Frost Heaved Decks and How to Avoid Them

By
Home Inspector with Structure Tech Home Inspections

One of the most common problems with decks in Minnesota is frost heave. Today I’ll explain how this works, why it matters, and what steps can be taken to help prevent damage from frost heave when building a deck.

How Frost Heave Works

When expansive soils freeze and expand, the earth rises. Wikipedia has a nice illustration of how this works, shown below.

Frost Heave

How much can soils heave?  It depends.  I once lived in a townhouse in Saint Louis Park with a patio that would heave about 4” during the winter; it got so bad that I could barely open my storm door during the winter.

Damage Caused by Frost Heave

Without a doubt, some of the worst frost heave I've come across has been here in Maple Grove. The photo below shows a Maple Grove deck that we recently inspected in the middle of an April snow storm. The right side had obviously heaved several inches. Click on the photo for a larger view.

Frost Heave at Deck

The deck shown below had severe frost heave in the middle and against the house, and had to be completely torn down and rebuilt because the deck was so severely heaved in the middle.

Frost Heave at Deck 2

Here's one more photo showing major frost heave at a deck.

Frost heave at deck 3

How Deck Footings Heave

When a deck is attached to a building, the part of the deck that gets supported by the earth needs to have proper frost footings. The most common deck footing is basically a big chunk of concrete poured in to a hole in the earth. The goal is to have this chunk of concrete go deep enough in to the ground so that the bottom rests on soils that never freeze, which should prevent the soils from pushing the footing up.

When deck footings aren't poured deep enough, decks can heave. Sandy, well drained soils aren't particular susceptible to frost heave, while soils with higher clay content are prone to frost heave. This is why it’s so important for deck footings to extend down below the frost line.

Preventing Frost Heave

How deep do footings need to be to prevent frost heave? There’s no magic number. When there is a lot of snow on the ground, the snow acts like a layer of insulation and reduces the frost depth. When it’s a winter with very little snow, like the type Minnesota experienced in 2011-2012, the frost depth will be much deeper than usual. Outdoor temperatures obviously make a difference, and so does proximity to water. Decks constructed near wetlands will have an increased potential for frost heave.

The Minnesota State Building Code requires footings to be a minimum of 42” deep in the southern part of the state, and a minimum of 60” deep in the northern part of the state. The diagram below shows the dividing line.

Frost depth map for Minnesota

While footings need to be deep enough in the soil to prevent frost heave, the depth of the footing is only one part of the equation. No matter how deep the footing is, if the surrounding soils expand enough to ‘grab on’ to the concrete footing, the surrounding soils can pull footings up out of the ground.

One step that can be taken to help prevent soils from grabbing on to the footing is to use waxed cardboard tubes for the footings, often referred to as sonotubes. The Family Handyman web site has a great cutaway photo of a footing with one of these tubes, used with permission below.

Deck footing cutaway view

Another step to prevent heaved footings is to ‘bell’ out the bottom of the footing, as shown in the photo above. The ‘bell’ shape at the bottom prevents soils from pushing the footing up and out, but it can also lead to a fractured footing. When expansive soils grab on to the walls of a footing and pull it up, the bell at the bottom will hold the footing in place until the pressure exerted by the frost exceeds the tensile strength of the concrete. Once that happens, the footing will simply break in half.

When you consider the relatively low tensile strength of concrete and the tremendous amount of force exerted by frost, it’s easy to understand how pier footings can break. One step that can be taken to help prevent this is to use rebar inside the footings. The diagram below, courtesy of the City of Maple Grove, shows a deck footing with rebar embedded to help prevent the footing from breaking.

Rebar embedded in footing

Next week I’ll have a follow-up post on a newer style of footing for decks that might just be the next big thing.

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Tags:
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Rainmaker
363,450
Joshua Frederick
Home Inspector for ASPEC Residential Services, LLC - Defiance, OH
Home Inspector in Defiance & all of Northwest Ohio

Good stuff, thanks for sharing.

