# Proper sizing for deck footings



## concretemasonry (Oct 10, 2006)

You did not post your location, so all you can get is some general dimensions.

By the way - Is it freestanding or connected to a ledger supported by through bolts.

The width of the footings (Sonotubes, I would imagine is what you will use for the main deck) depend on the soil bearing capacity. Smaller tubes can be used with a big-foot form if you want a smaller diameter Sonotubes. The frost depth gives the minimum required depth that is required for footings. If you are relying on the soil for lateral support for an 8.5' high deck, you might have to be deeper.

Good luck!!

Dick


----------



## stubits (Dec 30, 2008)

Thanks so much for the response. I thoughts I added my location to my profile, I guess not. Will do so now.

I am located in the Washington, DC area.

The primary deck will be ledgered to the house, yes.


----------



## Daniel Holzman (Mar 10, 2009)

Normally deck elements are not specifically sized based on detailed calculations, rather they are sized in the building code or perhaps in local codes maintained by the code enforcement officer. The idea of a 17 inch diameter footing is really odd, since as far as I know sonotubes do not come in that size, I am not sure how you would build it. Also, a 17 inch diameter footing is probably overkill by at least a factor of 4 in terms of capacity, given that you are extending it to frost, but if that is what the code requires, that is what you have to do.

If you do not have code mandated sizes, then you would size the footing based on the load and the soil capacity. Soil capacity can be estimated based on the type of soil you have (sand, gravel, clay, silt, ledge), and there is always a table in the code book that tells you the allowable capacity in pounds per square foot that soil of a particular type is good for. The vertical load on each pier is a function of the code mandated minimum loading on the deck, which is typically given in pounds per square foot of deck. You calculate how many square feet each post supports, multiply by the required load per square foot, and use that figure to compute the vertical load on each pier.


----------



## Gary in WA (Mar 11, 2009)

http://books.google.com/books?id=1g...=X&oi=book_result&ct=result&resnum=3#PPA31,M1

http://rockproducts.com/mag/rock_aggregates_prevent_frost/

http://www.awc.org/Publications/DCA/DCA6/DCA6-09.pdf

Be safe, Gary


----------



## stubits (Dec 30, 2008)

As always, many thanks to all of you for your input. I am definitely starting to understand things better now. I think I have the information I need now to figure out the footings.

I'd really appreciate your thoughts on using 4x4 posts for the deck stair landings. I've read that the newest version of the IRC requires 6x6 posts on decks, but it seems like tremendous overkill to use 4 6x6 posts on a 4x4 landing. Are landings treated as decks, as stairs or as something distinct?

Thanks!


----------



## Scuba_Dave (Jan 16, 2009)

I've always assumed the footing would be larger then the sonotube
So you might have a 2' footing with a 12" sonotube sitting on it ?
Just like the footing in a house - foundation wall


----------



## Daniel Holzman (Mar 10, 2009)

Generally the size of the footing when using sonotubes is the inside diameter of the sonotube. If you use a bigfoot sonotube, the bottom of the tube flares outward, so the area of the bottom is larger than the tube. With standard sonotubes, the hole is excavated a little larger than the tube, the tube is inserted and filled with concrete (and rebar if required), and the tube is typically left in place. I have never seen a sonotube installation where the tube was inserted into an oversized footing.


----------



## jlhaslip (Dec 31, 2009)

Daniel,

Lots of times I have placed 12" D sonotubes onto a 24" square, 8" Deep footing pad.
Build a frame 24" square, topped with OSB and then cut the 12" hole for the tube. Nail 2 x 4 cleats around the hole, slip the sonotube into the cleat opening, nail/screw the tube to the cleats, brace, pour, done.

Might be soil specific on the details, but we do lots of deck piers this way around here. The wider base allows for more bearing surface on the tube.
And most of our work is in a gravel/sand base. Clay is the exception around here.


----------



## stubits (Dec 30, 2008)

Yes, I think pouring a wider footing with a smaller pier is pretty common around here, although I have no idea why. The soil here has a lot of clay with a generally accepted bearing capacity of 2500 psf. 

Any thoughts on using 4x4 posts for the deck stair landings. I've read that the newest version of the IRC requires 6x6 posts on decks, but it seems like tremendous overkill to use 4 6x6 posts on a 4x4 landing. Are landings treated as decks, as stairs or as something distinct?

Thanks!


----------



## Aggie67 (Dec 20, 2008)

You might want to check with the town. A lot of towns in my area use altered versions of the AWC guides as their "standard" for deck construction. The AWC guides are a great starting point, but you never know, the town might throw in some local changes to cover variations like weather, geology, materials, etc. The AWC guides are just that - guides. If the town maintains modified standards, those will most likely be as close as you're going to get to a freely available best-practice type of construction standard for your area. It's a free phone call, in any case. Some towns post them on their website.

Here's just one fantastic example from my neck of the woods:

http://www.twp.howell.nj.us/filestorage/75/128/New_Deck_Information_package.pdf


----------



## stubits (Dec 30, 2008)

Thanks so much. I was actually born and raised in Toms River, so not too far from your stomping grounds.

Unfortunately DC hasn't produced anything like Howell has, they've just referred me to to 2006 IRC and IBC. I looked back over it and it seems like I can in fact use 4x4 posts. It also seems like the minimum footing is 12", which is certainly overkill, but that is fine. 

Thanks everyone!


----------



## Aggie67 (Dec 20, 2008)

Well, the AWC's DCA-6 mentioned by GBR and others is co-branded with the IRC Code Council.

Here's a blurb from my 2006 copy of DCA-6:


Where applicable, provisions and details contained in this document are based on the 
​_International Residential Code (IRC) _[bracketed text references applicable sections of the _IRC_]. Prescriptive construction methods recommended meet or exceed minimum requirements of the _IRC_. Provisions that are not found in the _IRC _are recommended as good industry practice. Where differences exist between provisions of this document and the _IRC_, provisions of the _IRC _shall apply. This document is not intended to preclude the use of other construction methods or materials. All construction and materials must be approved by the authority having jurisdiction. Every effort has been made to reflect the language and intent of the _IRC_. However, no assurance can be given that designs and construction made in accordance with this document meet the requirements of any particular jurisdiction.​ 
In the absence of any local jurisdictional requirements, I stick to referencing DCA-6 and IRC 2006 on the permit drawings I put together for residential jobs.

I think you'll be safe using DCA-6, but make sure you reference that you used it and IRC 2006. Good luck.​


----------



## Kevin M. (Nov 26, 2009)

Aggie67 said:


> You might want to check with the town. A lot of towns in my area use altered versions of the AWC guides as their "standard" for deck construction. The AWC guides are a great starting point, but you never know, the town might throw in some local changes to cover variations like weather, geology, materials, etc. The AWC guides are just that - guides. If the town maintains modified standards, those will most likely be as close as you're going to get to a freely available best-practice type of construction standard for your area. It's a free phone call, in any case. Some towns post them on their website.
> 
> 
> 
> ...


Hey Aggie67,

Wowser!! That is impressive. Beautifully laid out detailed wise. The efficiency of this type of a package should streamline some of the generalities. For an inexperience deck builder, this is a great guide for correct applications. 

They even illustrated an alternate nailing placement on the ledger shims, so as not to split them. Good details!

Thanks for posting that link. 

Kevin


----------



## stubits (Dec 30, 2008)

Thanks everyone! This has been great!


----------

