Wood movement

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  • #20920
    jgust747
    Participant

    Some interesting information.

    Wood Shrinkage

    Wood shrinks most in the direction of the annual growth rings (tangentially), about one-half as much as across the rings (radially), and only slightly along the grain (longitudinally). 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 the growth rings. Weight, shrinkage, strength and other properties depend on the moisture content of wood. In trees, moisture content may be as much as 200 percent of the weight of wood substance. After harvesting and milling, the wood will be dried to the proper moisture content for its end use. Wood is dimensionally stable when the moisture content is above the fiber saturation point (usually about 30 percent moisture content). Below that, wood changes dimension when it gains or loses moisture.

    Different woods exhibit different moisture stability factors, but they generally shrink and swell the 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). This means that plainsawn flooring will tend to shrink and swell more in width than quartersawn flooring, and that most flooring will not shrink or swell much in length.

    The numbers below reflect the dimensional change coefficient for the various species, measured as tangential shrinkage or swelling within normal moisture content limits of 6-14 percent. Tangential change values will normally reflect changes in plainsawn wood. Quartersawn wood will usually be more dimensionally stable than plainsawn.

    The dimensional change coefficient can be used to calculate expected shrinkage or swelling. Simply multiply the change in moisture content by the change coefficient, then multiply by the width of the board. Example: A mesquite board (change coefficient = .00129) 5 inches wide experiences a moisture content change from 6 to 9 percent ­ a change of 3 percentage points. In actual practice, however, change may be diminished as the boards proximity to each other tends to restrain movement.

    Calculation: 3 x .00129 = .00387 x 5 = .019 inches.

    .00411 Hickory
    .00396 Jarrah
    .00369 Red Oak
    .00365 White Oak
    .00353 Maple
    .00338 Yellow Birch
    .00300 Jatoba
    .00274 Ash
    .00274 Walnut
    .00267 Douglas Fir
    .00248 Cherry
    .00238 Santos mahogany
    .00212 Purpleheart
    .00201 Wenge
    .00162 Cypress
    .00124 Mesquite

    Wood Shrinkage

    from http://tonewooddatasource.weebly.com/technical-data.html

    Found it interesting that oak and maple has more movement than pine and fir.

    Dallas, Texas

    #20921
    Ken
    Participant

    interesting information Johan, thanks for posting

    #691372
    Jan Khmelnytsky
    Participant

    This is very useful information. Does anyone have ideas on how Paul’s standard shellac and wax finish might impact these coefficients. For example, could they by cut in half?

    #691420
    Cunha
    Participant

    This is very useful information. Does anyone have ideas on how Paul’s standard shellac and wax finish might impact these coefficients. For example, could they by cut in half?

    The coefficients would stay the same but the rate of change would differ. I have doors with layers of oil paint and they still move seasonally.

    Different finishes have different permeability but almost all will pass water vapor. I say almost because epoxy and super thick finishes may actually block vapor transmission.

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