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Questions about a ground level deck and build techniques as well as drainage

Questions about a ground level deck and build techniques as well as drainage

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  #1  
Old 07-12-16, 09:00 AM
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Questions about a ground level deck and build techniques as well as drainage

Good day everyone. I am embarking on a fairly ambitious project and have a million and one questions but will save you all the drawn out story and focus on the basics.

I originally went down the path of a roughly 1200 square foot concrete patio off the back of my walk-out basement. Seems like a fairly straight forward project for a competent contractor however; I have hit a number of major hurdles that have left my confidence shaken. The main concern with the area is the very high water table. I have had 6 potential contractors our to assess and quote and each one of them had a different approach to deal with the water in a climate that sees very cold (-30 degree) winters.

So, in no hurry to throw away a pretty good sum of $$ on concrete, I'm now thinking that a ground level floating deck is the way to go. At least this way if the water does create a problem I can lift a few boards and address it.

Now, on to the questions. I don't have a lot of ground clearance at the exterior door. There is currently about 2" from the grade to the door threshold. My thought was to make a ground level path out from the door to an area roughly 20 feet out where the space will open up to a fire pit. To the left and right of the ground level path would be a slightly elevated deck, roughly 8". The elevated areas of the deck are straight forward using beam and joist hangers. What is the best approach for the ground level path? I have heard of laying a 2x4 or 2x6 on edge directly on the ground and attaching your deck boards to them. This would work with the low clearance that I have.

1.) Is this an acceptable approach? Can I assume that the decking will move with the freeze and thaw cycles?

2.) The next thing I have to be concerned with is water drainage. My thought is to excavate 6" and try to achieve a 5%-7% grade away from the house. We are in a heavy clay based area so my thought is with that steep a grade the water will follow the clay away from the house. At the end of the deck roughly 20 feet away from the house I will install a French drain the width of the deck, 12" below the excavated depth. The water should follow the grade and drop into the French drain. Is this a feasible approach?

3.) Assuming number 2 is an approved approach, I now have to deal with the back fill of the excavated area. The raised portions of the deck are not a big deal. I can essentially backfill with anything that will provide good drainage down to my clay base. 3/4 washed rock perhaps? The challenge is what to do under the ground level portion of the deck. If number 1 is an approved approach I am going to need a sub base that is compactable. If it is compactable like 3/4 road crush, then it will not have the drainage characteristics that I may required. Perhaps I could move to a 2" shattered rock with a jagged face. I know that would provide drainage but would it provide the compaction that I need?

4.) Yes, an exhausting thread I know but another question has popped into my head. For the raised portions of the deck I would prefer not to have to go with pilings as we have a 4'-6' frost line here. If I'm going with the floating deck concept can I get away with footings? My thought is to dig 12" diameter, 12" deep holes and fill with concrete. I would expand the base of the hole to 14" to prevent the footing from heaving. Would this work or am I stuck with pilings?

I appreciate any input you may have to offer. This will an expensive build regardless so I want to make sure I'm on the right path.
 
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  #2  
Old 07-12-16, 09:13 AM
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Setting the 2xs on the ground and then the decking will work - for a short time! With no air circulation under the walkway, the boards will almost always stay damp and will rot prematurely [possibly a few yrs]

IMO deck blocks would be just as good as footings that only go a quarter of the way down to the frost line.
 
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Old 07-12-16, 09:30 AM
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A good point about the moisture and a definite set back. I'm reading about pressure treated wood that is specifically designed to sit on the ground, same issue? I'm not sure what other approach would work. I suppose I could excavate 8" and back fill 6". That would allow me to raise the deck boards on the walk way 2". The 2x would still be flat on the ground but at least there would be air flow.
 
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Old 07-12-16, 09:34 AM
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Most 4x4 & 6x6 PT posts are treated for ground contact, most 1x & 2x PT lumber is not. I'd work on figuring out how to get rid of the excess moisture first. Gravel and foundation drain pipe run out to a lower location often works best ..... if that's possible on your lot.
 
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Old 07-12-16, 10:08 AM
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For sure. The current plan it to slope the 20 feet to a perpendicular drainage pipe running the length of the deck. Not sure if that will be enough. Should I be adding additional drainage under the deck? My thought is if I can create a steep enough grade and a consistent slope one drain at the end of the deck should suffice. That drain will then run An additional 30 feet to the back of the property.
 
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Old 07-12-16, 10:50 AM
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IMO it's best to do the grade and drainage work and then give it time to evaluate it's effectiveness before proceeding with the deck.
 
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