Imported Wood Shrinkage Ratios: Stability in Epoxy Tables

Understanding the Tangential-to-Radial Shrinkage Ratio

When building epoxy river tables and solid slab tops, wood movement is the primary factor that dictates long-term stability. As timber gains or loses moisture, it moves unevenly depending on grain orientation. Radial shrinkage occurs across growth rings from the pith outward, while tangential shrinkage takes place along the curved arc of the annual rings. In the Wood Handbook (FPL-GTR-282) published by the USDA Forest Products Laboratory, Table 4-4 documents these dimensions from green to ovendry conditions across dozens of imported timber species.

The tangential-to-radial (T/R) shrinkage ratio reflects how evenly a species responds to moisture fluctuations across its cross section. A ratio close to parity indicates that a species contracts at nearly identical rates across its growth rings and along them. As this ratio rises, the disparity in movement between the two planes widens, creating internal tension whenever ambient humidity shifts.

Why Low Ratios Resist Cupping

Cupping occurs across flat-sawn or live-edge boards because the outer face exhibits higher tangential shrinkage than the radial movement seen toward the center. This uneven pulling forces the board edges to curl upward or downward. A low T/R ratio minimizes this structural distortion:

• Uniform contraction: When tangential and radial shrinkage rates remain close, growth rings do not pull against one another with severe force.
• Reduced cross-sectional warp: Woods with low ratios stay significantly flatter across their width during seasonal moisture cycling.
• Minimized bond stress: Flat slabs keep the cast epoxy channel seated evenly, reducing uneven leverage on the mechanical bond.

For example, Greenheart posts a T/R ratio of 1.1, while species such as Macawood and True Mahogany maintain low ratios of 1.3 and 1.4. In contrast, woods approaching a 2.0 ratio experience twice as much tangential contraction as radial movement, making them far more prone to cupping across wide, flat sections.

Balancing the T/R Ratio with Total Shrinkage

While a low T/R ratio prevents severe cupping, it is only half of the stability equation. Woodworkers must examine the ratio alongside total volumetric, tangential, and radial shrinkage. A slab can hold its flat shape while still undergoing massive overall dimensional changes that generate destructive shear force along the wood-epoxy bondline.

Consider the contrast between Greenheart and True Mahogany. Greenheart ranks at the top of the ratio list with a value of 1.1, but its total shrinkage values are aggressive: 8.8 percent radial, 9.6 percent tangential, and 17.1 percent volumetric. African Ebony presents a low ratio of 1.2, but pairs that with 9.2 percent radial, 10.8 percent tangential, and 20.0 percent volumetric shrinkage. Although these timbers resist cupping, their sheer volume change creates intense shear stress against rigid epoxy casts.

Conversely, species that combine modest ratios with low absolute movement represent balanced choices for hybrid epoxy assemblies:

• Macawood: Low T/R ratio of 1.3, combined with only 2.7 percent radial, 3.5 percent tangential, and 6.5 percent volumetric shrinkage.
• True Mahogany: Ratio of 1.4, with 3.0 percent radial, 4.1 percent tangential, and 7.8 percent volumetric shrinkage.
• Cocobolo: Ratio of 1.6, with 2.7 percent radial, 4.3 percent tangential, and 7.0 percent volumetric shrinkage.

Teak presents an interesting balance in the data. While its calculated T/R ratio is 2.3, its total movement is minimal, showing 2.5 percent radial, 5.8 percent tangential, and 7.0 percent volumetric shrinkage. Because its absolute shrinkage is remarkably small, the total movement remains manageable. Assessing both figures ensures that you select timber that neither cups violently nor pulls away from the resin interface.

Calculated from USDA Wood Handbook (FPL-GTR-282), Table 4-4.

