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Old 06-29-2014, 06:02 PM   #1
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Ground Level Deck (Ventilation issue??)


We recently had a ground level deck (made of Batu) installed and after about 30 days it went from looking great to needing to be replaced.
It’s been sunny & warm with no rain for at least 45 days so off the bat you wouldn’t expect any immediate & obvious moisture problems. Thx to a bit of overwatering from our neighbors (whose yard is a few feet above ours) the ground under the deck got pretty wet causing the boards to cup and in an extreme case it caused 2 boards to expand so much they came off.

It appears that poor ventilation is a likely cause of this problem as we have 0 vents installed, the deck butts up against the house on 2 sides and runs straight into an up slope in the rear of the yard and then butts into the fence on the 4th side. Other than the 1/8″ gaps between boards (which are non-existent in some areas where the wood expanded vs contracted we have what appears is very little airflow)

So now we have to rip it all up and do something…but what we do isn’t clear:
1) should we simply excavate a bit under the deck to create a bigger air space
2) do we put vents between each joist to improve airflow
3) do we stick with the Batu (3/4″ or go with a thicker board and face screw it)
4) or do we replace with a composite or PVC?

Any suggestion for a very confused homeowner on options to rebuild? The added expense will be annoying to say the least but we have to get it right…i couldn’t handle having to build this for a 3rd time.

Here are some of the deck specs:
- Batu (5.25″ by 0.75″)
- Installed with Clips vs Surface screwed (no glue either)
- Sealed with Penofin
- Gaps between boards were 1/8″
- Joists every 16″
- ventilation gap is approx 5″ to 6″ under a good portion of the deck (nearer the home it’s closer to 30″)
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Old 06-29-2014, 09:37 PM   #2
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Never a good Idea no matter the wood to not have any air flow to dry it out under it.
Should have been a stone, pored slab, patio blocks, ECT. not a wooden deck if t's to close to grade.
Knowing where on the planet would be a help with suggestions.
Decking never should have been installed that close to the siding.
No clue what Batu even is.
Even pressure treated 2X's are not rated for direct ground rated like that.
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Old 06-29-2014, 10:34 PM   #3
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I certainly agree with Joe, that should have never been built as a deck, a much better solution would have been a poured slab. I also have no idea what Batu is? Assuming it's some kind of pressure-treated wood is normal to wait until it completely dries before you put any sealer on it which may be part of your deterioration problem. I don't believe there's any way you can get sufficient ventilation to make this work.
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Old 06-29-2014, 11:34 PM   #4
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we are located in the San Francisco area. Batu is similar to Ipe if you're familiar with that.

Half of the deck has 30+ inches of clearance underneath (and no moisture problems) - from the great feedback everyone's provided across a few forums it's clear this should have been excavated to get more than 6" of clearance and we should have gone with composite (couldn't do a slab or stone as we have some existing retaining structures under the deck that cause the clearances to get much bigger closer to the house.
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Old 06-29-2014, 11:47 PM   #5
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I built a ground level deck back in August of 2010. It is about 12' x 14' and is holding up very well. I used pressure treated 2x8 for the joists, spaced 16" on center, with the structure resting on concrete blocks, a few inches above grade. The decking is 5/4" cedar. Before building it I removed all the grass and some soil and laid weed block fabric and then spread a layer of pea gravel. There is good drainage and so far (knocks wood) I have had none of the problems that others have mentioned. Did you prepare the ground properly to assure adequate drainage? I know you mentioned no rain for awhile but something is causing this problem. I wonder about this decking you used. It seems to be a dark material so maybe it is absorbing too much heat on top while staying cool below causing the material to warp. By the way, the gap below my deck averages only a couple inches.

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Old 06-30-2014, 10:15 PM   #6
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I am familiar with IPE, it's a very good though difficult decking material to work with. Based on that Batu should also be a good decking material. If you have ventilation on one side of the deck and the joists run so that the area between each joist pair are able to vent into that space you will probably be all right as long as you prepare the area under the deck like Dave suggested.
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Old 06-30-2014, 11:23 PM   #7
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Thx for all of the replies - I actually consulted with a soils engineer to confirm a suspicion that the water was a bit of a red herring. it turns out that when you have a tight, poorly ventilated space that water will actually seep up to the service. This is more commonly a problem in crawl spaces but also an issue with low level decks. The rug on our deck only exacerbated the issue as it further killed the little airflow we had.
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Old 07-01-2014, 09:35 PM   #8
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Quote:
Originally Posted by SF_Hokie
Thx for all of the replies - I actually consulted with a soils engineer to confirm a suspicion that the water was a bit of a red herring. it turns out that when you have a tight, poorly ventilated space that water will actually seep up to the service. This is more commonly a problem in crawl spaces but also an issue with low level decks. The rug on our deck only exacerbated the issue as it further killed the little airflow we had.
Thanks for the info on the rug. My wife has been talking about putting some sort of large mat down on top of the decking. It sounds like that may have been at least partially to blame for your problems. I will definitely nix this idea.
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