How to size footings

by Deborah Ford
(Annapolis, Maryland)

I am building a small deck at my back door (so I can let my dogs out). It will be 10 feet wide (against the house) and 6' 8'' out. It will be freestanding so I don't have to work with a ledger board.

I have dug 4 holes at 6'6" on center apart (wide) and the first two are 18" (centerline) from house and the next two are 48" (centerline) from those. The paper I have says to make the cylinder foundations 16" in diameter and 30" deep. The depth is not a problem.

All my neighbors decks have 12" diameter cylinders and I think 16" is too much for such a little deck. I can't find the building codes for Anne Arundel county Maryland. The web just keeps putting me to deck builders and I am doing it myself.

What size do my cylinders need to be? I can't even find 16" sono tubes.

Thank you,

Deborah (who is tired of walking my dogs around the house to the back yard)

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Apr 14, 2021
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starstarstarstarstar 		Your footing diameter NEW
by: Skyle

Rule of thumb:
Footing diameter depends on the post size

3x the diameter of the post

4x4 requires footing diameter of 12"
6x6 requires footing diameter of 16
Jul 14, 2011
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starstarstar 		Footing size and soil type are most important
by: Editor - Rich Bergman

A larger diameter pier or footing - the footing is that bottom of the pier that distributes the load to the soil - simply allows you to - possibly - use less footings to achieve the same weight distribution. That is because you are spreading the weight over more area per footing.

But in your case I don't think it will matter much. The real question in all of these discussions about how many footings and the size to use depends on how hard the soil is. A good average for most soil is 2000 psf. But it can get worse. I just built a new home on clay. Its called Leda clay and was a former sea bottom so its a real concern and the engineers said it could hold a maximum of 1500 psf. So we engineered the footings to be 52" wide, and thicker and full of steel throughout the footings and walls.

The risks for a deck failing are way less than a house and less expensive. But the principal remains the same. I assume frost does not extend below 30" in your area, otherwise you have to dig deeper. Or if your deck is not more than 6' high you could build it as a free standing or floating deck and avoid the need for subterrain footings.

12" diameter piers should be more than enough and you good put a bell shaped footing form on the bottom if you wish to spread out the weight even more.

But a great place to start is on my deck footings calculator. You can play with many different variables and soil bearing capacities to see what works best for you.

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Shallow Footings For Deck

by Steve Cara
(Ottawa, ON, Canada)

I'm building a deck for a client that is in two parts.

I've tied the main deck to the house and poured 6 footings for it, then tied the second (lower) deck to the first in an "L" shape. The second deck has 9 footings.

Most of the footings are between 3 and 4 feet deep. About three of them are between 1.5 and 2.5 feet in depth due to solid obstructions to both shovel and auger (Rock and Construction Debris). They are not, however, on bedrock.

Should I be concerned about these heaving? If so, what can I do to shore them up?

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Jul 09, 2008
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starstarstar 		How Deep Should Footings Be?
by: Editor - Rich Bergman

This is a great question.

In Ottawa you have cold winters and the frost line can go up to 4 feet deep or more in a bad winter.

I would say with the 1.5 and 2.5 foot footings you have about a 50% chance over the next five years of seeing some lift on that footing. But given the rocky terrain in some of that area there is not much you can do without some very expensive excavation.

I would advise your client of this. Its a risk. Not huge but it is a possiblity. If they want to pay for the equipment and time to remove the rocks that is the only real solution at this stage.

You might actually have less long term headache if those small footings were set on top of a surface footing. I would put down 1/2" granular stone, compact it, set down a 24"x24" patio stone and then a deck block on top. You should be able to compact the granular to at least 1500 psi without machinery and that would probably be more stable over the winter spring periods.

I guess its too late for that now.

But I know the City of Ottawa has new bylaws that demand that if you connect your deck to the house the piers must be at least 60" deep. That is new. And it is significant. But this may not apply if you are outside the city boundaries. So for other readers, check your local bylaws.

The other product that I really like and I have used myself and will not lift because of its design is the Oz-Deck Foundation system. Check it out at www.livoutdoor.com in Canada and www.oz-post.com in the US.

Hope this helps.

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Alternate method to burying posts in concrete footings???

