Understanding Log Checking vs. Cracking in Railings
When homeowners install their first massive log or structural timber railing, panic often sets in around the six-month mark. Loud “pops” echo from the deck, and long, deep fissures appear along the length of the beautiful peeled logs.
The immediate reaction is often fear that the railing is failing or that the lumber was defective.
In reality, this is a completely natural, unavoidable process. The wood is not breaking; it is releasing tension. To maintain a log railing properly, you must learn to identify the difference between normal “checking” and catastrophic structural “cracking.”
What is Checking?
A “check” is a separation of the wood fibers along the grain that occurs as the log dries.
When a tree is cut, it is full of water. As a solid log dries, the outer layers (the sapwood) lose moisture and shrink much faster than the inner core (the heartwood). Due to the geometry of a circle, this uneven shrinking creates massive tension. Eventually, the tension exceeds the strength of the wood fibers, and the log tears itself open to relieve the stress.
This tear is a check. It is the log’s way of finding a stable equilibrium with the surrounding atmosphere.
Characteristics of a Normal Check
- It runs parallel to the grain of the wood.
- It stops before it reaches the exact center (the pith) of the log. It will never go all the way through to the other side.
- It does not affect the structural integrity or load-bearing capacity of a large post or rail. The core remains solid.
What is Cracking?
A “crack,” in structural terms, is a failure of the wood that compromises its strength. Cracks are often caused by extreme physical stress, severe overloading, or advanced rot, rather than the natural drying process.
Characteristics of a Dangerous Crack
- It splits a piece of wood entirely in two (passing completely from one side to the other).
- It runs dramatically across the grain, or spirals wildly, severing the long fibers that give the wood its strength.
- It occurs directly through a critical structural joint, severely loosening a mortise and tenon connection.
If a six-inch main newel post has a single check running up its side that is a half-inch wide and two inches deep, the post is perfectly safe. If a two-inch vertical baluster has a crack that splits it entirely in half, that baluster must be replaced.
When to Caulk a Check
The primary danger of a check is not structural failure; it is moisture intrusion. Checks create valleys that can catch rainwater, funneling it deep into the unprotected core of the log where rot fungus thrives.
However, you should not caulk every check you see. Caulking incorrectly is worse than doing nothing at all, as it traps moisture inside the log.
The Upward-Facing Rule
The rule for maintaining checks is based entirely on gravity.
- Leave Side and Bottom Checks Alone: If a check is on the side of a log or underneath a handrail, leave it wide open. Water cannot pool inside it. Air needs to circulate through these checks to allow the core of the log to continue breathing and drying.
- Seal Upward-Facing Checks: If a check is on the very top of a horizontal rail, facing straight up toward the sky, it must be sealed. This is a funnel that will catch rain, snow melt, and debris.
How to Seal an Upward-Facing Check
Do not use standard silicone bathroom caulk or hard wood putties. Standard caulks lack the extreme elasticity required, and putties will pop out the following winter when the log expands.
- Clean the Void: Use a wire brush or compressed air to ensure the check is free of dirt, loose fibers, and trapped moisture.
- Install Backer Rod (Crucial): If the check is deeper than a quarter-inch, push a piece of closed-cell foam “backer rod” into the crack. The caulk must only adhere to the two sides of the check, not the bottom. This allows the caulk to stretch horizontally like a rubber band when the log moves.
- Apply Log Sealant: Use a highly elastic, specialized acrylic-urethane log caulk or textured chinking designed specifically to stretch up to 400% alongside natural wood movement.
- Tool the Joint: Smooth the caulk flush with the surface to ensure water sheds immediately off the rail.