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!

As a custom deck builder and home improvement contractor in Massachusetts, I’m often regularly asked these kind of questions once I’ve built their pressure-treated deck. So I will do my best to answer the most common question I can think of now. Understand this before I move on, staining your treated decking involves the same process as untreated-decking — the main thing is that the wood is clean and dry from any moisture or dirt.

cabots deck stain

Can I stain pressure treated lumber?

Yes, you can stain pressure-treated lumber with the one exception, that the decking has completely dried out and has been clean of any and all dirt.

How long should I wait to stain pressure-treated lumber and why do I have to wait?

The process begins at the lumber yard factories and treatment plants. The pressure-treated lumber goes through a process of using a water-based solution that carries the treatment of chemicals into the fibers of the wood while under pressure; this leaves the wood wet.

What if I decide to stain the pressure-treated decking or wood before it’s dry – are there any consequences?

If you were to go ahead and stain or paint the decking before it dries, you’ll end up with bubbles -or- blisters forming on your deck. Also keep in mind, you’ll end up locking in moisture inside of the wood creating fungus or rot to take hold and destroy the wood.

What are the time tables for staining the deck, how long should I wait?

It all depends on what region of the country you live in. If you live in a hot sunny state like Arizona, give it a few weeks and it will more than likely be dry. However, here in Massachusetts it all depends on what season your deck is being constructed in. In the spring and summer months here in MA. I would wait 30-60 days before I would treat your deck with stain.

What kind of stain should I use for pressure treated lumber?

There are stains on the market today. My favorite type of stain would be Cabot stain, it holds up to the rough seasons we see here in Massachusetts the best. Many people believe Thompsons is the best, I completely disagree –they only have the best marketing! Just read the backs of the can and follow their directions…

How should I stain pressure treated lumber? Or, how should I apply the stain?

Read the can because it varies with some stains but most stains can be applied with a brush, sprayer, or sponge. You will probably find a sprayer to be the easiest method.

Should I stain pressure treated lumber yearly?

Yes! If your looking to preserve the deck, porch, patio for years to come stain your deck annually.

My final thoughts:

Staining pressure treated lumber is not rocket science. In fact, there is nothing remotely complicated about it but it does take time and if you don’t do it properly, you’ll mess it all up. If I did not make this point obvious enough above then let me say it again here:

Treated wood must be dry before you stain or paint it.

Enjoy your project. Wear gloves. Wear goggles. Be careful with tools. Work in ventilated areas.

Feel free to comment on my page, I’ll answer your questions to the best of my ability. And if you live in Massachusetts or Rhode Island and in the market for a new deck or porch, visit my home page – the link will be highlighted below.

deck and porch contractor ma