Wood Checks: Why does lumber split and cracks?

Checks and splits in lumber and timbers, especially pressure-treated lumber, are often misunderstood when assessing the condition of a structure. Decking splits as it dries. As the water evaporates from the wood in the sun, the surface wood shrinks cracking around the un-shrunk core wood or fasteners. The solution to prevent decking from splitting is controlling the pace the wood dries and providing room around the fasteners for the decking to shrink.

What is pressure-treated lumber checks?

Checking is the separation of grain that occurs in lumber as moisture levels change and timber dries. It looks like cracks and splits in the wood. They are two different types of deck splitting: structural end decking splitting and cosmetic field cracking of the decking. Cosmetic decking or in the industry called “checks” are unsightly but will not impact the structural strength of your decking. It may look ugly, but your deck can still be enjoyed for years to come. End splitting is a concern as these splits can crack through the full depth of the decking. Weakening the decking, where a foot could break through the decking.

Why does my deck post crack / checks?

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Pressure-treated wood is softwood lumber, typically southern yellow pine, that's been chemically treated to resist rot, decay and termites. The boards are rolled into giant pressurized tanks where chemical preservatives are forced deep into the wood's fibers. The result is an exterior-grade wood that's ideal for building decks, fences, and other outdoor projects. Direct sunlight on a summer day can cause a timber to check dramatically, from all the moisture that was absorbed during chemical treatment process once exposed to the summer heat it will quickly dry out or shrink therefore creating checks.

Should I be worried about wood checks / cracks in my deck posts?

Checks and cracks actually help improve structural integrity because it releases the tension and pressure built up by shrinking wood. There is no way to prevent the naturally occurring process of checking from happening, especially when the wood will be used outdoors. Outdoor environments experience the most dramatic range of environmental exposure. From snow, inclement weather, pouring rain, howling winds, direct sunlight, freezing, thawing, high temperatures and low temperatures, wood that’s used outdoors experiences the full range of nature’s punishment.