Please take note that all cedar wood is subject to the natural process of “checking”. Checks are cracks or openings that form along the surfaces of wood, parallel to the wood's natural grain direction; seasonal fluctuations in wood's moisture content.Cedar wood has a number of beneficial qualities that make it ideal for use in shingles, fencing, outdoor furniture and several other applications. The wood has an aesthetically pleasing, straight-grain pattern; a pleasing fragrance and it naturally resists the damaging effects of moisture, insects and microorganisms.
Checking happens as wood releases moisture, and normally occurs across or through the annual growth rings. While outer layers of wood fiber are able to expel moisture quickly when temperatures warm up, inner layers shed moisture at a much lower rate. As a result, the outer layers shrink inwards, but crack when they bump into the denser, moisture-heavy inner layers. It is the usual result of wood seasoning. Checking occurs only on the surface of the wood, and does not affect the structural performance or the integrity of the wood.
The U.S. Department of Agriculture Forest Products Laboratory at Madison, Wisconsin has done considerable research on this subject. The following is quoted from U.S.D.A. Bulletin #1187: "Round items are considerably more difficult to season than sawed items. The difficulty arises because the round item contains the heartwood or center of the tree, frequently of large proportion to the whole, and the outer sapwood layer. The enclosure of the heartwood within the piece prevents the satisfaction of the inherent differences between tangential and radial shrinkage by distortion of cross section. As a consequence, stresses set up in different directions during drying. Thus, the item has a natural tendency to develop a V-shaped check toward the center."
In plain language - when a full round or square timber dries, the exterior surface (sapwood) shrinks faster than the inner (heartwood) heart of the piece, and something has to give. The result is the familiar "check" which is technically neither a split nor a crack. Users of full round timbers (posts, rails, etc.) or dimensional members (posts, boards, slats, etc.) can be assured that the natural checking, which is almost certain to occur, will have no appreciable effect on the strength of that member.