Hardwood Tabletop Movement: 36-Inch Slabs in Epoxy Builds

Understanding Seasonal Hardwood Movement in Tabletops

Solid wood continuously interacts with the moisture in the surrounding air. When relative humidity rises, dry wood absorbs moisture and swells slightly. When air dries out, wood sheds moisture and shrinks. These changes are natural, ongoing, and considerably smaller than the initial shrinkage that occurs when drying green timber down from the forest. However, when wood is locked into a composite build such as an epoxy river table, even minor seasonal dimensional shifts create mechanical forces that builders must accommodate.

Data published by the USDA Forest Products Laboratory in the Wood Handbook (FPL-GTR-282, 2021) provides woodworkers with reliable methods to estimate these shifts. By understanding the underlying calculations, makers can anticipate dimensional change and design mechanical attachments that protect both the wood slabs and the cured resin.

Calculating Dimensional Change with Equation 13-3

Within the normal interior service range of 6% to 14% moisture content, the relationship between moisture change and wood shrinkage or swelling is linear. To calculate expected movement, the Wood Handbook specifies Equation 13-3:

ΔD = DI [CT (MF – MI)]

In this equation:

• ΔD represents the estimated change in dimension.
• DI is the initial dimension (in units of length at the start of the change).
• CT is the dimensional change coefficient for the tangential direction, found in Table 13-5 (for quartersawn lumber moving in the radial direction, CR is used).
• MF is the wood moisture content percentage at the end of the change.
• MI is the wood moisture content percentage at the start of the change.

When the final moisture content is greater than the initial value, the resulting quantity is positive, indicating swelling across the width. When moisture drops, the quantity is negative, showing shrinkage.

Interior Moisture Scenarios Across Regions

Table 13-2 outlines average interior equilibrium moisture conditions across geographic zones in the United States:

• Dry Southwest: Interior moisture content averages 6%.
• Most Areas: Typical interior environments average 8%.
• Damp Warm Coastal: Humid coastal regions average 11%.

A flatsawn tabletop measuring a full 36 inches in width experiences different degrees of expansion depending on seasonal shifts between these baselines. Moving from 6% to 8% represents an interior seasonal swing in drier states, while shifting from 8% to 11% reflects seasonal humidity rises across typical to coastal zones. A shift from 6% to 11% represents the wide environmental spread a piece might encounter if transported from an arid climate to a humid coastal home.

Change in width = width × CT × change in MC (Wood Handbook Eq. 13-3), calculated for a 36-inch-wide flatsawn top. Coefficients valid between 6% and 14% MC.

