# Deck Foundation Sizing Survey



## Glenn (Dec 13, 2012)

I am working on a deck code proposal for the 2015 IRC with a group of folks from NADRA, NAHB, CLMA, VBCOA, AWC, ICC BCAC, CCICC and others.  There are folks that wish to require the minimum size pier/footing for decks to be about 17 inch diameter.

While this does calculate out based on the default 1500 psf soil compressive strength, I believe this size may be quite larger than what works just fine in many regions...even with 1500 psf soil.

Please provide your perspective and experience in this short online survey.

https://mountainllc.wufoo.com/forms/deck-foundation-survey/

Please respond here if you have questions, comments or suggestions regarding the topic, survey or questions asked.  You can also PM me.  If you are nervous about following the link, let me know and I will find some time to re-write it in this thread.

Thanks, and please keep this thread as a positive and constructive source for working together on this topic.


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## fatboy (Dec 13, 2012)

Done....

FWIW, we have always accepted 8" piers in this jurisdiction, no methodology that I am aware of, just what has always been allowed. Have never seen a situation where that size has proved to be inadequate.


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## mjesse (Dec 13, 2012)

Done.

8-10" with sonotube is typical here. 3500-4000 psi clay, 42" frost depth

mj


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## Pcinspector1 (Dec 13, 2012)

Done.... When do I get my cert?

pc1


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## Min&Max (Dec 13, 2012)

8" minimum here if using a 4 x 4 post. Going to 17" would not be well received by many including myself.


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## mjesse (Dec 13, 2012)

Pcinspector1 said:
			
		

> Done.... When do I get my cert?pc1


You need to pay $180 to get one of those


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## codeworks (Dec 13, 2012)

always used 10 or 12 inch in vermont, with a good bell footing at the bottom , well below frost line ( at least 4"6") here, we rarely get frost. i dont think a 17 inch pier would get over well, (especaiily here), but even in the coldest places. 10 or 12 inches seems seem plenty, reinforced properly and with a   bell at the bottom, and getting below frost that really counts. i've seen folks build decks on the ground on those prefab concrtete footings and be fine ? .


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## tmurray (Dec 13, 2012)

We require 14" at with 4' of frost cover up here in the Great White North.


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## Sifu (Dec 13, 2012)

I like the NC prescriptive code; 16x16x8, use of 4 solid 4x8x16 cmu's laid perpendicular permitted. (unless it has changed)  Here we pretty much go with the DCA6 manual.


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## globe trekker (Dec 13, 2012)

Glenn (and others),

Will the 2015 IRC requirements be for (downward) vertical & lateral loading only, or is

there any language regarding wind restraints, ..to prevent the deck posts & framing

from separating from the in-ground piers, or is this already addressed somewhere

else? Short question, but is wind restraint being included in the design

"requirements" of decks & associated framing? Thanks!

.


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## steveray (Dec 13, 2012)

10" usually is pretty close on 2k# soil here....maybe with a little bell......


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## DRP (Dec 13, 2012)

Same as Sifu, two 80# bags/hole. 12" pier if they insist on a hinge at grade.


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## kyhowey (Dec 14, 2012)

The Wood Deck Construction guide has a chart based on spans on page 9 showing footer sizes and thickness.

http://www.awc.org/publications/dca/dca6/dca6-09.pdf

You can also calculate it:

Tributary area: ½ distance from post to post or beam and/or house

Multiply area by 40lb live load & 10lb dead load=50lb

Total load divided by soil bearing capacity (1,500psf)

Square root = size of footing in square feet

Square feet times 1.12 give you the diameter if using round footing (nothing scientific about the 1.12 factor.  It just works and saves you steps.)


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## Glenn (Dec 14, 2012)

kyhowey said:
			
		

> The Wood Deck Construction guide has a chart based on spans on page 9 showing footer sizes and thickness. http://www.awc.org/publications/dca/dca6/dca6-09.pdf
> 
> You can also calculate it:
> 
> ...


