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We are having our two story deck fixed. The deck is built on the downward sloping side of a hill and the footings were failing. The footings were replaced with much larger tube-formed footings.

When the contractor installed the new posts, he actually cut off the old posts (approximately 20'-22' lengths) at the halfway point and put new posts (vertically of course) under the old/existing posts. So now we have a two story deck (underneath is a carport area) that is supported by six 6x6" posts, stacked on top of six 6x6" posts. This just looks wrong and dangerous.

Is it acceptable to stack posts on top of one another to create a total height of 20'+ for the purposes of deck construction and support? If so, how should the posts be secured to one another?

  • Welcome to Stack Exchange. You should take our tour so you know how to participate here with upvotes on helpful answers and check marks for accepted answers. – HoneyDo Apr 10 at 19:41
  • You say "two-story deck", but I suspect you actually mean a deck at the second story height. Which is it? Are there two levels to the deck? – isherwood Apr 10 at 20:01
  • You are correct. It is a deck at the second story height above a carport. – TBelen Apr 11 at 2:24
  • How much new post is on the bottom of this 20+ foot tall stack, and how much old post is on top? In either case, you've still got a hinge-point or slipping point, but how the joint is reinforced and how that reinforcement looks (esthetics are secondary, but they are important), my depend on where the splice is. – FreeMan Apr 14 at 16:01
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You’re right, vertical splices in posts need special attention, especially 1) when the posts are 20-22’ long, 2) when they support decks or spaces where people can gather, 3) where cars can impose “extra” load on old existing footings, and 4) lateral displacement depending on seismic activity or seasonal ground movement, 5) how square the two cuts are made at the splice.

1) When posts are 20-22’ long they tend to buckle when over loaded. This is called the “slenderness ratio” and depends on the species and grade of wood. (AND because it’s outdoors I suspect it’s pressure treated.) A typical 20’ 6x6 of spruce-Pine-fir (SPF) can support about 8,000 lbs. depending on its grade. (I’m assuming a middle grade of No. 1 and better, but not a “Select” or a “Dense Select”.) Likewise, a 22’ 6x6 of the same species and grade will support about 7,000 lbs. I’m also assuming the posts are braced in BOTH directions at the 20’ or 22’ splice. If not, it’s a hinge a a bigger problem develops because it becomes a 30’ post...or whatever the existing post adds onto its length.

2) When a post supports decks and other “living” spaces we take extra precautions. Depending on the size of your deck, you could easily overload the post at your next birthday party when the gang starts singing Happy Birthday and jumping up and down. We use 100 lbs. per square foot to calculate the load, but the Code allows 40 PSF. The Code allows an increase in stress (strength) for impact loads, which your party would qualify for, but there’s a reduction for pressure treated posts too.

3) We also worry about adding additional loads to existing footings that have “always been just fine”. You now have a carport and the weight of that car on the soil can make the existing footing settle or tip (lean).

4) If you live in a seismically active area there are special considerations that must be met. I doubt you do, because you say there are independent footings. If you lived in S. California or another active area you wouldn’t have independent footings. (If I’m wrong, I’d get that looked at immediately.) Remember, the post needs to be braced in both directions.

5) Finally, to answer your original question: “can posts be stacked and how should they be connected “. Yes, they can be connected and they should have an adequate connection. The Code does not stipulate how they’re connected, but says it needs to be adequate. If the two ends are not cut perfectly “square” and there is jumping on the post, it could “slide” past the other post, especially if it’s a little wet. (When I cut a 6x6 with my skillsaw I have to turn it a couple of times to cut all the way through. It’s never a perfect cut. Did you have skilled workmen you trust?)

That “sliding” load can be calculated if we know the load on the post. Short of that I would not trust wood 2x6 side blocks nailed into the 6x6’s for a connection. (Someone said blocks on two sides would be ok. I think not because the nailing could be in withdrawal. That’s bad. At a minimum I’d use blocks on 4 sides.)

Without knowing your exact circumstances, I’d prefer a steel connection with braces in both directions at the splice points. You could use something like Simpson ECCLQ. It’s not perfect, but it allows braces in both directions.

https://www.mudsupply.com/Simpson-ECCLRQ666SDS-p/909200801.htm?gclid=CjwKCAjwssD0BRBIEiwA-JP5rIXNWlhsWWIz4wEdUUQ_3SGkclQJnV6f7cRuEyGWCSqWmELi9uIOnRoCRVMQAvD_BwE

Remember, you just created a HINGE. You need to brace it so it doesn’t move.

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In my experience, the most common and aesthetically pleasing way to do this would be a half-lap joint with fasteners connecting them (nails, screws, carriage bolts, pegs, etc). At this point you could also use Simpson tie connectors, or two angle braces. Wood blocking would look pretty janky and would be my last choice.

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It Depends

  1. As you noted and is generally obvious, they do need to be connected together in a manner that they stay aligned and together to work as a single unit. This can be accomplished by nailing on 2x block to at least two sides or using a manufactured metal connector product
  2. Tall skinny posts that are not braced will fail by buckling where the middle of the post will move to the side suddenly and without warning, this is very dangerous of course, especially for a deck with the potential of having lots of people on it. To resist this sideways movement, tall posts are braced at the middle point (1 brace), and maybe even the 1/3 points (2 braces) or 1/4 points (4 braces) in both directions usually using 2x material. The ability to resistance to this sideways movement between supports requires continuous materials between the supports, that is, no connections. Therefore, the joint of your posts must also be braced even if it does not fall at the normal 1/2 etc points.

Edited to add this note:

Note that there is no requirement that the connect brace be an additional and completely separate and different brace than the others. For design the post height would be the greatest of the distances between supports. What this means is that the brace is not required to be at the 1/2, 1/3 etc locations, this is just done for efficiency in materials and because it usually looks the best. If your post to post connection is not at one of these locations then it might be possible to place one brace there if the longest unbraced segment of post is still under the maximum. The aesthetics that results from the brace location is not a structural issue and you'll want to consider it based on other criteria. Note I can't tell you the maximum post height here now for many reasons, sorry

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  • Under most building codes wouldn't the upper post be considered a separate post and, as such, require a proper footing to meet code? I don't know - just asking. – HoneyDo Apr 10 at 20:00
  • No. Lumber is stacked on lumber all the time. As long as it has proper bearing surface and lateral bracing it's fine. – isherwood Apr 10 at 20:03
  • The footing is basically the interface between the building and the earth. A post is a completely separate item and can be anywhere in a building, including on upper floor levels. Though a post and footing often go together, which makes it seem that they always go together – Ack Apr 10 at 20:10
  • I've edited my answer to address a secondary question that I think is likely to come up – Ack Apr 10 at 20:22

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