Proper Sizing For Deck Footings - Building & Construction - DIY Chatroom Home Improvement Forum

 DIY Chatroom Home Improvement Forum Proper sizing for deck footings

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02-09-2010, 06:03 PM   #1
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## Proper sizing for deck footings

I am in the planning stages of a relatively small elevated deck off the back of my house. The deck will be approx. 12' long by 6.5' wide and will be approximately 8.5' off the ground. To accommodate lower profile stairs down to ground level, we've utilized two landings, such that you will take one step down from the main deck to a 4'x4' landing, then 3 steps down to a second 4'x4' landing and then 7 steps down to ground. The main deck is supported by a 12' long beam composed of 3 2x10s, the beam rests on 2 6x6 posts each located 2' in from the edge of the deck, resulting in an 8' span with 2' of cantilever on either side.

My understanding is that the footings for the main deck 6x6 support posts will need to be 17" in diameter, set at frost depth (30" here). Does that sound about right?

I am more confused about the support posts for the landings. The beams here will be a double 2x10 with an approx. 4' span. I cannot figure out if the posts need to be 6x6 or if I can use 4x4, any thoughts? And regardless, what size footings? I have read that 12" diameter is correct, set at frost depth. Is that correct? I have been unable to find a calculator, formula or other precise guidance on this.

Thanks!

 02-09-2010, 06:26 PM #2 Member     Join Date: Oct 2006 Location: Minneapolis/St. Paul, Minnesota - Latitude 45.057 Longitude -93.074 Posts: 4,113 Rewards Points: 2,716 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

 02-09-2010, 06:30 PM #3 Member   Join Date: Dec 2008 Location: Washington, DC Posts: 355 Rewards Points: 250 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.

 02-09-2010, 07:19 PM #4 Civil Engineer   Join Date: Mar 2009 Location: Boston Posts: 5,832 Rewards Points: 5,246 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.
 02-09-2010, 09:28 PM #5 Registered User   Join Date: Mar 2009 Posts: 11,730 Rewards Points: 526 __________________ If any ads are present below my answer or words underlined/colored, I do not condone/support/use the product or services listed/linked to, they are there without my consent. 17,000 dryer fires a year, when did you last clean the inside of the dryer near motor or the exhaust ducting?
 02-09-2010, 10:32 PM #6 Member   Join Date: Dec 2008 Location: Washington, DC Posts: 355 Rewards Points: 250 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!
 02-09-2010, 10:57 PM #7 Member   Join Date: Jan 2009 Location: South of Boston, MA Posts: 17,248 Rewards Points: 2,000 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
 02-09-2010, 11:21 PM #8 Civil Engineer   Join Date: Mar 2009 Location: Boston Posts: 5,832 Rewards Points: 5,246 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.
02-10-2010, 12:39 AM   #9
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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.
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 02-10-2010, 08:25 AM #10 Member   Join Date: Dec 2008 Location: Washington, DC Posts: 355 Rewards Points: 250 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!
 02-10-2010, 10:09 AM #11 Licensed P.E./Home Insp     Join Date: Dec 2008 Location: New Jersey Posts: 757 Rewards Points: 844 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/filestor...on_package.pdf
 02-10-2010, 10:52 AM #12 Member   Join Date: Dec 2008 Location: Washington, DC Posts: 355 Rewards Points: 250 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!
 02-10-2010, 11:32 AM #13 Licensed P.E./Home Insp     Join Date: Dec 2008 Location: New Jersey Posts: 757 Rewards Points: 844 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.
02-10-2010, 12:09 PM   #14
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Quote:
Originally Posted by Aggie67
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.

Quote:
 Here's just one fantastic example from my neck of the woods:
http://www.twp.howell.nj.us/filestor...on_package.pdf
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!

Kevin

 02-11-2010, 12:26 PM #15 Member   Join Date: Dec 2008 Location: Washington, DC Posts: 355 Rewards Points: 250 Thanks everyone! This has been great!

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