Frost Heave on a new deck

brianluMarch 3, 2008

I built an 19x16 foot deck last summer. The deck height is about 6 ft with 8 6x6 posts. The footings already existed and passed an inspection from the local city inspector. There are 4 posts along the middle of the deck, and 4 along the outside edge. This winter, I've noticed the middle 4 footings heaved and the outside posts are now about 3-4" off of the footing. I'll be carefully watching them as they settle back this spring, but is there any way to address this without having to redo all 8 footings? The existing footings are 48" down in a clay soil in the Twin Cities, MN.

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What is your code for footings? I live in northern Illinois and ours is 42". When I set posts, I always go 6" deeper and place gravel so there is drainage.

Somehow moisture has settled below your excavation and frozen. Clay soil holds moisture. Since you say the footings were existing, there is a strong possibility this condition has been happening in the past. Or they may not be as deep as you believe.

How is the drainage away from the deck? Is water collecting there or flowing away? Can you work below the deck? Short of replacing the footings, you might want to excavate alongside at least one to be certain they are deep enough and then place some gravel to help keep moisture from the footing.

Hopefully someone can offer a better solution.

    Bookmark   March 4, 2008 at 11:24AM
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I'm with fnmroberts on the gravel thing. I have 6X6 sitting on concrete 42" down, all backfilled with 3/4" gravel.

I had somebody help with my footing (he's a deck framer). He shot galvanized fasteners (Remington 22 caliber) securing the 6x6s into the concrete piers , using some kind of joist hangers.

Your piers are cured, I don't know if you can use a gun. But it sounds like you need some mechanical attachment. But I'm no deck pro.

    Bookmark   March 5, 2008 at 6:02AM
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I had an inspector check the footings before I built the deck to avoid this problem I have now. I dug down next to one footing for a check and they are definitely 4 feet down. I spoke with a soil engineer that I know and he said that it looks like the footings weren't lifted from below, but from the sides. Ice grabs on to the sides of the concrete and lifts it up like a straw. What I plan to do this summer, as fnmroberts suggested, is to grade the area underneath the deck for better drainage. Also, the soil engineer suggested digging down a foot or two along side each footing and putting a foam insulation to limit the frost penetration.

It looks like after I try these, it will be the waiting game until next winter. Otherwise, I will have to replace the footings.

Thanks for your suggestions.

    Bookmark   March 5, 2008 at 10:38AM
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Tubes weren't used when the footings were poured then. Rough edges of the concrete in the soil create the effect of a shallow pour.

When you improve the grade, consider placing some visquene atop the soil and hold it in place with some pea gravel.

    Bookmark   March 5, 2008 at 1:26PM
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Tubes were used, but I have a feeling the previous owner went with cheap ones since they have since rotted away.

I will look into the visquene. Basically I'm looking to do anything to insulate the ground as much as possible to avoid the frost from going as deep.

    Bookmark   March 5, 2008 at 4:30PM
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You could also add an under deck drainage system of some kind that would keep the area dry.
My barn water freezes sometimes because of this problem - the ground expands, rises, and kinks the pipe. I dug down next to the house and filled w/ gravel, and that seems to have fixed it.

    Bookmark   March 15, 2008 at 11:37AM
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Hi Brainlu,

I am also living in twin cities. My deck footing raised a few inches last week. I know we have heavy clay soil and I tried my best to improve drainage. May you please let me know if form insulation solved the frost heaving? If it is, I will try this summer. Thanks!

    Bookmark   January 19, 2009 at 11:01PM
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I am also experiencing this issue with a deck in the Twin Cities at our house. I would also like to benefit from your experiences and correction. Were you able to resolve your issue? If so what did the trick?


    Bookmark   January 20, 2009 at 4:01PM
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" I spoke with a soil engineer that I know and he said that it looks like the footings weren't lifted from below, but from the sides. Ice grabs on to the sides of the concrete and lifts it up like a straw."

