Can I support 6x6" deck posts on 10" diameter poured concrete footings?

Question

I am building a new 13x10 deck. So far I have the ledger board in place, and yesterday I finished building 3 concrete footings, 9 ft away from the ledger board. I use tubing, 36 inch deep with gravel on the bottom (30 inch tube+ 6 inch gravel, as per city requirement).

The problem is that there were no 12" tube anywhere to be found so I decided to go with the 10" tubes. Now I will be mounting 6x6 posts (j-bolt + Simpson post bracket).

In terms of load, the 2x8 joists will span 9 ft and sit on the beam (double 2x10) + 1 ft cantilever.

I read that either 10 or 12in diameter tubing are fine, but now when I place the 6x6 post on them they seem so small. Did I just mess up the footings size? If so, is there a fix that doesn't involve demolishing the 3 pillars?

Answer 1

Summary - the post size seems questionable. Even at 12", it might have been insufficient. It depends very much on your soil type.

The remainder of this answer is a huge estimate based on lots of assumptions.

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For something like a residential deck the reason the size of a footing is important is not because of the strength of the concrete itself, but because of how much soil is directly below it (*). Soil is much weaker than concrete and the concrete footing has to transfer the weight of the deck to the soil, spreading that weight out over a large enough area that it won't gradually sink or move in some way.

We can make some broad assumptions to try and analyze your footings.

For a residential deck in the US (not sure where you are) I think 50 pounds-per-square-foot (psf) of deck surface is a typical design value. This estimate is supposed to include the weight of the structure plus any occupants, furniture, etc.

• A 13x10 deck is 130 square feet (sqft) in total
• 130 sqft x 50 psf = 6500 pounds

So where does all this weight go? Diagram shows a top-view of the deck, divided into sections. One half towards the ledger, the other half further divided into regions around the posts. These areas estimate how the weight will be distributed.

Assume half the weight of the deck is taken by the house ledger, leaving 3250 lbs for the posts (actually if the deck overhangs the posts they would take a bit more).

The 3 posts won't split the weight evenly: area B = A + C, therefore post B will take the most weight. Assuming A = C then we have A + B + C = A + (A + C) + C = 4A = 3250. So A = C = 812.5 and B takes about 1625 lbs.

A 10" post has an area of π r² = 3.14 * 5 * 5 = 78.5 sq inches = about 0.55 square feet.

(If the post were 12", then you'd have 113 sq inches = 0.8 sq ft)

Dividing the weight per post by the post's area, you get:

1625 lbs / 0.55 sq ft = 2955 lbs/sqft for post B.

(Using 12" posts, B would have taken 2030 lbs/sqft).

It comes down to what kind of soil you have whether this is OK or not:

• Clay: 1,500-2,000 lbs/sqft
• Sand: 2,000-3,000 lbs/sqft
• Gravel: 3,000-5,000 lbs/sqft
• Bedrock or solid rock: "unlimited" (?)

(reference for soil values: https://www.timbertown.com/building-a-deck-on-different-types-of-soil/)

The question doesn't have enough information to assess this further.

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Probably you should discuss this with your local code enforcement official. They should know what would be acceptable, typically.

If you do need to make changes, one option is adding one or more additional posts in between A-B and B-C. This would spread out the load.

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(*) Concrete might have a strength of like 5000 pounds per square inch. This means it is 1000-2000 times stronger than typical soils.

Answer 2

If you are willing to dig some big holes, while stabilizing the piers so they don't fall on you, and do some drilling you might be able to solve this more simply. Hmm - actually, those (10" dia x 30 inch tall) calculate out to only weigh 204 lbs or so, if I didn't slip a digit, so you might even be able to manhandle them around.

The piers you have are fine to support the post, if they are supported by soil.

The usual way to do that is to have a large pad (poured first - the actual "footing") that they set on, or to use an inverted funnel that the tube slips onto so that the area in contact with the soil is a couple of feet in diameter.

If you dig out that area and drill holes into (or through - if still green it may drill easily) the piers you have to run reinforcing steel that will connect the pier (remove the cardboard, too) to the "new actual footings" when you pour them around the piers, then your load will be spread over the area of the new footing. A disc 2 feet around and 4-6 inches thick would be a typical footing, or you can try to calculate out based on assumed loads and assumed bearing capacity of your soil to arrive at a required size.

The sooner you do this, the better the bond to your piers will be, but you're mostly going to be depending on the reinforcing steel to transfer the load, so don't skimp on that or drilling holes for it.

If you can manage to roll the piers around by hand (or using an engine hoist, which is equipment you can carry in pieces through a house into the backyard that will pick heavy things up and let you move them), you could just pour a pad (still reinforced) and have some steel sticking up from it, which you would drill into the bottom of the pier to accept (for positioning), and then "grout" (cement) the pier onto the center of the pad. Either the piers would come up the thickness of the slab more than currently and the wooden posts would be shorter, or you dig down the thickness of the slab more, in that case.