Sorry.  I forgot to include in my first post that the AWC is also part of the group I am working with on this.  I have edited that in now.  The DCA 6 is a big starting point for much of our work.

The problem is that calculating the bearing area is yielding some sizes that are well, well, well in excess of what appears to be working just fine across the country for decades.  This is the problem and why I am asking for input regarding how you size them in your region and how they have been performing.

It appears that regions with no significant frost depth, are pouring large bearing areas in their shallow foundations, likely because it's not that big a deal.  Also, when your not going very deep, it probably just "feels" weird to not put much concrete in the hole.  So it's likely still a two bag, wide footer.

On the other hand, regions with deep frost depth (3+ feet) are digging narrow and straight down.  Still a two bag hole, but not crazy wide.  If only calculating bearing area, these foundations would appear to be grossly insufficient.  However...they seem to be working just fine all over the place.  These regions will not take well to having to pour wide footers at the bottoms of their 3 foot holes.  Homeowners also do not want large amounts of exposed concrete at the surface, though there are other ways around that.

Please share your experiences and deck foundation methods...before one ends up in the 2015 IRC.  Why are 10-inch deck piers working just fine all over the place?  Why should they now be so much bigger?

ENGINEERS!  Could it be possible that even at only 3 or 4 feet deep that there is some skin friction working in favor?  How would that compare in an un-cased pier (straight against the dirt) vs. one that poured in a forming tube?

Thanks so much to all who have responded.  There is a large proposal of new deck code underway that will be submitted Jan. 3rd.  Whether a deck foundation sizing table is included has a lot to do with the group agreeing...which in my mind has a lot to do with how the country responds.  This survey is going out to many other places.

I encourage you to help by copying the survey link and sending it to your ICC chapter, contacts or any other group that is interested in responding and helping to shape the future 2015 IRC.

Thank you.


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## Glenn (Dec 14, 2012)

globe trekker said:
			
		

> Glenn (and others),Will the 2015 IRC requirements be for (downward) vertical & lateral loading only, or is
> 
> there any language regarding wind restraints, ..to prevent the deck posts & framing
> 
> ...


We discussed uplift on decks.  No one was able to produce any experiences where decks have experienced significant uplift from wind events.  It's usually topographical effects, as I am aware of some in the rocky mountains, but due to wind patterns in valley's coming up a mountainside to an overhanging deck.  Pretty rare.  Wind does not affect decks like it does roofs because there is no pressure difference from below the deck to above.  Actually a couple in the group spoke of homes with roofs removed from wind and deck left sitting just fine.

If you have any experiences or opinions about how wind affects decks and produces uplift, please share with me.  PM if you wish.


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## jar546 (Dec 14, 2012)

fatboy said:
			
		

> Done....FWIW, we have always accepted 8" piers in this jurisdiction, no methodology that I am aware of, just what has always been allowed. Have never seen a situation where that size has proved to be inadequate.


You must be on great soil/rock, otherwise, I do not agree whatsoever.


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## mtlogcabin (Dec 14, 2012)

18" bigfoot with an 8" sono tube is pretty common here. One contractor just did 4 decks on existing homes this year and used helical piers for the foundation. Said they where faster more accurate less disruptive to the existing yard and in the long run less exspensive.


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## Inspector Gift (Dec 14, 2012)

Glenn,the building department at Cowlitz County has a good handout for homeowner/builders to do deck design.  It requires a minimum deck footing of 12”x12”x8” for all decks.   Decks over four feet in height will require an 18”x18”x8” footing.

Bottom line, the footings should be sized to support the calculated design loads.   Using a soil bearing capacity of 1500# psf, a 17" diameter footing has a capacity of supporting over 2250#.   It is not uncommon for middle footings to have loads greater than 2000#.


