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Patrick Presti
Deck Footing Help


I am replacing my (4) 4x4 post with (4) 6x6 post because my 16x16 deck sways a lot when there are people walking around on it.

My question is how do I calculate the footing size I need to make in order to support the new 6x6 post? The deck is built on sandy soil on Long Island, NY.



Patrick Presti
Re: Deck Footing Help


Is there any type of membrane that needs to go in between the 6x6 and the post base to prevent water seepage into the 6x6?

Re: Deck Footing Help

You left out important info, such as the size and the height of the deck.

Since a deck on posts is an area where people can potentially get injured, it has to be approved by the bldg dept.

Patrick Presti
Re: Deck Footing Help

The deck is 16'x16' square and the current deck post are 10' in height. Attached in the first posting are pictures for review.

Would the town building department be able to tell me the appropriate footings I would need for a deck of that size?

Re: Deck Footing Help

Standing 10' above grade, you have a major deck. Whoever built it, obviously did it without a permit. Right now this deck is a disaster waiting to happen - don't let anyone on this deck until you fix it. One injury, to you, your family or a guest, and you will be wiped out.

Your bldg dept will tell exactly what you need to do, once you bring them a diagram/plan. You can draw it yourself, they'll check your plan, require changes and finally approve it.

You can also contact an architect if you don't know how to do it. It will be money well spent.

Armed with approved plans and a permit, you can start talking to deck builders/framers about all the changes/modifications/additions necessary. Or DIY, if you have a lot of experience. You will need your field inspector's approval and sighn off.

Good luck.

Re: Deck Footing Help

Many building departments have guidelines for decks. Usually posted on-line.
use a post base set into the footing to keep the post off the concrete.
Swaying may have to do more with cross bracing. 10' off the grade should be 6x6.
All parts of the deck are suspect at this point, including attachment to the house.

Re: Deck Footing Help

My guess is that lateral swaying has little to do with the size of the posts themselves, but more to do with the lack of adequate knee braceing. The crush strength of a 4X4 is immense. That being said, were I starting from scratch, I would use the thicker posts. Normally, the posts are sitting on top of a concrete footing with nothing to give lateral bracing, merely a bracket that keeps it from slipping off the pad. It is the angular bracing which gives the structure rigidity. Just like iron bridges of old, it is a series of triangles which brings strength and rigidity.

I would agree with the others here that you do need someone qualified to evaluate the structure. It may be possible to save what exists and merely bolster it with additional bracing. It may also be more cost effective to tear it down and start over.

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