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How to replace footing on seven posts supporting my porch. Should I have professional help or is this something the home owner can do?
Depends on what's wrong and how handy you are.
Footings need to be dug below the frost line and large enough to support the weight of the structure above them. A typical footing starts at around 12" x 12" x 12" with a wet set bracket to hold the base of the post that will be placed on top of it. Notice I said that this is a "typical" footing, as again, the load it must bear and the depth of the frost line and the stability of the soil are critical factors in determining the size of the footing.
If the posts are the type that just sit on top of a concrete slab, it can be a diy project. A jack will need to be used next to the existing post and raise absolutely no more than necessary.