Choose the right foundation for your deck

Build a sturdy deck from the ground up

By Steve Maxwell

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Your personal temperament, risk tolerance and expectations are almost as important as the technical issues when choosing the best deck foundation. You have a choice between either dealing with seasonal movement in order to save time and money, or a rock-solid deck, which requires investing more up front. Of course, the physical realities of your site and design are also factors.

If you’re unconcerned about your new deck moving a little, then deck blocks are probably for you. They’re precast concrete blocks that act like feet on top of the soil, supporting the 4×4 posts that hold up the deck. The best deck blocks also have slots cast in the top surface to accept 2x wood on edge. This is for the floor joists of low-slung decks that don’t need support posts.

Deck blocks can work well, but you need to consider a few things. Sand or gravel under the blocks works best because these materials don’t change with seasonal freeze-and-thaw cycles. They allow water to seep away in the fall, eliminating frost-related heaving in the winter. Heavy clay soils, on the other hand, can rise and fall substantially be­cause they hold so much moisture.

Another issue is the state of the soil. If it was backfilled within the past year or two, then the risk of settling is high. Wait for a couple of years, regardless of the foundation you’re using, but especially if you’ll be using deck blocks. They’re the most susceptible to settling because they have so much soil underneath them.

Is your site sloped? Even a small grade can cause your deck to tilt downhill over the years if it sits on deck blocks. They have almost no ability to resist any sideways tug of gravity.

If you lose sleep over the fear that your deck might heave 1/8″ each spring, or if you want to build a deck that you can leave to your grandchildren, then you should use a buried foundation.

The best type of general-purpose deck foundation uses thick cardboard tubes sunk into holes that extend below the frost line and are filled with concrete. Wooden posts sit on top of these piers, anchored with metal post saddles set into the wet concrete. Installing this system is a lot more trouble and more expensive than deck blocks, but concrete piers reduce the risk of movement and foundation failure to almost nothing if you put them in right. Staple some heavy, black poly plastic to the outside of each cardboard form tube to prevent frozen soil from gripping the outside and lifting it during winter.

To anchor the bottom end of stair railings, you need something more than a wooden post and concrete pier. This system cannot resist side-to-side forces. Posts require some kind of bracing, and that’s not possible at the bottom end of a set of stairs. The solution is to set pressure-treated posts into concrete poured directly into a hole that extends well below the frost line in your area.

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