Adding a Porch with Concrete Foundation
potton
| Posted in General Questions on
We are ready to form a 3’x7′ concrete porch in front of main entrance of a new house in zone 6, and are reading/getting contradictory opinions whether to attach it or not to the house foundation…
INFO: The house has a slab on grade with frost walls going 5′ deep, except on that side where they are only 10in high (because of bedrock). The footing on that side are on 2” EPS foam, extending 3-4′ from the wall. [see attached sketch]
The porch slab will be on an inside corner. It will have a 8×8 post (and beam) supporting the single-slope roof overhang, that is 5′ at this point.
The plan is to add 4 rebar dowels, join them with rebars, and pour the slab over an additional 2” foam installed above clean gravel, the French drain, and the footing itself [see attached sketch].
Any educated opinion in favor or against that plan?
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Replies
I'm not a structural engineer, so this is more a bump of the post than any sort of expert advice. I can say that our plans have a similar, but substantially larger, concrete slab porch. Our engineer specified (1) no connection to stem wall, but concrete (tube) pillars that support the house side of the porch slab and that sit on an expanded footer, and (2) rather large footings for the posts that support the roof over the porch. Our design may have, in part, been to accommodate the exterior foam on the foundation and avoid the thermal bridge of rebar into the stem wall.
Thanks for your reply.
In which zone is the house ?
Was it specified as well the slab’s concrete not to touch directly the stem wall (in order not to adhere) by inserting a band of sill seal foam, or felt, before the pour?
Potton,
I would isolate the support for the load-bearing post from the slab for a few reasons:
- You will probably get differential settlement, or cracking.
- The slab would otherwise need to be sized to take up that roof load.
- The post should sit on a base several inches above the slab to avoid moisture problems.
If it were me I'd pour a pad and concrete pier, and leave the slab unattached to either it, or the exterior wall, so any future movement is inconsequential.
Malcolm,
Would you form and pour the pad directly on the compacted gravel or dig 6´´ or so to reach the bedrock (slate) ?
If I understand well it means 3 pours : pad, pier, and slab around the pier ?
Potton,
If the gravel is properly compacted it doesn't matter.
I form up pads and piers as one pour. Then the slab can go in any time.
Great. Thanks for the pic.
Do you prefer the slab’s concrete not to touch directly the stem wall and the pier (in order not to adhere) by inserting a band of sill seal foam, or felt, before the slab pour?
Still, being zone 6, would you add rigid foam under, or around, the pad, extending 4’ to reduce frost penetration? Same question for the slab...
Potton,
Like any part of the load-bearing structure the pad needs to be protected against freezing, so it needs to either go down to the frost depth, or have foam above.
You have to be bit careful putting these elements on top of foam without the approval of your BI. Here it would not meet code and would need an engineer's involvement. Presumably you have already dealt with that issue for the main house slab?
A bond break between the pier and slab is a good idea.
Malcolm
Would the bond break be pertinent along the wall...?
Here in Québec there is no building inspection (!) The original design (R20 insulated hydronic slab with 4.5ft frost walls) was changed by the excavation contractor on the East wall to 1ft + footing over insulation with wings; very common he said... No engineer...
You mentionned to put foam above. What do you think if I cut the existing wing foam to install the pad on the compacted gravel instead of on the foam (see attached sketch) then add gravel and add foam on top of footing as a base for the slab?
Thanks for your input
PS: if it changes anything, the post to support the roof overhang is required only for winter time
Potton,
That sounds like a good plan.
I wouldn't be too worried about a bond break at the exterior wall. It will probably fall away a bit as it cures. You typically end up with a small gap there anyway.
Should that small gap be sealed (caulked?) ...? [and the one around pier]
Is rebar required in the pad and the 8x8 pier (16´’ high approx) ?
Thanks
Potton,
I don't see much point in sealing the perimeter of the pad.
You need rebar in the load-bearing pier and the pad it is supported by. Rebar in the slab would do no harm, but isn't mandatory.
By any chance would you have a schematic of proper rebar configuration?
Thanks
Potton,
I don't know the size of the pad necessary, as I know what the loading is from the roof. The generic reinforcement I use for pads is three rows each way of 1/2" rebar, located 2" from the bottom of the concrete. Two 1/2" bars vertically in the pier.
Given the proximity of the footing's French drain and its low depth, should I use only screened drainable 3/4'' crushed stone under the small porch slab (and its insulation foam) or rather some thickness of compactable CrusherRuns ...?