Jun 05, 2013 08:42 PM #1
Rainmaker
773,730
Ed & Tracy Oliva
West USA Realty - Arizona - Fountain Hills, AZ
The Oliva Team Arizona Agents

Good Morning: This is some great Info for all living in the cold country!! Keep the Info coming and good luck with your business,  E

Jun 05, 2013 08:44 PM #2
Rainmaker
233,824
Reuben Saltzman
Structure Tech Home Inspections - Minneapolis, MN
Delivering the Unbiased Truth.

Thanks Josh!

Ed & Tracy - you're lucky you don't need to worry about this stuff :)

Jun 05, 2013 09:19 PM #3
Rainmaker
1,418,577
Michael Setunsky
Woodbridge, VA
Your Commercial Real Estate Link to Northern VA

Reuben, when I built my deck the code called for digging the footings 42 inches deep. Here in Northern Virginia we don't have the Winter's that you do in Minnesota.

Jun 05, 2013 11:29 PM #4
Rainmaker
694,927
Clint Mckie
Desert Sun Home, commercial Inspections - Carlsbad, NM
Desert Sun Home, Comm. Inspection 1-575-706-5586

Hi Reuben,

Excellent post on frost heave. There is very little of that here in lower New Mexico. But it can be really cold in the winter with temps getting below zero for weeks.

Have a great day in Minnesota.

Best, Clint McKie

Jun 06, 2013 12:46 AM #5
Rainmaker
94,583
Roger Stensland
Keller Williams Realty Puget Sound - Maple Valley, WA
Let's Move!

I knew how to prevent it right a way, but I had no idea of just how far down the earth actually can freeze in Minnesota.  Wow.  Over here near Seattle the earth remains about 55 degrees year around ad a depth of about 5 feet.  Perhaps,  in the mountains it extends much deeper.

Jun 06, 2013 01:26 AM #6
Ambassador
1,338,813
Charles Buell
Charles Buell Inspections Inc. - Seattle, WA
Seattle Home Inspector

Reuben, frozen dirt can be very strong---I bet you don't see too many of those "carryem' home in your car" support piers :)

Jun 06, 2013 02:57 AM #7
Rainer
220,306
Suzanne Otto
Six Twenty Designs - Lansdale, PA
Your Montgomery County PA home stager

Good info Reuben!

Jun 06, 2013 03:03 AM #8
Rainmaker
1,632,648
Lyn Sims
RE/MAX Suburban - Schaumburg, IL
Schaumburg Real Estate

Wow, good info. Not knowing what you are doing can cause extensive future problems.

Jun 06, 2013 03:47 AM #9
Rainmaker
347,949
Kim Peasley-Parker
AgentOwned Realty, Heritage Group, Inc. - Sumter, SC

Being born and raised in Michigan I understand the frost heave issue.  No living in SC, it is something we don't even think about!  Many clients from a northern climate are amazed at the lack of frost line.

 

Jun 06, 2013 04:31 AM #10
Rainmaker
1,894,641
Jay Markanich
Jay Markanich Real Estate Inspections, LLC - Bristow, VA
Home Inspector - servicing all Northern Virginia

Here the recommended depth is 24".  But I have a better solution.

Your problem is the cold.  Simply move the deck to a warmer climate and it won't heave as much.

Thanks.

Jun 06, 2013 05:54 AM #11
Rainmaker
883,726
David Popoff
DMK Real Estate - Darien, CT
Realtor®,SRS, Green ~ Fairfield County, Ct

You gotta go below the frost line.

The Minnesota State Building Code requires footings to be a minimum of 42” deep in the southern part of the state, and a minimum of 60” deep in the northern part of the state. The diagram below shows the dividing line.

that is why we have building codes.

Jun 07, 2013 12:17 AM #12
Rainer
171,285
Robert Butler
Aspect Inspection - Montreal West Island, QC
Montreal Home Inspector | Aspect Inspection

As always, a nice article Reuben. Here we call that 'grabbing' adhesion.

Feb 01, 2014 05:57 AM #13
Anonymous
Sophie Miller

Dziękuję za informacje! Szukałem i nie mogłem znaleźć. Pomogłeś mi!
192.168.o.1

Nov 02, 2018 05:39 AM #15
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Reuben Saltzman

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