Solution for moisture problem in low deck build?

Old 08-07-14, 10:47 AM
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Solution for moisture problem in low deck build?

Hey all,

I'm putting in a deck which will be pretty low to the ground in an area which has had moisture problems largely due to poor grading/drainage and a relative lack of direct sunlight since it is blocked by the house on one side and woods on the other. The back of the house faces East, so the direct sunlight it does get is mostly at mid-day. The yard does not get all that much wind due to the same reasons.

I attached a picture to show you the area I'm working in. Rain water tends to pool up in front of the cement slabs, and you can see where moss has grown on them since the area is often damp.
I live in Philadelphia, PA which gets average amounts of precipitation and all 4 seasons.

The deck will be 16'x14' with the long side running parallel to the house. It will be 15" high coming off the rear wall of the house, and approx 24" high at the opposite end.

The sub structure will be pressure treated pine. I've decided to go with 5/4x4 ipe for the decking since i was told this would help with potential cupping.

I'm planning on fastening it with stainless screws through the face, but was wondering if it's possible to screw it on from the sides at a 45 degree to hide the screws, which is what a friend did with his pine deck, though the boards were a different size.

To help with the moisture problem, my plan is to re-grade the yard so water flows away from the house (like it's supposed to), then lay down landscaping fabric and cover it with a few inches of gravel where the deck will sit. I'll also treat the ipe with a sealant/moisture retardant on all sides.

That may be all I need to do, but I'm thinking of taking it a step further and waterproofing the underside of the deck using polypropylene membrane which would basically create a dry environment under the deck. The waterproofing method I'm considering is demonstrated here:
Low-Cost Deck Drainage: Landscape membrane and off-the-shelf gutters keep the space below new and existing decks dry - Professional Deck Builder Magazine

I'm the home owner and have never built a deck before, so I'm wondering if the under deck waterproofing (the membrane, not the sealant) is over-doing it, or if anyone has better suggestions.

Thanks in advance for your comments and suggests.
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Old 08-07-14, 06:28 PM
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Nothing is going to bother Ipe. That's used for the boardwalk at Jones Beach in NY. The stainless steel screws is another great idea not to mention regrading the yard. If you do all that, you probably don't need the membrane, under the deck. They are all great ideas. Just make sure that you use the right footings, sono tubes filled with concrete, to the frost line.

Edit:You will need to pre-drill holes in Ipe before you try to screw it, to the joists. That's how hard that wood is.
Old 08-07-14, 07:33 PM
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I wouldn't try to hide the screwheads by installing at 45-degree angles. Too much of a chance for splitting, and for individual planks to lift at the sides they aren't fastened along.
Old 08-15-14, 11:05 AM
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It looks like I wont be able to increase the grade all that much due to the elevation limitation at the rear of the house (only have 15" to fit in the beam, footer, and connection hardware), so I think I'll put down the membrane even if it turn out to be overkill. I'll be using 12" sonotubes and using the stainless steal screws to face fasten at 90 degrees as recommended.

Thanks again for your feedback, much obliged!
Old 08-15-14, 12:25 PM
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I'm assuming you get some runoff from the roof and being in PA I assume you know that gutters can be a pain in the winter. Basically, I see those troughs being full of ice that can't melt or be removed.

IMO, I would crown the dirt below the deck and cover the ground with a good black membrane, maybe some decorative gravel over that. With the crown and slope away from the house most of the water will exit and eliminate most of the moisture problem. Their trough solution only protects the bottom of the deck boards and not the deck framing. With the plastic on the ground and sloped away the entire deck will stay dryer.

Also, if you have any moisture dripping from the roof above you will ultimately have some moisture problems with the siding. No solution to suggest, but a frequent issue.

In winter, be sure to clear all snow and ice, even if you don't use the deck. Allowed to accumulate it can form a barrier that can force water past the siding into the house.

Not trying to be a pain, just been there and seen that. Hope it helps.


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