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Toronto, Canada I'm super novice DIY-er. I want to rebuild my porch/ deck in the front yard. The landing is very small 6 ft W x 6 ft L, the posts would be 4ft H above ground.

How deep should I dig the post holes? I googled and it said 1/3 to 1/2 hole depth (of the height above ground), so that would be 1/3 x 4ft = 14.4''. I also saw something about frost line and I can't quite understand if I need to dig 14.4'' below the frost line?

The existing posts have a very sturdy concrete base/ footing that I am unable to dig deeper than 16''. I dig all four holes down 14-15'' already and haven't set the posts yet because I want to ask this community for more input. If I can't dig more than 15'' deep (mostly shovel and hand...painful experience), what else can I do to make the posts more secure and not heave after a few harsh winter?

Thank you very much

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    Are you simply looking to replace existing posts on existing footings? If so, you existing footings should be (unless your deck has twisted due to frost heave) deep enough to meet local code. In that case, you'd simply attach the new posts to anchors on top of the existing footings. If you're digging new footings, then they'll need to be below the frost line as noted below. Please edit your question to be more clear about which of these situations you're looking at to get you a better answer (not that the linked one is bad).
    – FreeMan
    Jul 28 at 13:30

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You can check with your local building department as to what depth they consider the frost line to be. I expect it to be in the 1.2 meter range. The frost line is a point below which the ground does not freeze in the winter. If you go deeper you are safe, if you do not go that far depending on the freeze your porch/deck or whatever is above will move. This movement will be destructive over time.

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