Wood Cracks, Splits, & Checking Explained
If you have just had a new deck, porch, pergola, or any other exterior wood structure built, it’s important to set expectations about how all treated lumber behaves. It’s normal for cracks to occur over time as natural wood dries after installation. Wood cracks can develop as early as milling and processing and are a natural part of wood drying. Cracks are of little to no concern aside from aesthetics; furthermore, wood strengthens as it dries.
A wood “check” refers to a separation of wood fibers across the annual growth rings of timber. Conversely, a partial wood split will extend all the way through the lumber from the top to the bottom, resulting in near-complete separation, and should be replaced.
Another form of crack, called a “shake,” is rare but can form along growth rings as the tree matures. Many types of cracks can take place. Not all gaps are the same, but they all contribute to grading lumber. So, buying high-grade lumber is crucial if you deem it unsightly.
| Lumber Type | Triangle “Check” Risk | The Raleigh “Pro” Verdict |
|---|---|---|
| Pressure Treated Pine | High | Standard for Wake County decks; expect surface checking within 90 days as it acclimates to local humidity. |
| Western Red Cedar | Low | Highly stable for ITB siding; resists deep splits but requires tannin-blocking protocols. |
| Kiln-Dried (KD) | Moderate | Safe to paint/stain immediately; sourced from American Cedar to minimize structural movement. |
| Composite Material | Zero | Best for homeowners who want a “check-free” aesthetic and zero seasonal maintenance. |
*Pro Tip: Ensure deep wood checks are cleared of Raleigh pine pollen silt before staining to prevent the “wick effect” from drawing moisture to the core of the timber.
Water and Wood Splits
Most of the weight of a tree comes from water. Over 40-60% of a tree’s weight is water. Normal-size trees 30 -40 inches in diameter will contain 1-2 tons of water.Â
When timber dries, it takes longer to dry all the way through than for evaporation to initiate between the growth rings. As the growth rings shrink, the wood fibers inside begin to separate, creating tension and eventually visible cracks, known in our industry as “wood checking.”Â
The natural process of drying lumber causes stress, leading to splits, and it is often misunderstood or misdiagnosed as problematic.
When evaluating wood strength, consider that wood is weakest perpendicular to the grain, so radial cracks or stresses against the grain pose no structural issues or concerns. All natural wood will eventually develop some form of a crack; it’s accepted and considered part of its character.Â
The Time Wood Takes to Dry
Only when we understand that over half of a living tree’s weight consists of water can we appreciate how long it takes for green lumber to dry.
Lumber dries from the outside in, at 3/4 to 1 1/4 inches each year. So, the core of a decorative rough-cut 8×8 porch column in our luxury neighborhoods, like the Montreaux subdivision, or a 6×6 ground post for a deck, will take roughly 3-4 years to dry. Posts that are in soil will take 30% longer to dry.
Non-kiln-dried lumber is especially damp when stored inside climate-controlled buildings. If you have ever cut or driven a nail into treated lumber, you have likely experienced water seeping or quirking. Most lumber is so wet because the tree it was cut from contained over 300 gallons of water and was soaked in a tub to be treated, then stacked and stored on a shelf.
Wood is unique and versatile in many ways. One of the most impressive phenomena is its ability to act as a sponge. It uses water to grow, expand, and contract, acting like a sponge in its ability to hold and release moisture.
Pressure Treated Lumber Moisture Content
Atmospheric humidity percentages highly influence exterior wood moisture content. So, timber can only dry to the moisture level within its environment.
A timber reaches Equilibrium Moisture Content (EMC) when it neither loses nor absorbs moisture and is heavily influenced by whether its environment is indoors or outside.
In the US, average humidity varies by region. Here in Raleigh, NC, the humidity in our microclimate averages 85% in the morning and tapers to 71%Â in the evening. Humidity typically increases during the summer months, which causes wood to swell significantly.Â
Average wood moisture content once fully dry:
- South East: 11-13%
- National Average: 8%
- North West: 8-11%
- Mid-West: 12-15%
- North East: 7-10%
- South West: 5-7%
The best means of measuring wood moisture content is an electromagnetic scanner, which can read it from the core of the lumber.
