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Importance of Frost Lines when Building a Deck or Fence

If a there was a reality show about cedar decking and fencing, “When Water Expands” would be an excellent title for a horrifying episode. The show’s producers would understand that the temperature-driven expansion and contraction of water can devastate a poorly built deck or fence installation.

Portland homeowners usually understand the importance of resealing their cedar decking or fencing so as to seal out water and prevent mold and fungi from taking up residence. They may even comprehend the fact that water trapped inside wood (like all water) will expand 9 percent when it freezes, thereby causing splinters and checks. However, few people appreciate the damage that freezing underground water (i.e., frost) can cause to a cedar deck.

How Frost Heave Works

The physics of frost heave – the upward, destabilizing pressure of frost – is complex. We’ll simplify things here by saying that underground frost forms into thin horizontal layers of ice, called ice lenses. These ice lenses can be strong enough to lift an entire structure. Indeed, as water expands into ice, it exerts a pressure of 50,000 pounds per square inch – enough to lift entire buildings, and certainly enough to lift your cedar decking or fence installation!

Portland’s mild climate means frost rarely forms more than 18 inches underground, the official frost depth for the area. Colder parts of the country have lower frost depths; in some parts of Minnesota, the frost depth is 42 inches. And, as you’d expect, warmer locations have a shallower frost depth.

What this Means for Your Fence or Deck

When installing cedar decking or a new fence installation, Portland homeowners (and, indeed, homeowners in any area that sees frost) should ensure that post footings extend below the frost line. Otherwise, the ice lenses could lift fence or deck posts, damaging stability and potentially wrecking the entire structure. Every year, the posts will be pushed up higher and higher by frost; eventually, the entire cedar decking or fencing post can even be forced out of the ground!

To avoid this outcome, dig postholes that are deeper than the frost depth for your area (18 inches in Portland). Also, aim for a wider bottom on fence posts. Most decking and fencing posts are held in a concrete footing. Widening the bottom of the hole, where the concrete will sit, is a good way to prevent frost heave.

Rick’s Custom Fencing & Decking is the region’s expert on constructing solid, stable decking and fencing structures that will not heave with freezing temperatures. We’d love to provide a free estimate on your next cedar decking or fence installation, Portland residents.

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