Interesting... sounds like the sonotubes were placed all the way down to the bottom of the excavations. The tubes should be lifted 12" off the bottom of the holes, above whatever level backfilled with drain rock. This ensures the concrete pour will "mushroom" out into the bottom of the hole. A concrete pier footing should be like an upside-down mushroom anchored in the earth. I think many people these days use concrete tubes and set them all the way down the hole, basically creating a straw in the ground. So many people around here are reporting frost heave even though the footings go below the frost line.

Keeping the ground dry with proper drainage is equally important- so if you have an existing deck focus your efforts on channeling roof downspouts and surface runoff away from the footings.

    Bookmark   January 21, 2009 at 12:19PM
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I admitted that I dug the footing 4' deep and put the tubes all way down. How can I fix the aftermath? I did channeling roof downspouts. However, I cannot reduce the amount of water in the clay soil. When it froze, the heave happened.

Do you think the sub-soil insulation will do the trick?


    Bookmark   January 21, 2009 at 11:41PM
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melag, you need a moisture barrier, not insulation. Insulation only works, in this case, if there was a source of heat. Insulation on its own does nothing. You need to concentrate on keeping the moisture away and out.

    Bookmark   January 22, 2009 at 11:48AM
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First let me admit that I am not experienced with building in areas prone to frost heave. In my area we have to build foundations in all different soil conditions on hillsides near faultlines where liquifaction, landslides, and compaction are the main issues. But the funny thing is that the construction methods for earthquake zones are really similar to hard freeze zones. There are 3 main principals:
1.Create a foundation with a footing that is a stable shape. Picture an bell bottom or upside-down mushroom. This is why it is so important to lift the concrete form tube several inches off the bottom of the hole.
2.Get the footing of the foundation bonded on all sides into stable soil. This is why we dig down below the frost line or liquifaction zone. Backfilling is important. Layer the fill and compact each layer.
3.Keep water out. Compacting the backfill properly helps keep water from filling in the loosely compacted soil. In frost zones the use of drain rock at the bottom of the footing will help the liquid water drain deep enough into the soil that it won't freeze.

Look into diverting surface runoff away from the area around the deck, If the deck is in a low spot, install french drains around the perimeter and channel them to the storm drain (or to a drain well pit you dig on your property a good distance away from the deck.) Look at the grade of your entire lot and where does water puddle up consistently. Put the french drains where the puddles form or where the ground seems to stay soggy after a rainfall.

When you say you channelled the roof downspouts, did you tie them into a storm drain with solid pipe, run them underground into a perforated pipe, or just turn the downspout away from the house and discharge it onto a concrete block? They should be tied to the storm drain with solid pipe for your application.

Or you could dig up the footings and re-pour.

    Bookmark   January 22, 2009 at 2:44PM
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Thanks for all the suggestions from sierraeast and aidan_m.

I installed a sub-drain tile along the deck footing. Due to the grade limit, the sub-drain is 2' below the ground to have a daylight outlet. It helps dry the footing area. However, during the windchill 30 below, the ground frost deep than the french drain, the water/moisture in the soil made the frost heave.

I am not sure about sierraeast's suggestion. I used 6 mil plastic along the house and intend to make it dry. I found out that the moisture level is the unchanged all year along. What I mean is the soil under the sheet is not dry during the drought summer and is not soaked during rainy day. I have no clue how decrease the moisture level in heavy clay soil.

I though about re-pouring. But the cold join between new and old cement will not help me except I use rebar to tie them, which is almost impossible for deck footings.

That is why I am eager to know if the sub-soil insulation will work.

    Bookmark   January 22, 2009 at 11:07PM
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I believe you have your answer. Insulation won't work, the only way it would work was if the foundations were heated. Doesn't sound practical to me.

    Bookmark   January 23, 2009 at 2:22AM
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I have a similar situation up in Duluth Minnesota. I have a 120 year old house. This last year I built a deck off the back of a sunroom. The sunroom is tied into the basement on one end end the other end is on footings (in a heavy clay area). This year with the deck on the corner footing near the deck rose up enough to crack a little plaster and effect the squareness of the sliding door.

