wrong, what should I do now?

Thank you very much for your answer.

The honest answer from me would be to find out the source of the drainage problem as covering it is definitely going to cause you really big problems later on, maybe not this year but for sure, it will be far more expensive to fix and probably more damage done at foundation area BUT, that is my personal opinion and after having to deal with unattended water seepage and blocked drains, believe me, deal with the problem now before you have mega troubles.

It may well be that the drains are fine and other areas need fixing but when water wants to move soil, building materials like hard landscaping or other structures, there is nothing going to stop it and that's the type of problems you MIGHT end up having to pay for repairs AND perhaps neighbouring property also, just giving you some ideas and experiences weve had in the past when taking over a property we looked at in lovely dry weather, never agin, always ask for full reports re flooding and drainage.

Now to your idea of landscaping the area that gets mostly wet and run off water from somewhere. Think of a tent being pitched in wet soil, then you cover it with a groundsheet so you personally don't get wet, all you have done is trapped the water / wet soil, under the groundsheet, you don't see it, for a while you don't smell it, but be sure, that water is spreading further outwards and downwards IF there is a slight slope, under plastic is this NOW wet and
heavy soil that as it warms a little, being airless, it begins to turn sour and smells like lord knows all what, but for all you know, there could be even more water added to that area because it is still leaking and nowhere to flow to, so disperses as best it can and that could well be going outwards into other areas of garden land or backwards into and under your foundations.
Plastic sheeting wont allow the soil to breath, will help hold onto the moisture or flooding, the plastic will shed new rainfall off the plastic and into a wider area AND maybe move the stones /Gravel you have laid onto the plastic so in reality your adding to the water problem.

IF after all that advice you still want to just accept there is a unknown cause of a water problems and go ahead to add a cover for the wet soil then my personal advice would be to either make trenches for soak-away water but fill these with some of your gravel that will allow some water to escape lower down the property BUT retain the same depth or level under the deck.
On top of this, add a thick layer of WOVEN landscape fabric as this will allow the soil to breath, dry, and get aired and still prevent the Gravel from disappearing into the wet soil, it wont bring peace of mind with regards you foundations being constantly wet.

maybe the cost of paying for professional drainage advice, it will be cheaper in the long run and you wont need all that gravel in the first place.

Have you checked your home insurance ???? maybe they can guide you or offer where to seek advice, maybe it's a problem that is NOT your doing BUT your at the lowest end of water soak-a-way. These are all areas you need to look into first before you maybe cause more damage to your OWN property and perhaps someone else's as water has to flow to the lowest point.

Hope this all helps give you food for thought and you can decide the best course of action but covering a wet area with plastic unless you want a pool or water garden, is never a great idea.
Best Regards. WeeNel.

Ditto WeeNel:
FIRST step would be to fix the drainage issue.

Un-block the pipe, or run new pipe.
Add catch basins to collect surface water.
Regrade the soil under the deck so the basic slope is away from the house, and into the catch basins.
If needed, install what is commonly called a French Drain- a trench with drain pipe and drain rock to collect sub-surface water.

When there is no major way that water can get under there....
And whatever water does get in can be removed promptly...

THEN Plastic and drain rock is a very good way to protect the area under the deck.
Keeps animals out that might dig.
Prevents soil erosion.
Prevents weeds.
and so on.