Decks, Patios, Porches and Docks - Unsupported porch slab

I live in Canadian zone 5 area which means tons of snow. In the front of my house is a 5.5 inch cement slab attached to the house foundation on two sides and 4 feet footer on one side. The front side has no support. Under the slab is a crawl space of about two feet.

I want to install a porch in front of the house supported by 3 6 X 6. Top plate would be 3 2 X 6 on which would rest 2 X 6s which are attached to the house by a 2 X 6. All this exceeds the code by a country mile. The porch size would be about 30' X 5'.

My concern is the cement slab on which my 6 X 6 posts will be supported. This slab is supported on three sides but one 30' side has no support and there is nothing to support the 30' length of the slab on the front side. Witth the eight of the structure and 3 feet wet snow on the roof, will the slab cave in?

I went to the building dept and as with thousand other building depts, they were totally useless.

Should I dig under the spots where 6 X 6 posts will be and go down 4 ' (frost line in ours area), install a 8" Sonotube which is a foot shorter than reaching the underside of the slab, then fill the Sonotube with cement. When the cement sets, I could shim cement bricks on the top of the Sonotube to support the salb where the 6 X 6 posts will be. Or I could attach a Tubebase to the top of the Sonotube and have it stick out from the slab, then I can have the cement footer come right upto the slab.


I would be happy to email you the jpeg of the drawing if anyone wants to see a visual.

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This project requires a Professional Structural Engineer to evaluate : your soil conditions; your slab conditions (even then there is no way to tell); the loads exerted; and the biggest one of all---- the liability issue. Have him check your deck plans, too. Be safe, GBR

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Here are the photos:

Pictures by ArvindPradhan - Photobucket (http://s642.photobucket.com/albums/uu143/ArvindPradhan/)

Thanks a lot.

I know it sounds extreme, but no larger than that slab is, demo it:eek: and properly prepare the area for a new concrete pour. You will be better of and will be able to install tube structures below it as the pour is being made for your post supports.

Why not go outside the slab with the posts? That way you can do the proper footing for the posts and not have to bother with the slab at all. This will also give you a bigger overhang on the roof and more weather protection.

Why not go outside the slab with the posts? That way you can do the proper footing for the posts and not have to bother with the slab at all. This will also give you a bigger overhang on the roof and more weather protection.


Your idea is the cheapest and the safest. I will verify with the city that I am allowed to have the supporting posts 5' away from the house wall. (I think there is some rule about how far away the supporting posts can be from the wall). The big advantage for me is I don't have to pay a structural engineer to approve this. An architechural technician can sign off on this and I can get my building permit.

Thanks.