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

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

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.

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

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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Robert Shaw
Colorado Springs Custom Decks
Colorado Custom Decks
Custom Composite Decks

72chevy4x4

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!!!

AtlanticWBConst

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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" Of course I don't look busy, I did it right the first time "

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/publicatio...905/entire.pdf

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

RobertCDF

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.

Attached Thumbnails

 

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Robert Shaw
Colorado Springs Custom Decks
Colorado Custom Decks
Custom Composite Decks

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.

Driftwood

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.

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.

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.

Driftwood

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

RYANINMICHIGAN

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

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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Dick

Engineer, designer and consultant recently active domestically and internationally on construction and design in about 35 countries.

RYANINMICHIGAN

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.

dkillianjr

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

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