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Concrete Slab And Porch Addition

 
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Old 03-29-2019, 08:40 PM   #1
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Concrete Slab And Porch Addition


I am a recent Construction Management graduate and am tackling a personal project. I plan to add a porch over the sliding doors to this cabin in Northwestern Pennsylvania. I will come off the existing roof with a shed dormer style roof to cover a concrete pad I will pour. I plan to support the porch roof with 6x6 cedar posts that I will have secured with simpson style brackets I will set in the concrete. The concrete pad will be 16'x10 and I will ensure the forms drop 1/4" every foot to ensure drainage away from the cabin. The question I have is whether or not I should dig out and just pour the concrete below where the support posts will be in sonotubes that start below the frost line -or- just dig out the whole outer edge of the pad below the frost line and pour that whole edge deep to ensure it won't heave. I know that is likely overkill but wanted some professional advice. I mocked up the structure on sketchup and added an actual photo as a visual.


Thanks in advance for the help.
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Old 03-29-2019, 09:45 PM   #2
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Re: Concrete Slab And Porch Addition


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Old 03-29-2019, 09:50 PM   #3
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Re: Concrete Slab And Porch Addition


With Ground frost, you need the columns for the porch roof to be on footing BELOW the frost line.

One fourth the roof load(16/2 x 10/2 =40 feet square) x total load, on each post for sizing bottom of footings.

Backfill column footings then install expansion joint between slab on grade so slab heaving doesn't damage roof structure.

Cheap single truck pour, all together, then saw Control joint at brick faces that face SOG, with demo saw or walk behind- floor saw, fill with concrete joint filler....

Don't forget wind uplift, = need for strap anchors to trusses to ground footing. Ready for tornado near misses....

Under 3/12 roof pitch, use metal?
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Old 03-30-2019, 07:41 AM   #4
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Re: Concrete Slab And Porch Addition


I had planned to use metal for the roof as the pitch on the porch overhang was going to be shallower than the existing roof line with 4/12.

Thanks for the advice and detail in your reply.
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Old 03-30-2019, 08:59 AM   #5
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Re: Concrete Slab And Porch Addition


Build footing for the columns extending to below the frost line, then build the structure. Isolate the slab from the columns with expansion joint material and pour the slab all a uniform thickness and just let it "float" with freeze thaw independent of the columns. Use some porous compacted gravel under the slab and it won't move around as badly as if on dirt.

Also, know that your slab is going to move around some, so keep the edge up against building a little lower than door thresholds and slope it to shed rain water away.
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