# what size holes you dig for a deck?



## dkillianjr

I was just curious what size holes you guys have to dig when building a deck or elevated screened in porch and ect. ? In my area the code is 24" round or 24"x24" square and 3 feet deep. Just curious what you guys are diggin?

Dave


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## TempestV

for our post bases, we dig big enough to sink an 8" or 10" sauna tube in 3 feet, with a flat packed bottom


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## AtlanticWBConst

In our area, unless it is specified differently on specific architectural or engineer's designs, we go 12" dia. and 4' deep. 

12" dia. because most towns now require it per code. Tho, we were doing it before that.

4 ft deep also is by code requirement for our region.


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## TNGHomes

On the Cape its 4' down 12" wide (with a big foot for farmers porches and heavier load bearing applications) because its all sand.


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## dkillianjr

The reason I ask I cause our code used to be 12" diameter and 36" deep but now all of the codes are changing around my area 24" diameter seems a little bit of overkill just for a plain jane deck. If the deck has a roof load on it or something I can see that. 


Dave


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## RobertCDF

Piers need to be sized differantly depending upon where they are in the deck... For example the CENTER pier of a beam in the center of the deck carries the most weight of any pier in the deck system. And to see 20-30" diameter is not out of the question. Piers on the end of a beam carry the least amount of weight and can usually be done with 10-12" diameter with out much of a problem. 

Add a hot tub and the piers get bigger and closer together. Add a roof load and they usually go up by roughly 2-4" in diameter.


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## AustinDB

for pole buildings (4x4 or 6x6)-our county requires 16" diameter if post is OC, 18" if not OC. Either way, 36" deep. I once inquired as to using a telephone pole as a post for a pole building-would have had to dig a 36" diameter hole!!!


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## AtlanticWBConst

RobertCDF said:


> Piers need to be sized differantly depending upon where they are in the deck... For example the CENTER pier of a beam in the center of the deck carries the most weight of any pier in the deck system. And to see 20-30" diameter is not out of the question. Piers on the end of a beam carry the least amount of weight and can usually be done with 10-12" diameter with out much of a problem.
> 
> Add a hot tub and the piers get bigger and closer together. Add a roof load and they usually go up by roughly 2-4" in diameter.


Hi Robert,

I have seen you write about this point before. You are the only person that I have come across that believes in making certain piers different (center). 
I do find that this concept makes alot of sense. Is this something that is common practice in your region? 
What exact equation do you use to calculate the center pier dimensions?

Thanks


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## wolffhomerepair

Local code calls for 4" of pier all around posts. So if you use 4x4 it needs 12", 6x6 use 14". All piers must be minimum 42" for frost line.


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## mdshunk

The United States Army Corps of Engineers, Publication Number EM 1110-1-1905, _"Engineering and Design - Bearing Capacity of Soils"._ 

http://www.usace.army.mil/publications/eng-manuals/em1110-1-1905/entire.pdf

Page 20 of this document is most helpful, as it has the frost depth contour map of The United States.


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## RobertCDF

AtlanticWBConst said:


> Hi Robert,
> 
> I have seen you write about this point before. You are the only person that I have come across that believes in making certain piers different (center).
> I do find that this concept makes alot of sense. Is this something that is common practice in your region?
> What exact equation do you use to calculate the center pier dimensions?
> 
> Thanks


Yes it is common practice here... They have a span chart but they also will do a load calc to see exactly how much weight sits on a pier. 

To understand a little better I am ataching a pic that might help. 

This is just a simple 16x16 deck with 12" O.C joists with a beam in the center. I have outlined the areas of what is bearing what weight. And you can see how the piers support differant amounts of weight.


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## Same Old

Robert's got it. I don't know of any model code that has such vague requirements as the others describe. I suspect those are more common practice than anything based on sound engineering. 

This is a pretty good guide for designing standard decks. Scroll down and click the link at the bottom of the page for a very simple table of beam and footing sizes.
http://www.ci.inver-grove-heights.mn.us/dev/permits/decks.html 

Footing depths obviously are different depending on your location, but the sizes in varying soil types are universal.


