Need Help With Footing Size - Plan Pics Included

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harborremodel

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I'm trying to learn all the calculations that go into designing a deck. I have a design for a deck at my house and I'm trying to figure out the footing size i need. I've attached some pics of my roughed out design any help would be great! I'm in an area of the northwest that only requires 12" deep footings i'm just wondering about the diameter.
 

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Snoonyb

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What is the height of the door threshold above grade?
 

kok328

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Thinking that's going to be a local code question.
In my neck of the woods, you have to go 46" below grade.
 

harborremodel

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@Snoonyb the height of the door threshold above grade is roughly 36 inches. Local code requires footings to be 12" deep, I'll probably go 18 just to be safe but it's really the diameter of the footing that i'm trying to figure out.
 

Snoonyb

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Thanks.,
I'd use an 18"sq. pad 12" deep, at between 6' and 8' OC., supporting a double 2x8 girder, change the direction of the floor joists and use 2x8 floor joist, which would span 12'7" and leave the 4' section as a cantilever.

This will leave the decking running away from the structure and gave the appearance of greater length.
 

nealtw

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Size of footing really is decided by the weight and soil condition.
To the rest of the drawing, the finished deck level should be 6" below the subfloor inside the house.
I would move the center beam over 1 ft and lap the joist at least the thickness of the beam
Solid blocking between joist centered between bearing and no more than 7 ft from bearing.
Have you got a flashing detail for the ledger.
 

Snoonyb

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I'd actually not attach to the dwelling, but leave the deck as a floating platform.
 

bud16415

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This is the way I built our deck (free floating) and 1” away from touching the house. All that involves is a few more posts and footings and that way you don’t break the weather proofing layer of the house or add weight to the house footing. Our house is 150 years old and it was questionable as to how good a ledger needed to be attached also by being free floating it removed all the questions.


Of course if your house is relatively new construction the soil next to the foundation is back filled and not stable for a number of years.


We have a footing requirement of 48” here for very cold deep frosts and many people even recommend 5’.


As an experiment an I was told being a free standing structure it was not considered part of the house. I only put in footings (pre-cast) builder center type that have a socket for a 4x4 PT post to rest in. With the stacked construction @Snoonyb mentioned my longest post was only about 24” and column strength was not an issue with short posts. I kept all the spans short and used as many post as needed. Because it wasn’t attached to the house for stability I crossed braced it both direction down one section of posts. It is a very stable deck. I have a 6000 pound hot tub sitting on it. It is free to lift in the winter and it moves up to a half inch. It hasn’t shown any signs of problems in the last 5 years.


I would never think of building a tall deck with this method but for a ground hugger it has worked fine for me.


Not sure what code in your area would say about free standing but with a 12” footing requirement I would use the pre-cast and use as many as you figure your soil needs to spread out the load. Dig your holes carefully to not disturb deeper than you want to go and then use a 4x4 pole to tamp it flat. Toss in the pre-cast footing and start building.


Good luck.
 

harborremodel

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This is great info thanks everyone! @bud16415 I'd actually really like to make it free floating and keep it unattached to the house just not sure what code says here in Washington state and my county. With footings only having to be roughly 12" deep i wouldn't mind digging a few more holes to make it happen. The builder who built this place around 99 did a terrible flashing job in a number of areas but for the current ledger just bolted it right over the siding (of course). If I was going to use a ledger I'd do it right and flash it correctly bit i really like the idea of a floating deck and eliminating water entry points especially since our area tends to get a ton of driving rain.
 

bud16415

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Make a few calls and see what they say. On a low deck like yours there isn’t a major stability issue where you need the house to stabilize the deck. If you do stacked construction half your height will be beams and joists. Posts will be pretty short above grade. You would need some stability and depending on how you attach your posts to the beams it could be in that joint with the right pre-made hanger or you could add in cross bracing or some diagonal bracing.


I went with the understanding here that free standing isn’t a permanent structure. Not sure if I’m right on that or not but they are pretty relaxed in this county with decks and just about all DIY projects except septic and waste water. Part of the reason we bought here.


I really like the no need for flashing and potential water intrusion this way and in my case my ledger would be going into 150 year old hand hewn beams. It is harder for me to interpret the strength of that joint than it is to calculate additional posts and footings.


Let us know how you do it and post some pics when you get it done.
 

harborremodel

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Figured I'd post a follow up since i'm in progress now on this deck. I checked with a few local contractors and all said a 12" footing is fine since the ground doesn't freeze here in this part of Washington state. I ended up pouring 16 x 16 x 4" concrete pads reinforced with #3 rebar and then poured a 12" sonotube of concrete at a minimum of 12" high some higher in the spots where i had more room and again reinforced with rebar and Simpson CBSQ post base. I had to dig a decent amount for some of the footings to make sure i could have level beams and a single plane deck. Pics attached of the project in its current state.

2018-11-09 14.54.04.jpg 2018-11-09 14.54.19.jpg 2018-11-09 14.54.34.jpg
 

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