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Old 04-22-2012, 11:17 AM   #1
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Porch foundation options


Hi, folks.

I'm planning to add a screened porch to the rear of our home, and am wondering about the best type of foundation to use. The porch will be 12' x 15' with a gabled roof.
Right now there is a concrete patio in the area where the porch would sit, but I believe it's just a thin slab with no footers to support a structure. The issue is that the finished floor of the house is only about a foot above grade, and the patio area has little or no slope. The sliding door that opens to the patio has a 6" high, 4'6" square concrete step that splits the difference between the height of the interior floor and the patio slab.
My first thought was a concrete slab foundation, poured with integral footers along the edge, that would also act as a concrete floor. However, to bring the floor of the porch up to the same level as the finished interior floor would require 12" of concrete above grade (and presumably would need to extend below grade?), which, along with footers extending below the frostline (we're in New England) seems like a lot of concrete and an expensive pour.
However, with the floor of the house so close to grade, I can't picture how a post-and-pier foundation would work. I've read that concrete piers should extend 4-6" above grade to avoid rot and insects, which doesn't seem to allow enough room for a beam and joists (let alone any kind of posts!).
It seems like this must be a pretty common issue, but all the examples I can find either have the floor of the house much closer to grade and use a slab, or much further from grade and use post/pier.
Any ideas?

Thanks,

RAH

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Old 04-22-2012, 11:42 AM   #2
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Porch foundation options


Post a picture for better replys.
An on grade slab with no footers like you have now is most likly just going to be in the way when building the new porch.

And building on top of it may not be the best option. The walls would be to close to the grade so water will just come in under them, there's no footing to support the walls or post.
For the best look and far less chance of it sagging I'd be looking at building stem walls, back filling, poring a new slab on top of it.
http://images.search.yahoo.com/image...mb=cpCLpHoH11V
If you did it that way later on the porch could be enclosed if you wanted to.

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Last edited by joecaption; 04-22-2012 at 11:52 AM.
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Old 04-22-2012, 11:49 AM   #3
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Porch foundation options


You could remove the concrete sill under the house door. Remove the old slab and dig a footer. Pour the footer and slab with wire mesh and rebar as one pour to local building code specs. In that pour, form up a sill/step under the house door....

If the house floor is 12 in. above grade, make the slab 5 in. above grade( good to stay above heavy rain water) and pour a 7 inch tall door sill/step under the house door. It means one step down/up to get in the house which may or may not be welcomed. Just a idea. Wood deck would probaly be cheaper, but hard to insect proof as a screened in porch....

The structure you use will be determined a lot from what the local building codes require when you file for a permit. good luck.
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Old 04-22-2012, 12:29 PM   #4
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Porch foundation options


take out concrete 4 x6 step and cut holes in existing slab where footers will go. Use ground contact pressure treated lumber and depending on what type of decking you use build so that once it is built the decking is an inch below underside of door threshold. looks like 2x8 lumber would work for the framing with proper footing spacing as per load support.

The other option instead of cutting in footings on existing slab is remove slab altogether and build deck the same way,it just will not have the concrete slab under it. The slab being under the new deck will help with moisture control in my opinion. Either way because deck is close to the ground have spaces between decking boards to allow air flow and avoid trapped moisture.

with this design the deck support posts will be quite short depending on depth of footer below the deck. The deck joists would require all joist hangers with this type of build, no fancy exposed under deck beams here.

After frame is built and before putting decking down, staple insect screen down to top of joists for fully enclosed insect control.

Last edited by hand drive; 04-22-2012 at 12:31 PM.
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Old 04-22-2012, 07:49 PM   #5
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Porch foundation options


Thanks for the ideas!

My thought was most likely to remove the patio slab regardless.

It sounds like the stem wall would be the most solid- and involve the most work! Am I right that we'd need to:

1) excavate the entire 12' x 15' area to the depth of the footers (48" around here)

2) form and pour the footers (using vertical rebar and/or a key to lock in the foundation walls)

3) form and pour the foundation walls from the footers to a height above grade

4) backfill inside the foundation walls

5) pour the floor slab- essentially using the stem walls as forms?

What would we do along the existing foundation of the house? Would the ends of the stem walls perpendicular to the house foundation just need to be doweled into the house foundation with rebar? Also, would this leave the floor slab "floating" relative to the foundation walls? Could that cause problems?

What are the advantages/disadvantages of a stem wall foundation vs. a "monolithic pour" for the foundation slab?

Thanks again for the helpful input.
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Old 04-23-2012, 12:49 AM   #6
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Porch foundation options


The new foundation and slab do not get connected.
I would use block not a pored foundation for such a small job.

I would not use a monolithic slab in you area. The footing would not get you below the frost one, and you may need more height then you can with that style slab.
http://images.search.yahoo.com/image...mb=cpCLpHoH11V

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Last edited by joecaption; 04-23-2012 at 12:55 AM.
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