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Wood Movement

Wood Movement

Moisture

The 2x4 that you buy at the lumberyard began as a living tree. As a living tree, the trunk of the tree provided a mechanism to transport water (in the form of sap) from the roots to the leaves. When this tree trunk is cut down, the sap stops flowing, but the sap remains in the trunk. Wood that is freshly cut is referred to as green wood or wet wood, referring to its high moisture content (40% or more!)

This raw wood is then stripped of bark and branches, and then cut into various sized blanks. Traditionally, these blanks are air dried to approximately 15% moisture content, giving up its moisture to the atmosphere. Current practice involves using a kiln to dry wood to an even lower moisture content, approximately 7-10% moisture content.

But once you purchase that 2x4, its moisture content will not remain constant. Just like a sponge, the wood will absorb moisture from the air and expand, or give up moisture and shrink.

Wood shrinkage

Direction

Wood that expands and contracts due to changes in moisture content will do so along predictable lines. With a little bit of knowledge, you can predict the degree of wood movement, and take appropriate action to accomodate the movement. Wood shrinks most in the direction of the annual growth rings (tangentially), and only half as much across the rings (radially, or from the centre of the tree to the outer edge).

Wood that is harvested from near the center of the trunk will display less movement than wood that is harvested from the edge of the trunk. If you look at the end of a piece of wood, you will be able to see lines which are the so-called age rings of trees. Looking at the direction of the curves, you will be able to determine the direction of the heart of the tree, and of course the other side will be the outside of the tree. The combined effects of radial and tangential shrinkage can distort the shape of wood pieces because of the difference in shrinkage and the curvature of annual rings. The major types of distortion as a result of these effects are illustrated in Figure 3–3.

It shrinks most in the direction of the annual growth rings (tangentially), about half as much across the rings (radially), and only slightly along the grain (longitudinally). Flat-sawn - These boards will cup away from the heart of the tree. The shrinking will occur mostly in its width.

  • Riftsawn - These boards will warp and shrink into a trapezoidal or diamond shape
  • Quarter-sawn - These boards will shrink slightly in both length and width. The side of the board closest to the hart will not display much variance, but the side farthest from the hart will display most of the dimensional changes.
  • Square Block - Similar to the rift-sawn boards, these will warp and shrink into a trapezoidal or diamond shape
  • Through-and-Through - These boards are similar to flat-sawn and quarter-sawn boards, except the heart of the tree passes extremely close to the centre of this board. This causes some cupping, along with some reduced thickness along the ends furthest from the heart
  • Post or Dowel - Unless cut from the exact center or heart of a tree, these will shrink to a slightly oval shape. Smaller dowels won't display much change, however larger dowels will display greater variance.

Wood Species

Different wood species will display different rates of shrinkage and therefore different species will display different rates of movement. The following are an example of the most commonly used woods in Canada and the United States, sorted by their shrinkage factors.

These species are sorted by their tangential expansion factors, meaning how much they would expand along the direction of the wood's annual growth rings:

SpeciesTangentialType
Cedar, Northern white0.00229Softwood                 
Cedar, western red0.00234Softwood                 
Pine, lodgepole0.00234Softwood                 
Cherry, black0.00248Hardwood
Yellow-poplar0.00289Hardwood
Red oak: water, laurel, willow                  0.00350Hardwood
Maple, sugar0.00353Hardwood
White Oak, commercial0.00365Hardwood

These species are sorted by their radial expansion factors, meaning how much they would expand across the grain:

SpeciesRadial                 Type
Cedar, Northern white0.00101Softwood                 
Cedar, western red0.00111Softwood                 
Cherry, black0.00126Hardwood
Pine, lodgepole0.00148Softwood                 
Red oak: water, laurel, willow                  0.00151Hardwood
Yellow-poplar0.00158Hardwood
Maple, sugar0.00165Hardwood
White Oak, commercial0.00180Hardwood

From this you can see that species like cedar have a well deserved reputation for their stability, along with other species such as catalpa, mahogany, teak, and redwood.

Other species, such as the ever popular oak or maple, display almost double the expansion factors of the more stable species.

Please note that the numbers used to perform this calculation were taken from Wood Handbook: Wood as an engineering material (see bibliography).

