Do deck posts have to be 4 feet deep due to frostline?

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Posted by: from Belle River, Ontario
10/18/2014 at 8:33:02 AM

We are building a deck - 52 feet x 16 feet and bolting the frame to our steel breakwall. Do we need our support posts going down 4 feet due to the frostline? (We live in Ontario, Canada).

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Joseph from Kolten Landscapes in Windsor
Date/Time10/18/2014 at 11:30:33 AM

Yes most definitely. The frost will only push the post back out of the ground, 2 years from now. You can not guarantee a deck without the posts 4 feet deep. Just watch your contractor carefully as he does these holes. Or you won't know what your getting guaranteed.

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Date/Time10/18/2014 at 5:07:30 PM


Yes, you must anchor all of your posts properly. All the holes have to be drilled a minimum of 4'. And there should be about six inches of concrete added to the hole prior to setting the 4"x4" or 6"x6" in place. then fill to the slightly above the root line of the grass.

These posts act like the foundation of your home.

Keep an eye on the contractor that is doing the work for you. Because the vast majority of contractors will not dig a full 4'. To save time removing soil and to save expense on concrete.

Enjoy your new deck,

James Fram

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Date/Time10/18/2014 at 5:17:41 PM

Unlike another reply I saw... the answer is no. You might not need to go down 4 feet into the soil. It all depends on the location and the type of soil you have. Another factor is the height of the structure. I have been using deck blocks for the better part of 25 years and none have ever moved. The 4 foot requirement is something written into the Ontario and National Building code in part 9. It is intended for a heated buildings or a house.

If you look around, you will see many cottages where the supporting structure is no more than poured concrete slabs underneath piers. It is true that frost will move them about a wee bit, but when the frost leaves, they will go back to their original position.

I have seen many posts that have been set at 4 feet blow the soil which have heaved. This is where unfavourable soil conditions exist. The heaving is caused by a phenomena called Ad-freezing. This happens when frost attaches itself to the sides of the posts and lifts them. Once lifted, they cannot go back down the hole. You have surely seen many fence posts that seemingly 'walked' upwards.

Dow Chemical who are the manufacturers of the Styrofoam* brand of products have researched this in many places across the world. You might want to look at their website and search out 'Soils Insulations'.

If you do choose to go with deck blocks, I highly recommend you use a patio slab underneath it. This inexpensive addition will increase the load bearing capability of the deck block 3 fold and thus prevent any 'settling'. This knowledge I gained from being a past Dow Chemical employee and dealing with architects and engineers who use similar construction methods all over north America.

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Date/Time10/19/2014 at 5:43:02 PM

Yes however, deck posts should not be placed into the ground, rather a concrete pier should be poured and a post bracket should be used.

Unlike a fence post, where the post needs to be in the concrete due to horizontal pressure on the fence such as wind.

For decks the best solution is at least an 8" post hole, with a sono tube, filled with concrete with a post bracket inserted on the top of the concrete.

The sono tube is very important to prevent heaving. When a post hole is dug, it naturally takes on a cone shape, that is wider at the top and narrower at the bottom. This results in heaving because of the horizontal pressure on the concrete.

Sono tubes prevent a cone shaped hole and therefore the frost cannot get a grip on the post preventing heaving.

Sono tubes cost about $6/hole so they are well worth it.

Most importantly, be there when the holes are done, and don't be afraid to measure the depth, any self respecting contractor won't mind.

Good luck.

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Date/Time10/20/2014 at 9:44:17 PM

Yes, I agree! Sona tubes are a must! 48 inches minimum.

A permitted project will always be inspected by a building inspector.

Best regards,

Robert W. Barrer

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Date/Time10/21/2014 at 11:09:56 AM

Hi Dawn,

I thought I would chime in.

Yes, your foundation must extend below the frost line AND must bear on undisturbed soil that is rated for the capacity of the load being placed on it. In most of southern Ontario 4 feet deep is the norm (technically Part 9 of the Ontario Building Code states 1.2m which is 3.927 feet), however it is not sufficient to simply use depth as the only guideline. It is critical that the bearing pressure of the soil can handle the load being placed on it and rests on undisturbed soil. Undisturbed soil is loose fill that has been "aerated" by the excavation process and other loose material that has fallen back down the hole. This material most certainly cannot bear the mass of much of anything and will result in your foundation sinking sometimes up to a few inches.

I am not clear on your plans exactly but regardless there are specific exceptions listed in the code and please note that your Municipality down there in Belle River can add more restrictive requirements (not the other way around) and will have zoning requirements you will need to adhere to as well.

Frost heave is a very serious concern despite what anyone says to the contrary and should be taken seriously when designing any structure. Depending on the type of soil and water content, frost heave can destroy a structure and the structure it is attached too.

Regardless, it is best to have a Designer produce drawings with details and apply for a permit which will be reviewed by a Plans Examiner prior to starting construction as they are familiar with the local by-laws and soil conditions.

I am attaching two photos I found on the web that I think illustrate "frost heave" fairly well.

Best of luck with your project.

Jason Irving

The Cedarbrook Group

Do deck posts have to be 4 feet deep due to frostline?

Do deck posts have to be 4 feet deep due to frostline?
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Date/Time10/24/2014 at 12:34:22 PM

Hi Dawn,

I apologize, I made a typo and said "undisturbed soil is...", it should have read "Disturbed soil..."


Jason Irving

The Cedarbrook Group

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