Wood is a popular material used in construction and woodworking projects. However, one downside of wood is that it naturally expands and contracts with changes in moisture and humidity. This movement can cause problems like warping, cracking, and joint failure in furniture and other wood projects. When choosing wood, it’s important to consider dimensional stability – that is, how much the wood expands and contracts. So which type of wood expands the least?
The Factors That Cause Wood Expansion
To understand which woods are the most dimensionally stable, it helps to first look at why wood expands and contracts. There are two main factors:
As wood absorbs water, it swells and expands. As it loses moisture and dries out, wood shrinks. The amount of shrinking and swelling depends on the initial and final moisture content of the wood. Most wood is dried to a moisture content of 6-9% for use in furniture. Going from this level up to the fiber saturation point (about 30% moisture content) causes wood to expand dramatically.
Coefficient of Expansion
This refers to the degree to which wood expands and contracts with changes in moisture content. It’s expressed as a percentage of change per 1% change in moisture content. Different species have different coefficients of expansion, so some react more to moisture changes than others.
So when selecting wood for furniture or woodworking, you want species with low coefficients of expansion and stability across varying humidity levels. Following are some of the best options.
Most Stable Wood Species
Teak is a tropical hardwood native to Southeast Asia. It’s one of the most stable woods used in furniture making and construction. Teak has an exceptionally low coefficient of expansion, averaging less than 0.2% radial and tangential movement. This means a 6 inch wide teak board will only expand or contract by less than 1/16 inch with large humidity changes. Teak is also less prone to warping and checks. Its natural oils also help resist moisture. For these reasons, teak is commonly used for outdoor furniture, boats, and other applications where wood movement poses problems.
The main drawback of teak is the cost. It’s relatively rare and overharvested in some regions, so it commands a high price. There are also some sustainability concerns if not forest-certified. But when dimensional stability is a must, teak is hard to beat.
Cumaru is another tropical hardwood valued for its weather-resistance. It has a low coefficient of expansion around 0.4% radial and 0.8% tangential. This South American species has exceptional dimensional stability along with good hardness and decay-resistance. The reddish-brown heartwood of cumaru closely resembles teak in look and performance. It’s an excellent choice for outdoor furniture.
However, cumaru also shares some of the same sustainability challenges as teak. Responsible sourcing practices are advised. Cumaru costs less than teak but is still one of the more expensive wood species.
For a domestic hardwood, white oak is a great choice for furniture, flooring, and other uses where stability is needed. The coefficient of expansion is about 0.4% radially and 0.8% tangentially. White oak is naturally durable and resists warping and checking better than most other North American species. It has interlocking grain patterns which help enhance stability. The majority of antique furniture was made from white oak, which has stood the test of time.
White oak is more readily available than exotic species like teak. It’s also cheaper, making it a good wood for users on a budget. Just take care in selecting boards with straight grain and few knots. Quarter-sawn white oak is best for the most stable boards.
Jarrah is an Australian species sometimes referred to as the “Australian teak” due to certain similarities. The wood is deep red in color with an interlocked grain. It has fair dimensional stability with a coefficient of expansion around 0.35% radially and 0.65% tangentially. Jarrah works well for both indoor and outdoor use.
The supply of jarrah is limited globally. But it’s worth checking out for woodworkers who can source it locally in Australia. Jarrah does contain silica which can wear down cutting edges more quickly. Care is needed when machining it.
Black cherry is one of the better domestic woods for dimensional stability. With a coefficient of expansion around 0.5% radial and 1.0% tangential, it approximates white oak. But cherry has the advantage of working easily with hand and machine tools. It has a fine, close grain that polishes beautifully. The reddish-brown heartwood of cherry darkens beautifully over time.
Cherry’s stability makes it a wood of choice for cabinetry and furniture. It resists warping better than more problematic species like hickory or ash. The main limitations are availability of wide boards and cost for the best stock. But cherry is an exceptional find when quarter-sawn into lumber.
True mahoganies like Honduran and African varieties have excellent dimensional stability. With a coefficient of expansion of 0.4% or less, mahogany resists swelling/shrinking and warping. It has historically been the premier wood for quality furniture and cabinetmaking. The definition of luxury is often “mahogany.” While genuine mahoganies are overharvested, plantation-grown woods allow wider availability today.