Species (Table 13-5 name)	CT per 1% MC	36-in. flatsawn width: 6%→8% MC (in.)	8%→11% (in.)	6%→11% (in.)	CR per 1% MC
Alder, red	0.00256	0.18	0.28	0.46	0.00151
Ash, Oregon	0.00285	0.21	0.31	0.51	0.00141
Ash, black	0.00274	0.20	0.30	0.49	0.00172
Ash, green	0.00248	0.18	0.27	0.45	0.00158
Ash, pumpkin	0.00219	0.16	0.24	0.39	0.00126
Ash, white	0.00274	0.20	0.30	0.49	0.00169
Aspen, bigtooth	0.00278	0.20	0.30	0.50	0.00112
Aspen, quaking	0.00234	0.17	0.25	0.42	0.00119
Basswood, American	0.00330	0.24	0.36	0.59	0.00230
Beech, American	0.00431	0.31	0.47	0.78	0.00190
Birch, paper	0.00304	0.22	0.33	0.55	0.00219
Birch, river	0.00327	0.24	0.35	0.59	0.00162
Birch, sweet	0.00319	0.23	0.34	0.57	0.00226
Birch, yellow	0.00338	0.24	0.37	0.61	0.00256
Buckeye, yellow	0.00285	0.21	0.31	0.51	0.00123
Butternut	0.00223	0.16	0.24	0.40	0.00116
Cherry, black	0.00248	0.18	0.27	0.45	0.00126
Chestnut, American	0.00234	0.17	0.25	0.42	0.00116
Cottonwood, balsam poplar	0.00248	0.18	0.27	0.45	0.00102
Cottonwood, black	0.00304	0.22	0.33	0.55	0.00123
Cottonwood, eastern	0.00327	0.24	0.35	0.59	0.00133
Elm, American	0.00338	0.24	0.37	0.61	0.00144
Elm, cedar	0.00365	0.26	0.39	0.66	0.00162
Elm, rock	0.00285	0.21	0.31	0.51	0.00165
Elm, slippery	0.00315	0.23	0.34	0.57	0.00169
Elm, winged	0.00419	0.30	0.45	0.75	0.00183
Hackberry	0.00315	0.23	0.34	0.57	0.00165
Hickory, pecan	0.00315	0.23	0.34	0.57	0.00169
Hickory, true	0.00411	0.30	0.44	0.74	0.00259
Holly, American	0.00353	0.25	0.38	0.64	0.00165
Honeylocust	0.00230	0.17	0.25	0.41	0.00144
Locust, black	0.00252	0.18	0.27	0.45	0.00158
Madrone, Pacific	0.00451	0.32	0.49	0.81	0.00194
Magnolia, cucumbertree	0.00312	0.22	0.34	0.56	0.00180
Magnolia, southern	0.00230	0.17	0.25	0.41	0.00187
Magnolia, sweetbay	0.00293	0.21	0.32	0.53	0.00162
Maple, bigleaf	0.00248	0.18	0.27	0.45	0.00126
Maple, black	0.00330	0.24	0.36	0.59	0.00165
Maple, red	0.00289	0.21	0.31	0.52	0.00137
Maple, silver	0.00252	0.18	0.27	0.45	0.00102
Maple, sugar	0.00353	0.25	0.38	0.64	0.00165
Persimmon, common	0.00403	0.29	0.44	0.73	0.00278
Poplar, yellow	0.00289	0.21	0.31	0.52	0.00158
Red oak, black	0.00400	0.29	0.43	0.72	0.00151
Red oak, northern red	0.00304	0.22	0.33	0.55	0.00137
Red oak, pin	0.00338	0.24	0.37	0.61	0.00148
Red oak, scarlet	0.00388	0.28	0.42	0.70	0.00151
Red oak: water, laurel, willow	0.00348	0.25	0.38	0.63	0.00153
Sassafras	0.00216	0.16	0.23	0.39	0.00137
Sweetgum	0.00365	0.26	0.39	0.66	0.00183
Sycamore, American	0.00297	0.21	0.32	0.53	0.00172
Tanoak	0.00423	0.30	0.46	0.76	0.00169
Tupelo, black	0.00308	0.22	0.33	0.55	0.00176
Tupelo, water	0.00267	0.19	0.29	0.48	0.00144
Walnut, black	0.00274	0.20	0.30	0.49	0.00190
White oak, bur	0.00312	0.22	0.34	0.56	0.00151
White oak, live	0.00338	0.24	0.37	0.61	0.00230
White oak, overcup	0.00462	0.33	0.50	0.83	0.00183
White oak, white	0.00376	0.27	0.41	0.68	0.00194
Willow, black	0.00308	0.22	0.33	0.55	0.00112

What Wood Movement Means for Epoxy River Tables

The numbers in the table describe tangible dimensional changes across a 36-inch flatsawn hardwood slab assembly. For example, a stable species like sassafras moves 0.16 inches during a 6% to 8% shift and 0.39 inches over a 6% to 11% shift. At the higher end of movement, overcup white oak shifts 0.33 inches from 6% to 8%, and expands 0.83 inches across the full 6% to 11% span. Common river table selections like black walnut shift 0.20 inches from 6% to 8% and 0.49 inches from 6% to 11%.

In an epoxy river table, cured epoxy resin adheres directly to the live edges or seam faces of the wood. However, cured epoxy does not expand and contract with humidity changes the same way solid hardwood does. The polymer matrix remains dimensionally stable in response to ambient relative humidity, while the adjoining hardwood expands tangentially across its grain. Because wood moves and cured resin does not match this moisture-driven movement, internal stress concentrates directly along the glue line and epoxy interface.

Fasteners and Base Attachment Design

Because the total width of a 36-inch tabletop can move well over several tenths of an inch, rigid restraint will cause failures. If a tabletop is screwed tightly to a rigid metal or solid timber base without room to slide, seasonal expansion can buckle the tabletop or split the wood away from the epoxy river. When the wood attempts to shrink in winter, rigid anchoring can tear open wood grain or crack the cast seam.

To prevent joint failure:

• Allow movement in fasteners and base attachment points rather than locking slabs down rigidly.
• Use slotted holes or mechanical expansion clips on table bases so screws can glide laterally as moisture levels fluctuate.
• Ensure that the finish applied to the wood slows down moisture exchange evenly on both top and bottom faces.
• Follow the resin manufacturer data sheet regarding surface preparation and mechanical keying before pouring to achieve the highest possible bond strength along every timber edge.

Sources

• USDA Forest Products Laboratory, Wood Handbook (FPL-GTR-282, 2021)

Published October 28, 2026. Sources are linked in the text.

Header image: original graphic by BigEpoxy.com.