Grading soil underneath a deck

[FS7]

I'm building a deck/patio (yes I know the risks, yes I've done it before, and it lasted for 25 years). My home is properly graded and I have never had any moisture problems. This is going in the back, where the grade slopes in two directions. If water pooled against the foundation, it would be below the slab.

I did (and do) have to excavate to make room for the footings. At most, it's about 7 inches of soil. My plan is to have a "straight" grade away from the house for the excavated area - the deck will be about 12'x40', so it should be something like two inches of slope from the house to the rim joist.

I don't want any water problems. There is already corrugated drain pipe around the foundation, but I still plan to grade it away. Does it matter if I don't follow the existing grade? I assume not as long as the drainage direction is away from the foundation (even if it does deviate from the present slope). Also, how much leeway do I have given that I already have a drain installed?

[Bryan11]

When planning this for our deck, my goal was to make water drain as far away from the house as possible. I first graded the soil so it all declined going away from the house, probably about five degrees. Next, I put some heavy, 6mil black plastic on the soil and then used a hose to verify water drained as expected and there weren't any low spots. Finally, I added some heavy river rock onto the plastic to protect it from dogs and such. The deck was built on top of that and it's worked well so far.

[Splinter Puller]

When I build my own decks I like to grade the soil underneath with a decent pitch away from the house... I lay plastic down and then add a could inches of nice pea rock over the plastic. This looks good, is easy to crawl on if I need to go under the deck and keeps the water out.

I've had to retrofit a few installations because the soil settled near the house under the deck and water drained towards the house causing moisture issues in the basement.

[FS7]

I've been told specifically not to use any sort of vapor barrier (it's low enough that I won't have access underneath). I'm using wood so it will be spaced sufficiently to allow for breathing.

I also found out that I don't actually have a drain on that side. The other three sides, yes, but not for the walkout basement. I think I'm going to grade out about four feet and then bury a drain in there.

[Snipe Hunter]

The rule of thumb for grading is 1/4" pitch per foot. I wonder why there's no drain tile on that side of the house?

[FS7]

Snipe Hunter said:

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The rule of thumb for grading is 1/4" pitch per foot. I wonder why there's no drain tile on that side of the house?

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According to the building supervisor, they don't do drain tile if the finished grade is below the level of the basement floor. The soil, however, is graded appropriately. This makes some level of sense. At its highest point the rear grade is probably an inch or so above the level of the basement floor at most. I guess they felt that proper soil grading combined with Virginia clay would preclude the possibility of water coming under the slab. They do four side drain pipe with walkup basements - just not walkouts (which I have).

It rained a good bit today and the current excavation does not pool near the house but near the end (which I expect, because it's a four inch clay wall at that point). So that's good. I'll still probably put a drain pipe in about four feet from the house to make sure water doesn't stand under the deck since I have to redirect a gutter anyway.

[Snipe Hunter]

Okay, that makes sense.

[crokett™]

I'm confused as to why you are re-grading under the deck if it's already properly graded. If it's already graded, any water that makes it through the deck will still drain away from the house. When you say excavate for footings are you planning on a solid continuous footing or post holes? If it's post holes, they won't care about the grade either.

[FS7]

crokett™ said:

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I'm confused as to why you are re-grading under the deck if it's already properly graded. If it's already graded, any water that makes it through the deck will still drain away from the house. When you say excavate for footings are you planning on a solid continuous footing or post holes? If it's post holes, they won't care about the grade either.

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The deck is basically a patio. I would need to excavate for any patio, be it wood, pavers, or concrete, as the step down from the walkout (which is the lowest point of the yard) is about 5-6 inches. It slopes up, and the soil there was about 1 inch below the level of the walkout. Obviously I want the deck to be level (or close to level) with the walkout, so I had to excavate that soil. I am doing beams directly on footings using Simpson anchors, so I have to have the clearance for the beams (2-2x8) and the joists. Now it does slope down, so technically I wouldn't need to excavate for the outer half of the deck, but I do want to make sure I don't get water under the slab. I also want to make sure that I get sufficient airflow under the deck to aid in drainage.