How Much Will It Shrink?

To help you estimate how much movement you can expect from a piece of wood, enter the size, select the type of wood, then click the button

Size: (decimal units only, i.e.: 9.5, 23, or 254, etc)Please enter a starting dimension in decimal units

Wood Type:

 

Shrinkage from % to % humidity: along the grain lines and across the grain lines

Expansion from % - % himidity: along the grain lines and across the grain lines.

Please note that the numbers used to perform this calculation were taken from Wood Handbook: Wood as an engineering material (see bibliography).

Dealing With Movement

So now that you know wood moves, and how much you can expect it to move, you can now focus your attention on dealing with the movement of the wood.

As a woodworker, you might design and build a dresser with a solid white oak top which is 24 inches wide. If you build the dresser during the humid summer months when the wood has acclimated to the ambient humidity of 11%, when the dresser is finished and installed in the bedroom and acclimates to the dry winter air at 7% humidity, your table top will have shrunk down to 23.65 inches (that's a loss of 3/8"!).

That means that methods of installation that don't take into account the movement of the wood will lead to the cracking, warping, bowing, or some other deformation of the surface. This is definitely not something that you want on a piece that you spent so much money and time working on.

The solution to this problem is usually a simple one: don't stop the wood from moving, but account for its movement when you make a project. Allow the wood to move and instead fasten the piece using some mechanism that lets the wood move yet still holds the component together.

Ultimately, as a designer and builder, you should evaluate your piece for the effects of expansion and build in the appropriate measures to deal with it.

Non-Woods

This look into wood movement deals only with solid wood. Many materials that you will use are not actually solid wood, and do not display much expansion or contraction due to moisture. In some ways, this can be a boon to you as a woodworker, and you may design with these materials to overcome some wood-movement issues. These alternatives are usually not as pretty as solid wood, and their edges must always be concealed or trimmed in some manner.

MDF (or medium density fibreboard) and HDF (high density fibreboard, or hardboard) is basically sawdust that has been moistened with a glue and then compacted. The fibres that form MDF and HDF are not significantly affected by atmospheric moisture, and so you do not need to account for its expansion. What you do need to protect it from is direct contact with water, since prolonged exposure with water will cause this material to degrade, flake, and lose all ridgidity.

Particleboard is essentially wood chips that have undergone the same process as MDF. There is a great range in the quality of particleboard, and the products offered by some manufacturers is far stronger or workable than others. But since the wood chips are small and randomly oriented, the overall board is stable in all directions.

Plywood is created by many thin layers of wood glued and pressed together. Plywood has more stability than solid wood since the layers are glued with alternating grain directions, so any expansion is minimized, and therefore the final plywood product is considered very stable.

Frame-and-Panel

Individual pieces smaller than 2 or 3 inches usually are not a factor. The amount of expansion is not particularly significant, so the frame of most frame-and-panel constructions are not affected. The panel in the center, however, is often made of solid wood material, and the solution is to allow a gap in the mortise/groove along the side stiles. The amount of gap can be calculated by using the calculator above, since it may vary depending on how large the panel is.

Panels made of MDF, hardboard or glass do not need to account for expansion since these materials do not deform significantly due to atmospheric humidity.

Chest / Case

Chests and cases are basically boxes with solid wood sides and tops. If the box has all the grain in the same direction flowing from side to side, the box will essentially get taller or shorter as it exapands and contracts with changing humidity.

Any time two pieces of wood are attached with grains running in different directions, the method of attachment needs to be a sliding mechanism, such as oversized holes or slots on cleats allowing the screw to slide.

Joints

Any time two pieces of wood are attached with grains running in different directions, the joint between the two becomes a factor. Joints that link end grain to end grain are safe since the expansion of both pieces will be identical (assuming they both use the same type of wood!). Edge-to-edge joints (long grain to parallel long grain) are similarly safe for the same reason.

Problems arise, though, whenever you have end-to-edge joints, or perpendicular edge-to-edge joints (where the long grain of one piece is joined across the long grain of another piece). In these situations, using solid joints such as dovetails or even glued joints will lead to eventual joint failure or fracturing of the wood. An alternate means of attaching the piece should be considered.

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