Crawl Space Size

Is there any rule of thumb on size of crawl spaces? I am building a three season 10'x24' porch on piers off the family room. The family room has a cement foundation with a concrete floor. How much room do I need between the floor joists on the porch and the dirt below them? Do I need a vapor barrier? Does the crawl space need and what size vents?
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for buffalo ny: under my porch is concrete. for a porch or deck think about rainwater, snow, and a permit. those piers holding up my second floor porch of your size 10 x 25' are 8"x8" pressure treated sunk 4 feet into the ground. go look at the neighbors incorrect deck made of 2x4" and make all yours 2x8" at 16" on center with 2 x 6" deck. make it twice as sturdy as any carpenter says, and it will be just as sturdy at 20 years of age as the day it was built. locally you would in buffalo ny 2 years ago see your local city hall permit office, phone ahead to see what paperwork you need to take there such as 5 copies of your survey and permit fee money as required. some city websites have online permits.
snipped-for-privacy@dva.state.wi.us wrote:

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Around here (Pacific Northwest), code requires 18" between the ground and the floor joists, though that can be reduced to 12" under beams. However, if you ever have to crawl under the porch, you'll wish you had more space.
We designed 24" stemwalls on 6" thick footings for a total 30" high crawlspace. Our footings actually ended up being thicker in places, so we have a little more than 30" in many areas. I find 30" to be quite comfortable to move around on my hands and knees, as well as dragging pipes, insulation, or whatever through the crawlspace.
As for venting, we have to have 1 square foot of vent for every 150 square feet of floor area. There are supposed to be vents close to the corners, and allowance made for cross ventilation.
Of course, if your porch is built on open piers, venting won't be much of an issue.
Anthony
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why on piers?.........to answer your question your building code is going to dictate all of these concerns. Id just exstend the familyroom slab and build on this. If you cost it out a concrete slab is MUCH less exspensive and eaiser.
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