Besides stability, mahogany has exceptional workability. It’s easy to machine, sand, and finish. With its reddish-brown color and lustrous grain patterns, mahogany remains a top choice for movable furniture likely to see wood movement issues. It’s more stable than oak or walnut and comparable to cherry or maple.
Least Stable Wood Species
On the other end of the spectrum, some wood species are notorious for their dimensional changes. Here are some common examples:
Aspen and Cottonwood
Woods in the poplar family like quaking aspen and cottonwood have very high coefficients of expansion, in the 1.5-2.5% range tangentially. They easily warp and shrink, making them poor choices for any application requiring stability. They may twist, cup, or show large gaps after drying. Aspen and cottonwood are best used for enclosed applications like cabinets or drawers.
Beech and Sycamore
These domestic hardwoods look nice but have only mediocre dimensional stability. Their coefficients of expansion are 2-3 times higher than white oak or cherry. They tend to warp and check more than woods like maple. Quarter-sawing improves stability, but MAPLE may still be a better option if minimizing wood movement.
Hickory and Ash
These strong hardwoods with open grain patterns also rate poorly for dimensional movement. Coefficients of expansion over 1% are common. Warping can be an issue after milling. Special care is advised for using hickory or ash in applications where appearance after drying is important. They make better structural lumber than furniture woods.
Softwoods like pine have much higher expansion coefficients than hardwoods. The typical range is 1-2% tangential and 4-8% radial. This makes pine more prone to problems like cupping and twisting as moisture levels change. Proper sealing and finishing is important for pine to help stabilize the wood.
Tips for Minimizing Wood Movement
Here are some other tips for dealing with wood movement in projects:
- Let wood acclimate – Allow boards to equalize to the humidity conditions before use. This helps minimize later shrinkage/swelling.
- Control humidity – Keep humidity constant with HVAC systems or dehumidifiers. Wood moves less when humidity levels are stable.
- Allow room for expansion – Leave room between boards and at joints for expansion and contraction. Floating floors and panel products account for movement.
- Select quartersawn boards – Cutting boards from the log quarter-sawn reduces the coefficient of expansion significantly.
- Use stable edging – Edge boards with a more stable wood species to control expansion on the most visible edges.
- Finish back sides – Applying finishes to exposed endgrain helps minimize moisture changes.
- Use engineered wood – Products like plywood and LVL are designed to limit wood movement across their widths.
The Bottom Line
When selecting wood for critical architectural and furniture uses, stability is an important factor along with appearance and cost. Tropical species like teak and cumaru offer exceptionally low dimensional change, but cost more. Among domestic species, white oak, cherry, and mahogany provide the best stability for furniture and interiors. Avoid notoriously unstable woods like aspen, cottonwood, or beech when dimensional movement may cause problems. With careful wood selection and construction methods, you can minimize expansion issues.
Frequently Asked Questions
What type of wood is the most stable?
Teak is widely regarded as the most stable wood species. It has an extremely low coefficient of expansion, averaging under 0.2% in each direction. This makes it highly resistant to swelling and shrinking.
Is red oak or white oak more stable?
White oak is considerably more stable than red oak. It has lower expansion coefficients both radially and tangentially. White oak has interlocking grain that helps resist warping. It’s one of the better domestic hardwoods for furniture and flooring uses.
What is the most stable wood for doors?
The best woods for exterior doors are teak, mahogany, and white oak. They resist warping and swelling in changing weather conditions. For interior doors, good choices are poplar, birch, maple, cherry, and alder. Avoid more unstable species prone to twisting like pine or aspen.
What wood is least likely to rot?
Teak has exceptional resistance to decay fungi, even when exposed to moisture. Other rot-resistant species include white oak, black locust, cedar, redwood, and cypress. These woods contain extracts that make them durable for outdoor use. Some pressure treated woods are also highly decay resistant.
What is the strongest and most stable wood?
Oak species like white and red oak are exceptionally hard and strong while still having good dimensional stability. Other very strong and stable woods include hickory, ash, and maple. Exotic hardwoods like ipe and ironwood are also top choices where hardness and durability are priorities.