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## Driftwood

*Best to keep sona tube out of the hole !!*

Here in Ca . inspectors won't accept the s. tube going into the hole !
On hillside homes we drill 18" holes 6 - 10 ft into rock My eng. pointed 
out side friction from the irregular drilled hole gets 1800 lbs, additional
carrying load every foot of hole.


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## Same Old

Good point. 

Sono tubes are essential in freezing climates for exactly the opposite reason. You don't want the soil to grab onto the sides of your pier when the ground freezes and heave the structure up. 

I did a caisson foundation where we never reached a good solid base for a couple of the holes but the engineer determined that the holding pressure from the friction on the side of the hole was adequate for those areas.


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## Tom M

Robert hit the nail on the head. To find point loads you need to find the tributary load which in his drawing show the center carry the largest amount of wieght. The IRC, IBC charts recommend pounds per squareft live and dead loads that get multiplied by the area. This tells you how large of spread footing you need. The depth is a minimun 12" for everyone and frost depth come into play seperatly by region or municipal. Mine is 3 ft. No footings are required if the deck is free standing.


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## Driftwood

I forget some people live in cold ! Use to in Mass. Hole different building world in shakey Ca,


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## RYANINMICHIGAN

dkillianjr said:


> I was just curious what size holes you guys have to dig when building a deck or elevated screened in porch and ect. ? In my area the code is 24" round or 24"x24" square and 3 feet deep. Just curious what you guys are diggin?
> 
> Dave


24 inch around would make for a PIA getting plumb. 4x6 = 8 inch hole.


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## concretemasonry

Ryan -

8" for a 4x6? That is not much room the get good concrete filling or coverage.

Dikillianjr -

The 24" sounds like a code "catch-all" that will work for everything and every condition. The only good thing is that it protects some people from themselves.

What about if you have design signed by an engineer? Its expensive since you might need a soil test or augering. Some areas will allow variances if you have a signed plan.


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## RYANINMICHIGAN

concretemasonry said:


> Ryan -
> 
> 8" for a 4x6? That is not much room the get good concrete filling or coverage.
> 
> .


Code in Michigan is 42 inches deep, biscuit in bottom fill hole with dirt removed while digging. Back in the day we poured crete or dry packed and was skeptical to change but post are very solid after a couple of days.


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## dkillianjr

Just judgeing by the responses it looks like the average hole size is 12" or so. 



> The 24" sounds like a code "catch-all" that will work for everything and every condition. The only good thing is that it protects some people from themselves.
> 
> What about if you have design signed by an engineer? Its expensive since you might need a soil test or augering. Some areas will allow variances if you have a signed plan.


Thats what I was thinking I guess if they just pick the biggest number you can go wrong haha. As far as an engineer goes Ive used them a couple times building some homes along the river and in the 100 year flood plane , the code requires an engineers stamp. Its just so much hassle and expense I would rather just dig the holes. I deffinetly overbuild my decks theres been so many collapseing here latley and I would never want one to be one of mine. I think thats why they changed the codes. 

Dave


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## Same Old

Same Old said:


> http://www.ci.inver-grove-heights.mn.us/dev/permits/decks.html


This will give you all the information on fdeck footing sizes you need to know. All you need to do is click on it. I assure you in many cases 12" will not do it.


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## AtlanticWBConst

Same Old said:


> This will give you all the information on fdeck footing sizes you need to know. All you need to do is click on it. I assure you in many cases 12" will not do it.


Thanks.....good calculations table on that link


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## ecooke21

I've never seen it in writing but, In all my deck plans i submit, i use 8" tubes, 40" deep.. Ive never had and inspector say anything.. BTW, I'm in Rhode Island


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## RobertCDF

ecooke21 said:


> I've never seen it in writing but, In all my deck plans i submit, i use 8" tubes, 40" deep.. Ive never had and inspector say anything.. BTW, I'm in Rhode Island


8" Dia is very much undersized. Unless you are talking about a 6x6 deck... Then the 8" Dia would be ok.


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## Same Old

Who needs engineering any-who?

Me pappy built 'em this way 'fer years and years.


