Softwood Lumber Sizes for Epoxy Table Bases

Why do nominal and actual softwood lumber sizes differ?

When planning a table base, leg assembly, or subframe for an epoxy river table, lumber sizes specified at retail are nominal designations rather than finished dimensions. As outlined in the Wood Handbook (General Technical Report FPL-GTR-282) by the USDA Forest Products Laboratory, rough lumber undergoes machining to attain smoothness of surface and uniformity of size. This surfacing operation removes wood fibers, resulting in a minimum dressed size that is smaller than the nominal tag.

Assuming a nominal board matches its stated label will lead to alignment errors in table aprons, joinery misfits, and uneven base support for heavy wood and epoxy tabletops.

What are the dressed sizes for softwood boards?

Table 6-6 outlines the American Standard Lumber sizes for yard and structural lumber. For boards, the finished thickness and width depend on whether the lumber is dry or green:

• Nominal 1 in. thickness: Minimum dressed dry thickness is 19 mm (3/4 in.), while green is 20 mm (25/32 in.).
• Nominal 1-1/4 in. thickness: Minimum dressed dry thickness is 25 mm (1 in.), while green is 26 mm (1-1/32 in.).
• Nominal 1-1/2 in. thickness: Minimum dressed dry thickness is 32 mm (1-1/4 in.), while green is 33 mm (1-9/32 in.).

Face widths for boards also scale downward when dressed dry:

• Nominal 2 in. width finishes dry at 38 mm (1-1/2 in.).
• Nominal 3 in. width finishes dry at 64 mm (2-1/2 in.).
• Nominal 4 in. width finishes dry at 89 mm (3-1/2 in.).
• Nominal 6 in. width finishes dry at 140 mm (5-1/2 in.).
• Nominal 8 in. width finishes dry at 184 mm (7-1/4 in.).
• Nominal 10 in. width finishes dry at 235 mm (9-1/4 in.).
• Nominal 12 in. width finishes dry at 286 mm (11-1/4 in.).

What are the actual dimensions for dimension lumber?

Dimension lumber is commonly selected for table stretchers, internal runners, and base framing beneath an epoxy river tabletop. Table 6-6 defines dimension lumber thicknesses and widths as follows:

• Nominal 2 in.: Minimum dressed dry thickness is 38 mm (1-1/2 in.); green is 40 mm (1-9/16 in.).
• Nominal 2-1/2 in.: Minimum dressed dry thickness is 51 mm (2 in.); green is 52 mm (2-1/16 in.).
• Nominal 3 in.: Minimum dressed dry thickness is 64 mm (2-1/2 in.); green is 65 mm (2-9/16 in.).
• Nominal 3-1/2 in.: Minimum dressed dry thickness is 76 mm (3 in.); green is 78 mm (3-1/16 in.).
• Nominal 4 in.: Minimum dressed dry thickness is 89 mm (3-1/2 in.); green is 90 mm (3-9/16 in.).
• Nominal 4-1/2 in.: Minimum dressed dry thickness is 102 mm (4 in.); green is 103 mm (4-1/16 in.).

Notice that for widths up to nominal 6 in., the dry finished dimension is 1/2 in. smaller than nominal, whereas for nominal 8 in. through 12 in., the dry width is 3/4 in. smaller (for example, nominal 8 in. finishes at 7-1/4 in., and nominal 12 in. finishes at 11-1/4 in.).

How do timber sizes apply to heavy table legs?

When selecting large posts or solid timber members for structural trestles and legs, Table 6-6 establishes standard deductions based on timber thickness and width:

• 5 & 6 thick or wide: Dry lumber is 13 mm (1/2 in.) off nominal; green lumber is 13 mm (1/2 in.) off nominal.
• 7 to 15 thick or wide: Dry lumber is 19 mm (3/4 in.) off nominal; green lumber is 13 mm (1/2 in.) off nominal.
• Greater than or equal to 16 thick or wide: Dry lumber is 25 mm (1 in.) off nominal; green lumber is 13 mm (1/2 in.) off nominal.

For example, a nominal 6 in. post purchased dry measures 1/2 in. less in thickness and width, yielding a finished dimension of 5-1/2 in. by 5-1/2 in.

Why do dry and green lumber dimensions differ?

As green wood dries, shrinkage occurs across the thickness and face width. Table 6-6 provides slightly larger minimum dressed dimensions for green stock so that when the wood seasons to dry conditions, it does not drop below required standards. For instance, a nominal 4 in. face width is surfaced green to 90 mm (3-9/16 in.), while dry dimension stock finishes at 89 mm (3-1/2 in.). For interior table bases supporting an epoxy top, dry lumber should be used to avoid ongoing shrinkage after joinery is fastened.

What surfacing designations should builders check?

Surfaced lumber is designated by the faces planed at the mill:

• S1S: Surfaced one side.
• S2S: Surfaced two sides.
• S1E: Surfaced one edge.
• S2E: Surfaced two edges.
• S4S: Surfaced four sides (two sides and two edges).
• Combinations: S1S1E, S2S1E, and S1S2.

For table legs and base frames that remain visible beneath an epoxy top, S4S material ensures all four faces are planed smooth, reducing rough mill marks and primary sawing imperfections.

Sources

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

Published October 29, 2026. Sources are linked in the text.

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