I am planning to build/rebuild a deck that is attached to the house and hangs over a daylight basement. The deck is about 9 ½’ above the ground at its highest point. It appears that Olympia, WA code requires 18” diameter footings 4’ deep with 6 x 6 posts buried in the concrete a minimum of 3 ½’.

This seems like complete overkill in every way but I’m guessing this is done to add lateral stability to the posts. I really don’t like the idea of having wood in contact with concrete due to the fact that the posts will be more prone to decay.

It also makes it very difficult if there was ever a need to replace a post.

Does anyone know of an alternate method of attaching a post to a footing that will have good lateral stability without actually burying the post in the concrete?

I’m thinking of something like a bracket that encases the outside of the post with a flange that bolts to the top of the footing or something similar.

I want to be prepared with other potential options to offer the permit office when I apply for a permit. Anyone know of brackets or hardware manufactured to deal with this situation?

P.S. This is the link to the Thurston county building code pertaining to deck construction.

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Where to Locate Holes for Deck Piers and Footings

by Alex

When I measured where to auger my holes for the deck footings, the post will sit right on the end of the deck...

Should I have left an overhang? So the deck would overhang the post underneath...you know what i mean? Or is it OK that i have the post on the end of the deck?

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May 01, 2010
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starstarstar 		An overhang is preferable
by: Editor - Rich Bergman

You can build your deck that way if you like, as long as the joist span limits are satisfied. But typically you should design your deck so that there is an overhang beyond the last beam.

The typical limit that is recommended is Lj/4. Lj is the maximum span that your joists can go between each run of beams. So let's say you have to have your beam at 6' centers from each other your Lj=72" or 6".

72"/4=18" or 1'-6"

Hope this helps. If not your deck will look a bit odd and perhaps amateurish but it should still function perfectly well.

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Planning for deck footings and layout

by Curtis Bader
(Franklin, Mass.)

I am planning to add a 16x16 four season porch attached to my existing house.

I know the town inspector has final say but would like to lay footings out and size corner and support posts.

Right now the plan is to attach one end to sill board of existing house and put 3 12" around and 4' deep concrete footings on the outer end.

It is planned to use 2"x10" joists with the vertical supports being pressure treated 4"x4" ranging from 5' in height on one down to 3.5' on the other end due to terrain.

I have had a friend help me lay out the basics to bring to the town but think 4"x4" corner supports on the side away from the house spaced at 8' seems to small.

Your expert thoughts would be appreciated.

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Jan 14, 2011
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starstarstarstarstar 		Use our Deck Footings Calculator Tool
by: Editor - Rich Bergman

This new tool under the Calculators tab should help you a lot with this problem. Or just click here.

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Deck support posts sunk in concrete or sit on footings?

by Bill Alvey
(Conover, NC)

I am building a deck that is above grade, the highest elevation about 14 feet from the floor to the ground(on a hill). All recommendations I read are to put the posts (6 x 6) on footings, bolted with brackets. It seems that burying the posts and filling around with concrete is more stable. Why is this not recommended? Is it an alternative way to put the posts in?

Thanks in advance for any help.

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Apr 20, 2012
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starstarstarstarstar 		not so fast!
by: Anonymous

> So my recommendation is to dig a deep hole, build
> a wide footing for good weight displacement, pour
> the cement into a cardboard tube form and even
> put in some rebar wired together to unify the
> footing with the pier. Let it cure and then
> secure the support post to a good post anchor on
> the concrete. That is your best long term bet.

Not as long term as you think! If you put iron (rebar) in the concrete it is going to rust,
expand and split the concrete, crumbling it to rubble and dust in a decade.
Oct 28, 2010
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starstarstar 		Secure the post to a concrete pier
by: Editor - Rich Bergman

Sinking a deck support post into a hole and then embedded in concrete will do the job fine if you have granular stone on the bottom of the hole to assist and water drainage and if you dig it well below whatever your frost line is. If it moves with the frost heave you will have problems.

But the main reason why I would not do this is because it will certainly shorten the life span of that post. It simply will not last as long as support post that rests above a concrete footing and remains essentially dry most of the time. Even in a worst case scenario a post that is secured to a footing rather than set into concrete can more easily be removed if rot were ever a problem.

So my recommendation is to dig a deep hole, build a wide footing for good weight displacement, pour the cement into a cardboard tube form and even put in some rebar wired together to unify the footing with the pier. Let it cure and then secure the support post to a good post anchor on the concrete. That is your best long term bet.

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