MBM Construction

How to Fix a Heaving Deck

A reader wrote in with questions about why her deck is heaving up and down though seasonal changes

Dear Mike,

Two years ago, my husband and his good friend built a above ground deck for our home. All has been good, except last winter we experienced our deck start to heave upwards quite noticeably. Then in the spring, my deck returns to normal. My husband has looked under the deck for damage, nothing out of the ordinary so far. So my question for you, what causes my deck to move up and down like that?

Laura,

Manchester, NH

Dear Laura,

Thank you for writing in with your concerns regarding your deck problem. Without having a set of your existing deck plans and concrete footer depths, I can only give you a broad answer. I’ll do my best to give you some examples and solutions of what should have been done to avoid your deck heave.

Main Causes:
The most common causes of deck heaves, are caused by not having your concrete footers set deep enough below the frost line. In order to know what depth you should set your concrete footers, you should first check with your local building department. Your building department will provide you with a table chart based on your exact location. Here in Massachusetts, we have to dig our footings to a depth of 48″ inches, to be below the frost line.
How can frost make a deck heave?
How can frost make a deck that is so heavy move upward? Most all types of soils contain water and when that water freezes, the water expands to 9% it volume. The result of this volume increase, can have the ability to exert up to 50,000 lbs of pressure per square inch –now that a lot of power!


Solution:

Laura, to accurately solve your problem, tell your husband he’s gonna have to get dirty – grab a shovel and start digging! Tell your husband to follow these steps below.

1. Visit your local building department and get a copy of your building code’s footer size, depth, frost line chart;
2. Set up temporary bracing to support the area in which you plan to excavate, to evaluate your existing footer depth;
3. If your existing footer is less than the required footer depth, you need to re-pour your concrete footers, so set up temporary bracing around your entire deck;
4. Go back to your building department and apply for a building permit to repair your footings. Why? Because a building inspector is trained to properly evaluate the depth and size of your footers, to ensure the job is correctly done to code before you pour new concrete – you don’t want to do this 3 times, do you?
5. Once you have removed and dug out all of your holes to the proper depth and width, compact the soil with a tamper, so the soil isn’t loose on the bottom;
6. Go ahead and shovel in a few scoops of crushed stone around the perimeter of the footer hole;
7. Next, install your new sonotubes but don’t place it completely on the bottom, allow enough room so the concrete when poured can mushroom out – picture having a mushroom upside down;
8. Don’t pour any concrete just yet! Have your building inspector come out and evaluate your size and depths of your footers, once you pass then you can pour concrete;
9. Start reassembling your deck posts with electrogalvanized post brackets and hardware;
10. That’s it, you’re done!