So why did the 120 year old sunporch decide to move all the sudden?

All winter long the deck has had zero snow accumulate under it, while the whole yard is covered in a few feed. It essentially took away the insulation near that footing and so the frost got deeper then it typically would have in that area and lifted that footing a little bit.

I think this is why you are seeing just your center ones heaving, they are not insulated at all.

Insulation should work. The heat that you need is provided by the constant 50ish degrees of the ground and the trick is to slow the loss of that heat down so the frost doesn't get down to your footings.

If you look up frost protected shallow foundations there is some good information.

    Bookmark   March 13, 2011 at 11:46PM
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Adam, I am in Canada (near Montreal). I have almost exactly the same situation as you. My house is a few inches above the ground on cinder blocks. I added a sun room about 8 years ago, and then added a deck.

The sun room is 14' x 8'. The 14' end replaced the end wall of one room.

The 8' wall has a sliding patio door in it. The heave in winter is so bad that the door has to be forced shut by February. By March it starts to drop, and it is now (in April) almost level.

The 14' side of the deck is attached to the 8' side of the sun room (plus 6' of the rest of the house).

The footings have been lifted and/or adjusted 2 or 3 times. The moron who put in the footings 8 years ago put 2 or 3 of them along the outside wall of the sun room, and sunk them 2.5 feet (says he hit water so had to stop). So I dug those up and went down 4 feet.

This winter, the room heaved again (this is fairly normal). But for the first time, the deck heaved something awful. The deck has already been removed twice and rebuilt.

We had the deck up on twice as many of those little blocks that hold a 4x4. They were on 16" patio stones, and those were on gravel.

The middle of the deck heaved up. The end by the house was connected to the house, so the far end went up about 8" ... maybe a foot.

We had heavy snow cover that kept the rest of the yard insulated, but I realized that by removing snow from the deck, and the deck itself, no snow underneath meant no insulation.

When we removed the top a few weeks ago, I was shocked by how high the soil was. I was sure we had not built on soil that high. Yesterday, a good rain finished off the thaw, and the soil under where the deck was is now level with the rest of the property.

I believe that the "mushroom" concrete pour (or the Bigfoot form they sell now that is cone-shaped, wider at the bottom), set into gravel, plenty deep is what I need to do.

I am planning (and reading all these related topics) to remove the clay, bring in a LOT of gravel, put 3 new, deep, anchored footings, and run a drain (O-Ring drain pipe) out to where I have a gravel bed for other drains (runs 200 feet away from the house).

So, since clay does not absorb water, the water just sits there, freezes, and heaves. So, getting rid of the clay seems important.

Gravel, as stated here, allows the water to run deep and run away (in the drain pipe). I am on the top of a hill, so it is all downhill from here.

Deeper footings, better water flow, improved soil, new drains.

I have some lighter soil that I can put back instead of the clay (soil that absorbs and holds water). Maybe mix the two soils. I had not thought of compacting the soil in layers when it goes back down.

I am guessing and I think that a lot of what we find in public forums is people trying to guess what to do.

My backhoe buddy has a lot of experience with draining clay soils, and with foundation work, so I will take him everything I find and let him do what he does. So far, what I just described seems to be the thing to do.

    Bookmark   April 11, 2011 at 9:53AM
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How would I remove a post from here? The one previous to this one. I do not want my email address posted for harvesting. This system should never post an email address publicly and not even inform us that it will be broadcast to every spammer on the planet.

    Bookmark   April 11, 2011 at 10:18AM
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ok///live in saratoga ny area....frost got down real deep last least 5 ft. im heaved my sunroom and usually goes back down....but some spots did in clay soil.i know im down 4 ft.but sonotube sticks way up on top like 2 ft in its like concrete peirs.any ways i dont believe i have any mushroom types and no gravel.the sunroom moves,the deck moves....and the deck that floats moves.any suggestions to a built delema....??

    Bookmark   June 19, 2014 at 12:10AM
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