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## concretemasonry

In the absence of prescriptive code standards, engineering guidlines would control.

They are common sense -

1. The area of a concrete support would be determined by the bearing capacity of the soil and future loads on the post. Loads should include possible additions and uses to avoid costly work later.

2. The size of the Sonotube should be enough to give you at least 2" and preferably 3" of cover at the corners of the wood post. This gives the concrete room to get in and not bridge.

You "pappy" knew what worked because they did not have hot tubs and gazebos and the soil probably was the same in the area he worked in. In those days they probably weren't avoiding a little digging.


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## Luke's Dad

It was always "suggested" by local inspectors to go with 8" bigger than the post you are setting. eg. 4X4 needs a 12" hole and a 6X6 needs a 14" hole. And I always use sonotubes too! :thumbup:


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## Same Old

concretemasonry said:


> You "pappy" knew what worked because they did not have hot tubs and gazebos and the soil probably was the same in the area he worked in. In those days they probably weren't avoiding a little digging.


I was being sarcastic. peir footing sizes are so easy to calculate there is no reason not to.


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## Builtbybeck

Wow I'm glad to be in the south. The frost line around here is only about 12" deep. Most any foundation (deck, house or whatever) is only 16" deep! We don't really have much soil though it's mostly limestone chunks held togeather with red clay. We don't dig deep but it's rough digging.


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## boman47k

Builtbybeck said:


> Wow I'm glad to be in the south. The frost line around here is only about 12" deep. Most any foundation (deck, house or whatever) is only 16" deep! We don't really have much soil though it's mostly limestone chunks held togeather with red clay. We don't dig deep but it's rough digging.


I still am somewhat amazed when I read about 42" footings. I have to remember different climate and soils. Thinking of building a deck here and just let it sit on the ground. Have one house with a deck like that. I didn't know this til I got to looking at it. Been there about 6 years now. A nephew built it for my ma-in-law and I was wonder8ing how he dug the holes. There no holes. Very few damned joists either! :blink: Seems sturdy enough except for the railings. Another "to do" thing.


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## concretemasonry

If you have only been in the south, it is obvious you have never seem what frost can do. The old phrase "Don't fight with Mother Nature" is absolutely true. - The same thing is when a northerner looks at and can't comprehend the damage from a hurricane storm surge.

Northern rural country roads can heave 5 feet upward from frost, which is no problem in the winter. They can be totally impassible in the spring with a 4x4 unless you have big floatation tires and a long pole. - The problem only lasts for a few days or so in the spring while things return to normal.

Wait until your first tornado or hurricane and then take a look at the deck. The code (minimum standards) often treat things like decks as expendable and replaceable since it is cheaper to have mother nature take them away instead of you and the replacement cost is so low.


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## boman47k

> Wait until your first tornado


That has definitely crossed my miind. One reason why I can't understand floating decks. Have had one tree blow down when one went over. Came from the direction of the proposed deck too. I always thought that if that tree ever came down, it would hit the house. It knocked one window pane out. Had one huge pine removed a few years back for fear of it falling on the house during ice storms/winds. Might be a change in plans here. I feel there might be liability issues if that thing was to go flying through the neighborhood. Slim chance, but....


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## concretemasonry

Mobile homes never move in the face of wind either!! - Maybe it is just because they attract tornadoes and hurricanes.

The wind on the side of a deck is not much of a problem, but when it once gets under to create uplift, it is gone unless it is anchored.


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## salvatore Paul

*bullet proof plan...*



dkillianjr said:


> I was just curious what size holes you guys have to dig when building a deck or elevated screened in porch and ect. ? In my area the code is 24" round or 24"x24" square and 3 feet deep. Just curious what you guys are diggin?
> 
> Dave


Dave, Each area where you live has different codes and guidelines that homeowners and contractors should follow. There is a reason for the size of the post footer you need. Each part of the country has different soil content and make up. The area you live in has been studied and codes and guidlines have been set. Besides it's not a good idea to upset the code enforcement officers in your area , and complete a project, that is not up to your areas code standards. Hope your project runs well. Sal


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