Water leaking in via cantilevered deck joists.

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In my new house being constructed, an engineer-designed balcony is standard 2 x 6 pt decking placed on cantilevered joists made of double 2 x 12s nailed and screwed. the joists extend about 7' out and twice that length inside the home where they're attached alongside Trimjoists at 16" o.c.

Problem? During a recent Texas thundercloud, we had excessively heavy rain and high wind. Workman on site noticed that the small groove formed by the side-by-side 2 x 12s allowed water to be channeled along full length of interior portion of the joist cantilevers, dropping significant water on what will be my living room ceiling.

Of course, we're going back and trying to seal/caulk everything possible and we're glad this happened now and not in six months, when I'm moved in.

I'm wondering if the entire 2 x 6 deck plank assembly should be covered with ply and a butyl or epdm suitable for foot traffic and roof decks.

I'm fearful that, while beefier caulking and sealant may hold for a while--we'll water test, believe me--, the next indication I will have that the caulking has succumed to the elements or the inevitable give-and-take of a cantilevered wood deck will be a 2-foot circle stain on my ceiling. Not a pleasant thought for a new homeowner.

Any suggestions for a way to cover the decking (forming, basically, an umbrella) to protect the 2 x 12 supports that are the problem?

Ideally, the material should be lightweight and light in hue. The engineer has said we can cover the existing deck with 1/2" ply--sloped away from house--and then seal the entie assembly. The balcony can support that much more weight but the sealant is the question, as foot traffic and deck furniture must be considered.

Another solution was to use a 'big stretch' and foam filler rope to seal the gaps between the existing 2 x 6 planks and then paint or torch down an elastomeric over what we have. This eliminates extra weight from additional plywood. Of course, heat gain and maintenance are a problem with some coatings.

This issue is huge because we have two decks--each 30' x 7'--and these decks are a key design feature, having a view of a park below and the downtown skyline as well. They'll be used a lot and must provide comfort for guests while not subjecting my house to inclimate water seepage.

Suggestions on engineering, materials, prayers?????

Thanks.

Kevin