# Footing for a deck.



## Batknee (Jul 15, 2020)

Hello, I am new here, I live in Maine and am building a deck. I am trying to figure out what code is for the footings for the deck. I looked at 2015 IRC r507 but it doesn’t give an exact for the diameter and I can’t figure out depth. If anyone can that that would be great.


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## Darren Emery (Jul 15, 2020)

Batknee said:


> Hello, I am new here, I live in Maine and am building a deck. I am trying to figure out what code is for the footings for the deck. I looked at 2015 IRC r507 but it doesn’t give an exact for the diameter and I can’t figure out depth. If anyone can that that would be great.


You really need to get with your local jurisdiction for this answer.  Too many variables, and need a full plan to answer that question.


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## Batknee (Jul 16, 2020)

That’s the problem is what I am being told I can’t find/read anywhere in the irc. I am building a 12x12 deck attached to house. Then a 10x8 deck off that attached to house. (Wrap around)


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## tmurray (Jul 16, 2020)

Footings are typically based on the beam and joist spans.

I'd tell you what I require, but you're on the wrong side of the border.


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## steveray (Jul 16, 2020)

You need to find the load going to each pier and divide by the soil bearing capacity per square foot....Not rocket surgery...What the municipality may use for default soil bearing is the variable...Where in Maine? I know some of those guys...


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## classicT (Jul 16, 2020)

The basic formula is as follows:

(Tributary area) x ( (Live load) + (Dead load) ) / (Soil Bearing Capacity) = (Minimum area of Footing)

Sq. root of the minimum footing area will give you the width dimension for a square footing.


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## rktect 1 (Jul 16, 2020)

Ty J. said:


> The basic formula is as follows:
> 
> (Tributary area) x ( (Live load) + (Dead load) ) / (Soil Bearing Capacity) = (Minimum area of Footing)
> 
> Sq. root of the minimum footing area will give you the width dimension for a square footing.



This is the correct answer above.  If you do not know your soil bearing capacity, we allow the use of 2500 psf without soil test report.


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## jj1289 (Jul 16, 2020)

Go to AWC.org and search for DCA6.  This is a free publication on how to design and construct a deck including footings.  As an FYI, much of the DCA6 manual is incorporated into the 2018 IRC and soon to be published 2021 IRC.


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## Sifu (Jul 17, 2020)

2018 IRC now has a prescriptive deck footing table.  It is based on tributary area so you still need to follow the guidance given to find that.  It is conservative but easy to follow.  Your AHJ would probably let you use it in the absence of a locally accepted method.


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## jar546 (Jul 17, 2020)

rktect 1 said:


> This is the correct answer above.  If you do not know your soil bearing capacity, we allow the use of 2500 psf without soil test report.



I thought the code said to use 2500psf in the absence of a soil report


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## e hilton (Jul 17, 2020)

jar546 said:


> I thought the code said to use 2500psf in the absence of a soil report


Jar ... clean yer spectacles ... he said the same thing.


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## jar546 (Jul 18, 2020)

e hilton said:


> Jar ... clean yer spectacles ... he said the same thing.


Damnit, I meant to type 1500psf


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## rktect 1 (Jul 20, 2020)

jar546 said:


> Damnit, I meant to type 1500psf



I have seen many soil reports for the village that I work in and none have ever come back under 3500 psf.  If the code says to use 1500 psf, and you are worried about your area, then you should use that.  But, those piers are going to be very large.


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## Glenn (Jul 20, 2020)

rktect 1 said:


> I have seen many soil reports for the village that I work in and none have ever come back under 3500 psf.  If the code says to use 1500 psf, and you are worried about your area, then you should use that.  But, those piers are going to be very large.


The entire deck industry appreciates your ability to administer the code appropriately for your region.  I appreciate you making code industry professionals appear sensible and likely building increased trust from your community.  THANK YOU.

