Still trying to waterproof area below my Deck

[Tom H]

I have found a couple of under deck methods of channeling water from a deck but they all go under the deck boards or joists. This won't work for my application.

Earlier I posted a possible way to water waterproof under a deck by cutting spline pockets into the edges of composite deck board and using silicone to seal the splines. A friend suggested using space ball (used to keep door panels from rattling) to keep the spline centered and allow for expansion.

Another suggestion was to cut a piece of vinyl siding 'J' channel into a 'U' channel with the top two bars of the 'U' about 1/2" long. Then making a table saw cut into both sides of the bottom of each deck board about 1/2" deep and 3/8" in from the edge.

As each board is installed, silicone would be inserted into each saw cut then inserting the 'U' channel. Water would fall between the deck boards and be channeled off the deck through the 'U' channel.

I would use an under deck fastening system to secure the deck boards.

Does anyone think either of these methods would work?

[Al]

I am not sure where I am coming into this at but is there no sloping for dranage?

Al

[Charley]

I'm coming in late too, but the U channel idea sounds like it has way too many joints and places that can leak, even if an attempt is made to seal them.

A relatively level surface that's impossible to get to for maintenance, because there's a deck above it, will likely collect a lot of small debris that will form small dams for the water flow and will make the tiniest pin hole become a big leak. Only a single sheet of thick plastic or something with no holes in it at all is going to work under a relatively level deck and stay waterproof for any length of time. Even that type of surface will require some maintenance, eventually, and you won't be able to get to it without major dis-assembly. If you were to build the storage space with a slightly sloped roof and then add deck platforms above it that could be removed for roof maintenance I believe it would be the best way to go. Decks that are built on top of flat gravel roofs are built in sections so they can be moved easily when the roof needs repair.

[J. Clarke]

Any method other than building a proper roof under the deck is going to be a kluge--if it works reliably then you may have come upon a new building method that will eventually get into the codes, if it doesn't, well, then you have to try something else.

The way that a boat is waterproofed is to cut the seams v-shaped and pound in some caulking then cover that with caulking compound. This leaks a little until the wood swells, then it is pretty much watertight. Trouble is that when it dries out it leaks again and the caulking has to be replaced periodically. With synthetic lumber and flexible sealants that _might_ work as a long term solution in your application or might not.

With your U channel you need to figure out how you are going to clean it and what is going to happen when it gets clogged and how you are going to fix it when a hole gets knocked in it and make sure that it has enough carrying capacity for the worst-case rainfall in your area. In an ideal world some cup in the boards and sloping them a little would be a good idea so that most of the rainfall is carried down the center of the board instead of being channeled into the cracks.

[Knotbob]