Wood Shrink Percentages
Even though Douglas Fir is a softwood, it’s used in construction for good reason. Douglas Fir only shrinks at 4% of the total size of the board, while oak and other white hardwoods will shrink over 7% of their full size.
Redwood and other dark, denser hardwoods shrink significantly less but are not sustainable building materials due to price and availability.
Wood Cracking Sounds
If you have ever been in a building with an open-cell wooden roof, you likely are familiar with the sound of cracking wood. Wood expansion and contraction are often heard, but wood drying also makes a sound.Â
Drying houses, wood stoves, and kilns are all examples of the sounds that wood can make.
If your structure makes sounds, you need not worry about loose nails or fasteners—that is just the sound of wood taking its course.
Plywood Cracks & Splitting
Plywood is made from thin layers of Southern Yellow pine that are glued together. Treated plywood is a softwood treated like an outdoor deck board to resist rot and decay.
The plywood is rolled into a pressurized tank, forcing the chemical used to treat the wood deep into its fibers. During indoor or outdoor use, heat, sunlight, and moisture can cause plywood to crack, just like any other wood.
Kiln-Dried Treated Lumber Splits
In short, yes, kiln-dried lumber does split. In part, kiln-dried lumber is slightly deceptive. There are multiple types of kiln-dried lumber, each with different drying levels.Â
Conventionally, dried lumber (KD) dries the outer 1 inch or so of the lumber, so the lumber is not completely dry. Rather, the board is merely dry enough to paint or stain immediately after application. More intense kiln drying, such as Radio Frequency Kiln Dried (RFKD), utilizes microwaves to dry the entire timber to its core.Â
(KD) Kiln-dried lumber, measuring less than 1 inch thick, is less likely to split and is paint-ready. Lumber over 2 inches thick will benefit most from a thoroughly dried core. Otherwise, we recommend installing wet lumber, as the benefits of thicker (KD) lumber are marginal and it costs significantly more than damp lumber.
If you want lumber that has naturally reached Equilibrium Moisture Content (ECM), there is none better than kiln-dried lumber. American Cedar on Tarhell Drive in Raleigh stocks one of the widest selections.
Trying to Limit Wood Cracks
The best way to reduce the chances of early wood checking is to coat exterior wood substrates with stain or paint within 3-6 months of installation.Â
A quality local exterior painter is instrumental in maintaining surfaces and resisting premature weathering. Staining allows the wood to breathe and repels water and UV. Light-colored stains are the best initial means of protection and require little effort to create color consistency, as new wood does not take stain as well.
Covered decks and porches, or any form of shelter, are always optimal for wood protection and making the stain last longer.
Repairing Cracks & Splits in Wood
Bondo, wood filler, caulking, and exterior compounds are the best ways to fill cracks, splits, knots, and seal wood surfaces. Proper cleaning and killing all organic growth is the best means of preparing the surface before all applications.
Also, consider sealing or recoating the wood with the existing paint or stain to reduce the chances of future cracks. Keeping the surface dry will also help reduce the chance of cracks increasing in size. Horizontal surfaces are more susceptible to standing water, which exacerbates gaps.
Don’t Want Any Cracks?
If you don’t want cracks, do not build with wood. Instead, construct with a quality composite material that will not blister, peel, or crack over time. Composite deck flooring, for example, doesn’t crack and doesn’t require painting and staining.
Glulam beams are another option for framing and structural applications. Reclaimed lumber is another option with minimal cracks that will not worsen over time.
Conclusion
Green timber will shrink, check, and sometimes twist over time. The good news is that builders, engineers, and manufacturers are already considering this movement. For example, we install wet deck planks tightly together because we know they will eventually shrink, providing the appropriate spacing.
Do not be alarmed if you have large or small cracks in the lumber, especially in large timber and treated lumber. Over the years, the openings will likely increase in size as the wood slowly cures.
Wood checks always begin at the surface, usually stop at the timber’s core, and are not a structural or warranty concern.
Our deck repair services extend to Cary, NC. View more of our articles for information on building codes, practices, and material reviews.