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## Papio Bldg Dept (Dec 14, 2012)

8 inch diameter minimum here...only good for 500 PSF with 1500 soil bearing capacity.  We sometimes allow for reductions of footing diameter depending on prescriptive soil analysis in the IRC.

http://www.egovlink.com/public_documents300/papillion/published_documents/Building%20Department/Residential%20Permit%20Folder/Deck%20Brochure%20updated%206%2025%2011.pdf


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## Papio Bldg Dept (Dec 14, 2012)

one thing we have seen, although not prescriptive is the use of engineering to include coefficient of friction in analysis for depths exceeding frost depth.  This allowed one developer to use all 8 inch dia footings by making them 56 inches deep versus the 42 inch frost depth required.  If I remember it was a 12 x 9.5 deck.


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## fatboy (Dec 14, 2012)

jar546 said:
			
		

> You must be on great soil/rock, otherwise, I do not agree whatsoever.


Just good old sandy, little to no clay farm ground. Like I said, been the same requirement for the 30 plus years I've been in/around the trade, have not seen any issues with it.


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## Pcinspector1 (Dec 14, 2012)

Papio, your deck handout is one of the best I've seen, as Garth Algar would say "Excell..........ant!

pc1


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## ICE (Dec 14, 2012)

fatboy said:
			
		

> Just good old sandy, little to no clay farm ground. Like I said, been the same requirement for the 30 plus years I've been in/around the trade, *have not seen any issues with it.*


Over the years as the code changes, I see some things appear and others go away.  Now and then I get the feeling that some of the changes are done not for any reason other than that the people that make changes need to eat too.  They need to do something rather than nothing or they might not be needed in the future.  A lot of stuff that ain't broke gets fixed.


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## Keystone (Dec 14, 2012)

Wow, 8" diameter piers - haven't seen those since BOCA. No concerns over cross sectional area of the post, centering of the post, etc?

Using a 4 by post = 10" minimum diameter

Using a 6 by post = we have allowed 12" diameter but strive for 14" minimum diameter


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## DRP (Dec 14, 2012)

That's what has worried me in the past, not settlement but a slender poorly footed pier to grade on solid ground with loose fill around its' tube and an eccentric post placement at the hinge between pier and post. I try to talk clients into running the posts down to the footing to avoid that hinge and potential lateral problem. I noticed in Papio's version that this is prohibited and am curious about the reasoning. BTW it is showing the ledger lags above the joist hangers, tension perp failure, I've seen pics of that deck on the ground with lags and the top of the ledger still on the building.

Those of you requiring a larger diameter pier, is it for footing size or lateral resistance?


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## Architect1281 (Dec 15, 2012)

So Pap you do not deduct the weight of the footing as dead load ? and there is NO safety factor in your analysis at all?

The friction thing only works in friction piles not in shovel applied backfil as well

you are giving a gift at those areas and weights


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## Architect1281 (Dec 15, 2012)

Not to be a pesemist but you are trying to teach a rock to sing. All your honorable work will only keep honest people honest and smart people smart. unfortunatly stupid abounds, and ya can't fix legislate or change stupid; you can only dis-approve upon inspection.


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## incognito (Dec 15, 2012)

Standard practice in my area is a 8" diameter hole, 42" deep filled with concrete with post anchored to top. Soil conditions range from sand to clay. I have yet to see any pier footing of this design fail in any way. If this type of BS finds a way into the 2015 IRC/IBC it will only be another reason for jurisdictions to NOT adopt yet another code cycle.


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## Keystone (Dec 15, 2012)

Papio, your deck format is one of the better ones I've seen, going to tweak & utilize that as a templete.

Clarification- Using a 6 by post = we have allowed 12" diameter... We allow this when the deck is low to the ground and capable of utilizing a 4 by.

 One item we make abundently clear to applicants on submission, if they can center every pier with the post connection then they are welcome to utilize the minimum sizes but if the pier to post connection is even slightly offset they must step up the size appropriately.

