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I want to use the space under my deck for entertaining when its raining. What would happen if I used one of the thick bed liner type wood coatings and simply applied it to the decking and also into the gaps, After a rainstorm I was thinking the water would evaporate or I could squeegee it off, I also thought about installing a composite type material underneath the floor boards between the joists to eliminate the coating from dripping through the cracks while it is drying. After it all dried I would possibly remove the composite material and coat the underside of the deck.

So could you give me any reasons not to do this?

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I have installed membranes underneath decks and ran them to gutters to provide an under deck patio space. To answer your question, no you can't do that - for the easiest reason - because it won't work and your guests will get wet. And for a hard reason, it will tear up your deck with wood expanding and warping due to pressure or wood rotting due to standing water.

I would ask a follow up question that asks possible ways to divert water, so that people can answer that.

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I don't think this will work. When the decking expands and contracts it will tear apart your coating. What they do in apartments where upper decks overhangs lower ones is use a membrane. First you need to apply sheeting, then you need to tape/seal the sheating joints. Then you apply several coats of the membrane with a roller. Most home improvement centers sell the kits (membrane tape and grit) but it pretty expensive. I have done two decks this way and they have lasted for years. Another thing I have done, but it is much more of a hack, is to install a corrugated roof under the upper deck.

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You don't want to completely seal up the spaces in between your boards. As @joncon said in his answer, that kind of coating won't last very long because the boards expand and contract with the seasons.

You also don't want to do this because the deck will not drain properly. You most likely would have issues with water pooling on the deck boards and causing them to stain, warp, and eventually rot. Standing water could also be a hazard in the Winter. There could be a sheet of ice on there which could cause a deadly slip and fall.

The best way to use the space below the deck if it is raining is to install a roof or awning over the deck. There are tons of options for this. A retractable one might be perfect for you since you can close it in the Winter, or in storms.

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