How should I build the concrete footings for my deck?

Question

I plan on making a small deck and I live in Howard County Maryland. The deck code here says that the concrete footing must be 30 in below ground level and have a thickness of at least 8 inches (i.e. to 22 inches below ground level). This which means the wood post is in the ground. This is also how all the contractors I have talked to would do it. (Deck guide for my county (PDF))

The books on deck building I have only suggest having the concrete footing extend above ground level by a little bit (I think this is called a tube footing). This method has the post not touching the dirt.

Is the second method better? Do I need to worry about the post rotting with the first method? I am assuming that I should be able to do it the second way because that exceeds the minimum requirement here. Is there anything better about the first method? Should I consider a different type of footing than these two? My deck with have about 3 posts and footings and will also attach to a ledger on the house.

Answer 1

Burying the wood post will cause it to rot prematurely. Even ground-contact rated Pressure Treated lumber won't last forever.

The only benefit of burying the post (besides it being easier to do) is lateral stability. i.e. you can sometimes neglect lateral bracing this way.

Best practice for wood-post decks is to have the concrete extend above grade and use a stand-off post anchor to hold the wood post to concrete.

Here is an example of this with some knee-braces for lateral bracing. The braces don't match so I find this somewhat ugly, but the image is clear and easy to understand.

Footings here don't need to be deep so I often form my own square ones but sometimes I use bigfoot forms and I really like those. Although Shirlock's "mushroom" method is more than adequate if you are only required 8" wide.

Answer 2

Your instincts are right. Do not bury the wood post!! We always use either a 6 or 8 inch Sonotube filled to slightly above grade or a 36 or 48 inch precast porch post.

If you decide to use the tubes, here is a little trick: when you start filling the tube, put in about 12 inches of concrete, then lift the tube slightly so some of the concrete flows out of the bottom of the tube forming a "mushroom" at the bottom. This mushroom will help keep the footing from lifting with frost in the ground, sort of a kind of anchor.

Another neat way is to use a precast pyramid concrete porch post. The concrete post drops into the hole. Seat it firmly, plumb it and backfill. Done. Most of these have a threaded metal insert to accommodate a bolt to hold a metal bracket that secures your wood post. I couldn't find a pic of exactly what you need in your area, but here is a vid of what we use here in Maine on larger projects:

Smaller versions are available and can be handled by hand, even though a small backhoe is cool to have. A 3 foot post is about 100 pounds 8 inches square at the bottom and 4 inches square at the top with a threaded hole in the center of the top.