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Old 10-20-2012, 11:22 PM   #1
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House addition over existing concrete porch slab


I have a small 10' x 4' porch off the front of my house that I would like to convert and close it in for some additional interior house space. There is an exterior door for access from the small concrete porch into the house already and it is fully sheltered but the house roof. I want to frame the perimeter walls on top of the existing concrete slab for some added living space. The finished slab is approx 5" above ground level and hoped to avoid the need to set a row of block or pour a concrete curb in order to raise the height and keep proper clearance from the bottom sill plate of my new exterior wood framing. I am going to use pressure treated lumber for the bottom plate with sill gasket where it will attach to the concrete slab. After I secure the bottom PT plate with the gaskets I was planning on framing the walls with 2"x6" spruce that will sit on the bottom PT plate and use house wrap (Tyvek) first to fully wrap the underside of the spruce before it rests on the PT bottom plate. Install the O.S.B sheathing and using this same piece of house wrap that is under the spruce to continue running under neath and up the face of the O.S.B with no seams. If I take this route will it protect the walls from rot cause by the weather elements we will get here in Canada??

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Old 10-20-2012, 11:27 PM   #2
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House addition over existing concrete porch slab


I doubt that's going to work unless there's a footing below the frost line.
Believe it or not that slab moves up and down a small amount as the seasons change.

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Old 10-20-2012, 11:57 PM   #3
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House addition over existing concrete porch slab


Yes there are existing footings under the slab

Last edited by Hardwood Head; 10-21-2012 at 04:15 AM.
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Old 10-21-2012, 08:44 AM   #4
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House addition over existing concrete porch slab


If there is a footer under that slab--then you should be just fine---

A picture would help --but isn't critical----treated bottom plate is required---you might want to double that if a leveling compound will be poured over the existing slap to level out the pitch after the wall is built-----
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Old 10-21-2012, 10:11 AM   #5
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House addition over existing concrete porch slab


is the outside of the wall going to sit flush to the outside of the slab or is it going to sit in onto the slab some? you will want to work it out so that you are flush to the edge so that water does not get under the wall... Also, put tyvek last and it does not go to the interior of the wall just onto the sheathing last and lap over the slab some. Does the grade of your land around the porch slope toward the house or do you have a positive grade away from the structure? post a pic and maybe you will get more opinions to help your project along
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Old 10-21-2012, 10:22 AM   #6
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House addition over existing concrete porch slab


Good point Hard Drive.
Every DIY deck I've ever seen where they tryed to enclose it they put the wall sitting back from the edge of the slab. Everyone of them leaks under the wall and rotted out the sheathing.
I just redid one that was set back 1-1/2. It rotted out the flooring, rim joist, sheathing. I had to shim out all the walls to get the sheathing to run past the slab.
One long wall was 2, 4 X 4's stacked on top of each other with no real rim joist and it bowed out 3" in the center. The two end walls were 6" differant in length.
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Old 10-21-2012, 11:01 AM   #7
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House addition over existing concrete porch slab


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Good point Hard Drive.
Every DIY deck I've ever seen where they tryed to enclose it they put the wall sitting back from the edge of the slab. Everyone of them leaks under the wall and rotted out the sheathing.
I just redid one that was set back 1-1/2. It rotted out the flooring, rim joist, sheathing. I had to shim out all the walls to get the sheathing to run past the slab.
One long wall was 2, 4 X 4's stacked on top of each other with no real rim joist and it bowed out 3" in the center. The two end walls were 6" differant in length.

Many times the wall for the new closed in porch is lined up with the beam of the porch which usually sits in from the edge of the slab so people do the best they can with it but it still leaks
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Old 10-21-2012, 11:04 AM   #8
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House addition over existing concrete porch slab


In most cases all they had to do was add a 2 X 4 to the post to space it out that's already there and use 2 X 6's for the wall studs so it comes out even.
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Old 10-22-2012, 11:06 PM   #9
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House addition over existing concrete porch slab


I have a new plan that I think will work better, please let me know your feedback!
I want to keep the snow and weather away from the wall as best as I can by framing a 2x6 PT knee wall 15" high, for added protection I will wrap the knee wall with a super 6 vapour barrier and a 2nd wrap with Tyvek and secure it to the slab with a sill gasket and a continous bead of exterior sealant between the wrap and the slab. Once secured I will caulk the perimeter of the bottom plate at the slab and add a 1" ridgid foam on the exterior side and then a layer of 1/2" concrete board. I will caulk and seal the perimeter at the slab between each layer. I will then parge the concrete board for a consistent smooth finish and use plastic drywall corner bead on the 1 outside corner. I do plan to set this back 1-1/2" from the slab but after the foam and concrete board are installaed I will be flush or very close with the existing slab. I will then frame on top of the PT knee wall with spruce, sheet with 11/16" OSB and wrap the exterior sheathing with the Tyvek. I am going to be using a 8" beveled pine siding and will start the fist course at the top of the 15" parged knee wall to keep the wood siding away from the exterior grade.........What do you guys think?
Yes the grade slopes away from the house, there is a flower garden that meets the slab on 1 side with new interlock on the other.

Last edited by Hardwood Head; 10-22-2012 at 11:23 PM.
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Old 10-22-2012, 11:58 PM   #10
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House addition over existing concrete porch slab


If your that worred about it I'd be running two rows of 6" block on top of the slab instead. Parge the block and slab on the outside.
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Old 10-23-2012, 08:46 AM   #11
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House addition over existing concrete porch slab


Quote:
Originally Posted by Hardwood Head View Post
I have a new plan that I think will work better, please let me know your feedback!
I want to keep the snow and weather away from the wall as best as I can by framing a 2x6 PT knee wall 15" high, for added protection I will wrap the knee wall with a super 6 vapour barrier and a 2nd wrap with Tyvek and secure it to the slab with a sill gasket and a continous bead of exterior sealant between the wrap and the slab. Once secured I will caulk the perimeter of the bottom plate at the slab and add a 1" ridgid foam on the exterior side and then a layer of 1/2" concrete board. I will caulk and seal the perimeter at the slab between each layer. I will then parge the concrete board for a consistent smooth finish and use plastic drywall corner bead on the 1 outside corner. I do plan to set this back 1-1/2" from the slab but after the foam and concrete board are installaed I will be flush or very close with the existing slab. I will then frame on top of the PT knee wall with spruce, sheet with 11/16" OSB and wrap the exterior sheathing with the Tyvek. I am going to be using a 8" beveled pine siding and will start the fist course at the top of the 15" parged knee wall to keep the wood siding away from the exterior grade.........What do you guys think?
Yes the grade slopes away from the house, there is a flower garden that meets the slab on 1 side with new interlock on the other.
Your thinking is wrong. If built correctly there should be no need for treated anywhere except on the bottom plate. if you are planning to combat moisture any where else in the wall you are asking for mold and trapped moisture inside the wall even though the treated studs will be fine the rest of the wall will mold.

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