The 2018 deck footing table is not so good.  Many issues were fixed in the 2021 edition, specifically the minimum 14" diameter regardless of soil bearing capacity.

PLEASE accept the 2021 for prescriptive design.  PLEASE consider the 1500 psf default for decks is perhaps unnecessary.  Decks can tolerate subtle movement better than plaster coated houses.  They do not need to be as conservatively designed.  Similar to how accessory structures up to a limited size don't need to be frost protected.  It's a risk assessment for the most appropriate MINIMUM code. I hope to see this more specifically addressed for decks in future code editions.


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## Flexo (Jul 20, 2020)

Show me a open deck that won't have a roof in another decade and be incorporated into the home within 30 years. Porches and decks almost always evolve in my neck of the woods. I believe that foundations for decks need to anticipate the eventual roof load that will come.


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## steveray (Jul 21, 2020)

Ohhhhhh....So we should regulate what might happen...Got it.....


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## tmurray (Jul 21, 2020)

Flexo said:


> Show me a open deck that won't have a roof in another decade and be incorporated into the home within 30 years. Porches and decks almost always evolve in my neck of the woods. I believe that foundations for decks need to anticipate the eventual roof load that will come.


I respectively disagree with this statement. The responsibility of the building official is to evaluate whether the current design meets the code, not to hypothesize what may come.

If we see things that may restrict options in the future, we do make recommendations; "If you think you might want to enclose your deck, you should really use this as your footing. Otherwise, you would need to replace it later.", but stop short of requiring it.


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## rktect 1 (Jul 21, 2020)

Flexo said:


> Show me a open deck that won't have a roof in another decade and be incorporated into the home within 30 years. Porches and decks almost always evolve in my neck of the woods. I believe that foundations for decks need to anticipate the eventual roof load that will come.



Been there, done that.  They cannot use the post holes for a roofed over structure that were designed for a deck 10 years ago, unless they can prove the loading.  Since most have been designed here for 2500 psf, they can easily get a soils report.  This report will then probably show at least 3500 psf if not more and they can have an architect or qualified professional do the calculations.  Maybe it comes out ok, maybe not.  So far though, the new loading has meant that I send them a letter stating that they will need new piers.  I have not had an issue yet.


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## classicT (Jul 21, 2020)

Flexo said:


> Show me a open deck that won't have a roof in another decade and be incorporated into the home within 30 years. Porches and decks almost always evolve in my neck of the woods. I believe that foundations for decks need to anticipate the eventual roof load that will come.


While we are at it, lets go ahead and assume they will add a second story too....

Nope! You can only review what is presented, not come up with your own arbitrary requirements based upon what you think may happen. And if it is that prevalent in your area, ask the question and perhaps give a recommendation.


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## Glenn (Jul 21, 2020)

A minimum standard controlling the choices of free people probably shouldn't include assumed, future choices of those people.

However, that would also include requiring a pre-wire for solar panels or an outlet for an electric car.  Both are assumed future choices.  Hmmm...I'll stop there and keep this on topic.

There are assumptions of human behavior and choices throughout the code.  However, adding a roof to a deck or turning it into habitable space requires a permit.  This would prompt the analysis of the new construction being requested for permit.  If someone builds a roof over a deck without that request, the foremost problem is work without permit, not foundation design.  That makes that "assumed future choice" into an "assumed, *illegal*, future choice".


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## Rick18071 (Jul 21, 2020)

What about free standing decks like one next to a aboveground pool. I don't make them do a frost footing for this, just 12" deep. 
What do the rest of you do about footings for free standing decks?


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## Keystone (Jul 21, 2020)

Rick18071 said:


> What about free standing decks like one next to a aboveground pool. I don't make them do a frost footing for this, just 12" deep.
> What do the rest of you do about footings for free standing decks?



Same as you under the sq ft requirement, I do specify & look for WYE bracing.


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## steveray (Jul 22, 2020)

Same....less than 600sqft


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