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Old 09-18-2009, 08:29 PM   #1
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Deck clearance over concrete




This is what is left after tearing out the old deck. The door is 10 inches above the 10-ft wide by 8-ft deep concrete aggregate pad. The intent is to put in a patio on 2/3 of the level area beyond the pad and a free-standing deck extending over the pad and the contoured area. It is a 30-inch drop from the top to the bottom. There is about 26-in strip of dirt between the wall and the lower concrete area.

You can see a little erosion area at the pad/wall corner that needs to be shored up. Perhaps I will put in one or two terraces between the top and bottom, but it is a difficult area to work with - for me.

Here is a tentative first plan for dealing with the deck area over the pad.

1) 4x8 beams on each side of the pad, running out from the house. The ground-level side will lowered for the patio base and to accommodate two or three precast footings laid on the packed dirt or a little crushed stone. Beam span is about 12-ft.
2) Top of the beams will be about 1.5-in above the pad. (To accomplish this, most of the footings will be buried below grade and the posts will sit on adjustable anchors.)
3) 2x8 hem-fir joists will sit on the beams at 12-in OC spacing and a span of about 10.5 ft.
4) Add 1x6 composite decking.

Span tables conflict and I don't know if this plan is reasonable. Is there any other way to do it aside from a concrete grade beam? It isn't much of a joist clearance (1.5-in), but I hope GC rated PT hem-fir will work. Could I get away with 2x6 joists to increase the clearance?

Any ideas about what to do with the rest of the area are also very welcome.

PS - Rain is an issue.

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Old 09-19-2009, 11:24 AM   #2
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Deck clearance over concrete


Lay the deck joists right on the pad extending out. Keep the deck at one level and build a staircase to the place you want it.
Ron

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Old 09-19-2009, 11:52 AM   #3
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Deck clearance over concrete


Building 2/3 beyond the pad towards the pergola?
Or over the lower concrete pad?
How big overall will the deck be?
Need some more info.....but----

10" drop from door to pad
1" thick decking? not sure on composite
2x6" across the pad = 5.5"
Cement block between pad & 2x6 joist = 1.5"
Total = over 8", so only a 2" drop between the slider & the decking
I like a 6" drop - but I am in a snow area

I have a small 12x12 out back that is near ground level
I used small concrete cloks (free off craigslist) to support most of the joists
I'd do the same with your joists over the pad area
That way the entire length of the beam is not in contact with the cement

But with pressure treated you can lay then right oin the concrete as Ron said
But that would mean that if you are building toward the pergola they would be at ground level
Careful on digging out dirt near the house- you don't want to create a slope or lower area near the house for water



This was built almost completely out of someone's pool deck I took apart



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Old 09-19-2009, 02:54 PM   #4
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Deck clearance over concrete


With the new code your joist span (w. 2x6) and beam span are too much. Do as Ron said with the joist direction. Check with your local Building Department as most require p.t. wood when that close to grade. http://www.awc.org/Publications/DCA/DCA6/DCA6.pdf
Be safe, Gary
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Old 09-19-2009, 07:24 PM   #5
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Deck clearance over concrete


Thanks for the comments so far. Let's see if I can give you more to work with.

The proposed upper deck level will extend from about two feet this side of the porch pad to 6 or 8 feet on the other side toward the arbor. Some step platforms will take it down to the lower level on this side.

Joist span would be 10 ft across the pad. Because the beams run away from the house and are not over the pad, span for them is not an issue. I would like to use 2x6 joists to increase the distance from joist to concrete, but the span tables I have seen suggest 2x8 is the smallest I should consider for this span.

All of the composite decking I have looked at is 1 to 1-1/8 thick (true measurement).

I am reluctant to put even GC PT wood in direct contact with the concrete for three reasons: 1) The concrete aggregate stays wet after a rain longer than flat concrete, 2) drainage would suffer and fir needles, maple "copters" and other junk from overhead trees would collect between the joists, and 3) from what I understand GC in the MCQ, CA and other forms isn't as durable as it used to be in the CCA days. In other words, "ground contact" treatment is better than above ground, but don't push it too hard.

I can see the potential problem of digging near the foundation. The first footing hole would be centered about 1.5 to 2-ft from the house. All of that area on the other side of the pad would be taken down a few inches to make a better bed for a patio. The upper level crawl space floor is on the same level as the lower level slab. There has been no moisture problem in the crawl space - so far. (Knock on my wooden head.)

Does this give enough information to work with?

Thanks, again.
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