DRP & Architect1281 bring another question to mind, primarily in our area we see augered holes without the use of sonotubes, what method are you seeing in your area?

Contractors in my area are reluctant to countersink posts into piers due to snapping at the top of the pier. Now correct me if I am wrong but wouldn't the introducation of countersinking the posts eliminate the necessity of lateral tension devices?


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## Glenn (Dec 15, 2012)

Keystone said:
			
		

> Now correct me if I am wrong but wouldn't the introducation of countersinking the posts eliminate the necessity of lateral tension devices?


It can.

I researched this, and found a formula in the IBC that has been there for decades for freestanding signs.  I wrote an article about this in the Professional Deck Builder.  I also put together a table of embedment/# of posts/height combinations that would yield 3,000 lbs of lateral resistance.

Have a read.  The part about sinking posts is on the third page.  I think you will find it interesting.

Handling Lateral Loads - Building Science, Structure, Design, Framing - Professional Deck Builder Magazine Page 1 of 3


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## Architect1281 (Dec 16, 2012)

Yes lateral loads are reduced or eliminated by adequate embedment; next issue wood posts must then be rated for direct burial

and not ground or above contact.


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## Architect1281 (Dec 16, 2012)

Jus a little tool I use when teaching Deck / residential plan review Note NO SAFETY FACTORS APPLIED

Footing / Pier			Soil PSF/area TOTAL LOAD LBS

dia	Area	DL conc	1500		2000		3000

	Sq.Ft.	Lbs.	Total 	Net	Total 	Net	Total 	Net

8	0.349	209.33	523.33	314.00	697.78	488.44	1046.67	837.33

10	0.545	272.57	817.71	545.14	1090.28	817.71	1635.42	1362.85

12	0.785	392.50	1177.50	785.00	1570.00	1177.50	2355.00	1962.50

16	1.396	697.78	2093.33	1395.56	2791.11	2093.33	4186.67	3488.89

Dia		Live Load	Tributary Deck area in SF per pier at Live Load + 10 DL

8		40		6.28		9.77		16.75

10		40		10.90		16.35		27.26

12		40		15.70		23.55		39.25

16		40		27.91		41.87		69.78

8		50		5.23		8.14		13.96

10		50		9.09		13.63		22.71

12		50		13.08		19.63		32.71

16		50		23.26		34.89		58.15

8		60		4.49		6.98		11.96

10		60		7.79		11.68		19.47

12		60		11.21		16.82		28.04

16		60		19.94		29.90		49.84


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## Architect1281 (Dec 16, 2012)

Sorry bout above it looke wonderfull from spread sheet unyil posted so feel free to delet


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## 97catintenn (Dec 16, 2012)

I think that it should be viewed as a footer period.  What ever the requirements are for house pier footer, the deck footer should meet.  It should be that way so that in the future a roof can be added and the footer will still be adequate.  Otherwise, you will have minimal footers can't handle the additional load that the home owner puts on it when he builds his roof without a permit, or adds a second level deck to it.

I've worked in areas that required 6"s of concrete under a post for a deck and 8"s if its a covered deck, and of course the other areas out in the wild Tennessee county side where there isn't any inspections or permits required for decks


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## 97catintenn (Dec 16, 2012)

Architect1281 said:
			
		

> Yes lateral loads are reduced or eliminated by adequate embedment; next issue wood posts must then be rated for direct burial and not ground or above contact.


I haven't seen any treated 4x4s or 6x6s that were not rated for direct burial.


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## rktect 1 (Dec 17, 2012)

We use an assumed soil bearing of between 2500-3000 and calculate each pier out.


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## Glenn (Dec 17, 2012)

Thank you all for the responses.  They will provide a great perspective and pulse of what is really going on out there, before any changes to it are proposed.  I only wish I had more time to keep collecting data.  Plenty of time for the 2018 edition...but running out for the 2015.

Know where you are, before you decide where you're going.


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