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Fix a sagging deck


How can I deal with my sagging deck? It's built on concrete deck blocks. The problem was pointed out to me during a home inspection five years ago and it has gotten worse since then. Cost is an issue, but I'd like to get the deck brought back to level permanently, if possible.–Paul Lovie, New Glasgow, Ont.

Sagging deck foundations are common because many deck builders take chances using foundation blocks that simply sit on top of the soil. In your case, the gamble backfired. On other kinds of soil, it might have been satisfactory.

The first thing you need to decide is how cheap you’d like the fix to be. As long as the wooden structure is sound, a sagging deck with an above-ground foundation-block system can be brought back to level fairly easily by simply jacking up the structure immediately be- side each settled post, adding patio slabs or pressure-treated wood spacers beneath the foundation block, then easing the structure back down and checking for level. You’ll probably have to make several attempts before you get the deck orientation just right. The weights involved with a deck aren’t large, so a tire jack will probably be more than strong enough to do the job. If the sagging is more than 6″ or so, and if it extends across several posts, then two or three jacks might be necessary to raise the area without putting too much stress on the structure.

While jacking and shimming will get the job done, they won’t solve the underlying problem with soil conditions. Over time, your newly levelled deck will probably sink slowly. To eliminate the problem once and for all, you’ll need to install the kind of permanent foundation that should have been used in the first place. Depending on how high your deck is off the ground, that could be difficult or not too hard. The higher your deck is, the easier it’s going to be to get under there and dig a hole for the 6″- or 8″-diameter cardboard tubes that you’ll fill with concrete. Don’t attempt this option unless you’re willing to dig below the frost line (that’s 36″ to 48″ deep in many parts of Canada).

The first step in building a permanent foundation involves removing the sagging posts and installing temporary supports on each side of the old post locations while you work. And the best tool for this job is a reciprocating saw, although you probably won’t be cutting wood. Fit the saw with the longest hacksaw blade you can find, then sneak in between boards, posts and braces to slice the nails or screws that hold everything together. The parts you need to remove will simply fall apart undamaged. You can reuse the wood again later after building the concrete-pier foundation.

Steve Maxwell

Steve Maxwell lives on Manitoulin Island, Ontario and has worked remotely as technical editor of Canadian Home Workshop since 1990. He uses his experience as a cabinetmaker, carpenter and stonemason to prepare projects for the magazine, to write stories of his own, and to test and review products and tools in his workshop. Steve has a readership of about 2 million people across Canada and the US, and takes photos and creates videos to accompany his work.

When Steve’s not working with words, wood and stone, he likes to spend time gardening, cutting firewood, and showing his five kids how to make things.


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Nov. 29, 2013

3:13 pm

Paul here. I rebuilt my deck last year (2012) and min code in Ontario is 4' deep X 8" wide cylinder with a bell footing on the bottom. I used the bigfoot system, which works well. It is not cheap but my deck is better supported than my house.

regards Paul


Nov. 29, 2011

12:35 pm

Putting in a "permanent" pier footing may require a building permit, and possibly raise your taxeable evaluation. Possibly why it was not done in the first place; apart from the increased labour. Worth a phone call to City Hall.

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