Why Exterior Wood Cracks

Wood Cracks, Splits, & Checking Explained

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 have little to no concern aside from aesthetics—furthermore, wood strengthens as it dries.

A wood “check” refers to separating wood fibers across the yearly growth rings of timber. Contrarily, a partial wood-split will carry all the way through from the top to the bottom of the lumber, resulting in near complete separation, and should be replaced.

Another form of crack called a “shake” rarely occurs but can occur 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.

How 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. 

When timber dries, it takes longer to dry all the way through than evaporation to initiate between the growth rings. As the growth rings shrink, the wood fibers inside start to separate, causing tension and eventually resulting in visible cracks referred to in our industry as “wood checking.” 

The natural process of drying lumber causes stress, leading to splits, and often is 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 cause no structural issue or concern. Natural wood will crack and is a part of its character. 

How Long 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, drying at 3/4 – 1 1/4 of an inch each year. So, the core of a 6×6 ground post will take roughly 3-4 years to dry if it’s not touching the ground.

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 the lumber was cut from contained over 300 gallons of water and was soaked in a tub to be treated, 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 equally to the moisture 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.

Here in the US, each region varies in average humidity. We are located in Raleigh, NC, and areas in the South East average 79% while northern regions averages are slightly lower at 71%. Humidity is higher during summer months and can cause wood to swell. 

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 with an electromagnetic scanner, which can read moisture content from the core of the lumber.

How Much Does Wood Shrink

Even though Douglas Fur is a soft wood, it’s used in construction for good reason. Douglas fur only shrinks at 4% of the total size of the board, while oak and other white hardwoods will shrink over 7% of its full size.

Redwood and other dark, more dense 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 can often be heard, but wood drying also carries a sound. 

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 clued together. Treated plywood is a soft wood 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 are all elements that cause the plywood to crack, just like any other wood material.

Does Kiln Dried Treated Lumber Split?

In short, yes, kiln-dried lumber does split. In part, kiln-dried lumber is slightly deceptive. There are multiple types of kiln-dried lumber, all of which contain different drying levels. 

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. 

(KD) kiln-dried lumber, measuring less than 1 inch thick, is less likely to split and is paint-ready. Thicker lumber over 2 inches thick will benefit best from a thoroughly dried core. Otherwise, we recommend installing wet lumber as the benefits of thicker (KD) lumber are marginal and cost 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.

How To Control 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. 


A quality local exterior painter is monumental in helping maintain surfaces and resist 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 in terms of application.

Covered porches or any means of shelter are always optimal for protecting lumber.

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.

I 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 frames and structures. Reclaimed lumber is another option that offers minimal cracks that will not increase over time.


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 and provide 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 more information, including building codes, practices, and material reviews.

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