Rank	Species	Tangential %	Radial %	T/R ratio
1	Greenheart	9.6	8.8	1.1
2	Ebony, African	10.8	9.2	1.2
3	Ipe	8.0	6.6	1.2
4	Kempas	7.4	6.0	1.2
5	Caribbean Pine	7.8	6.3	1.2
6	Macawood	3.5	2.7	1.3
7	Azobe	11.0	8.4	1.3
8	Mahogany, True	4.1	3.0	1.4
9	Limba	6.2	4.5	1.4
10	Mora	9.8	6.9	1.4
11	Jarrah	11.0	7.7	1.4
12	Bubinga	8.4	5.8	1.4
13	Avodire	6.7	4.6	1.5
14	Kaneelhart	7.9	5.4	1.5
15	Okoume	6.1	4.1	1.5
16	Balata	9.4	6.3	1.5
17	Bulletwood	9.4	6.3	1.5
18	Spanish-Cedar	6.3	4.2	1.5
19	Sucupira	7.0	4.6	1.5
20	Marishballi	11.7	7.5	1.6
21	Rosewood, Brazilian	4.6	2.9	1.6
22	Karri	12.4	7.8	1.6
23	Cocobolo	4.3	2.7	1.6
24	Piquia	8.0	5.0	1.6
25	Sapele	7.4	4.6	1.6
26	Ebony, East Indian	8.8	5.4	1.6
27	Ocota Pine	7.5	4.6	1.6
28	Benge	8.6	5.2	1.7
29	Hura	4.5	2.7	1.7
30	Peroba Rosa	6.4	3.8	1.7
31	Angelique	8.8	5.2	1.7
32	Roble	6.1	3.6	1.7
33	Manni	9.7	5.7	1.7
34	Peroba De Campos	6.6	3.8	1.7
35	Sande	8.0	4.6	1.7
36	Manbarklak	10.3	5.8	1.8
37	Mahogany, African	4.5	2.5	1.8
38	Obeche	5.4	3.0	1.8
39	Ekop	10.2	5.6	1.8
40	Ilomba	8.4	4.6	1.8
41	Roble	11.7	6.4	1.8
42	Lauan, Light Red And Red	8.5	4.6	1.8
43	Opepe	8.4	4.5	1.9
44	Courbaril	8.5	4.5	1.9
45	Goncalo Alves	7.6	4.0	1.9
46	Purpleheart	6.1	3.2	1.9
47	Banak	8.8	4.6	1.9
48	Wallaba	6.9	3.6	1.9
49	Lauan, White	7.7	4.0	1.9
50	Albarco	5.4	2.8	1.9
51	Ceiba	4.1	2.1	2.0
52	Parana Pine	7.9	4.0	2.0
53	Ramin	8.7	4.3	2.0
54	Gmelina	4.9	2.4	2.0
55	Determa	7.6	3.7	2.1
56	Lauan, Dark Red	7.9	3.8	2.1
57	Apitong	10.9	5.2	2.1
58	Keruing	10.9	5.2	2.1
59	Angelin	9.8	4.6	2.1
60	Afrormosia	6.4	3.0	2.1
61	Rosewood, Indian	5.8	2.7	2.1
62	Pilon	11.7	5.4	2.2
63	Meranti, White	6.6	3.0	2.2
64	Cativo	5.3	2.4	2.2
65	Rubberwood	5.1	2.3	2.2
66	Kapur	10.2	4.6	2.2
67	Imbuia	6.0	2.7	2.2
68	Cuangare	9.4	4.2	2.2
69	Mersawa	9.0	4.0	2.2
70	Teak	5.8	2.5	2.3
71	Meranti, Yellow	8.0	3.4	2.4
72	Jelutong	5.5	2.3	2.4
73	Andiroba	7.6	3.1	2.5
74	Balsa	7.6	3.0	2.5

Shop Fabrication and Resin Best Practices

When incorporating imported slabs into epoxy river tables, thoughtful shop preparation helps manage cross-grain forces:

• Moisture acclimation: Allow lumber to equalize thoroughly to the local indoor climate before surfacing and casting.
• Edge preparation: Clean and abrade live edges thoroughly to maximize mechanical adhesion before pouring resin.
• Manufacturer data sheets: Follow the resin maker’s technical data sheet for pour depths, mixing ratios, and cure schedules to develop full bond strength.
• Joinery allowances: Use slotted fasteners or tabletop clips when securing tops to metal bases, allowing natural expansion without cracking.

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.