Outdoor Equilibrium Moisture Content Across the Northern Plains
Working with live-edge lumber for epoxy river tables requires an exact understanding of wood moisture. Wood is hygroscopic, meaning it constantly exchanges water vapor with the surrounding atmosphere until it reaches equilibrium moisture content (EMC). According to the USDA Forest Products Laboratory in the Wood Handbook (FPL-GTR-282), outdoor EMC values represent averages of 30 or more years of NOAA weather data.
Across the Northern Plains, outdoor EMC fluctuates significantly from season to season. The regional data covers six representative cities:
Outdoor equilibrium moisture content (%), USDA Wood Handbook (FPL-GTR-282) Table 13-1 (averages of 30 or more years of NOAA data).
| City | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | Low | High |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Fargo, ND | 14.4 | 14.8 | 15.2 | 12.8 | 11.8 | 12.9 | 13.1 | 13.2 | 13.4 | 13.4 | 14.9 | 15.3 | 11.8 | 15.3 |
| Sioux Falls, SD | 14.1 | 14.3 | 14.3 | 12.6 | 12.6 | 13.0 | 12.9 | 13.6 | 13.3 | 12.9 | 13.9 | 14.9 | 12.6 | 14.9 |
| Omaha, NE | 14.2 | 14.2 | 13.4 | 12.1 | 12.7 | 12.9 | 13.2 | 13.8 | 13.4 | 12.7 | 13.5 | 14.5 | 12.1 | 14.5 |
| Minneapolis–St. Paul, MN | 14.0 | 13.8 | 13.3 | 11.8 | 11.7 | 12.3 | 12.4 | 13.1 | 13.4 | 13.1 | 14.2 | 14.9 | 11.7 | 14.9 |
| Des Moines, IA | 14.3 | 14.2 | 13.4 | 12.6 | 12.5 | 12.9 | 13.2 | 13.6 | 13.4 | 12.8 | 13.8 | 14.9 | 12.5 | 14.9 |
| Wichita, KS | 13.7 | 13.3 | 12.6 | 12.4 | 13.3 | 12.7 | 11.7 | 11.9 | 12.5 | 12.4 | 13.0 | 13.8 | 11.7 | 13.8 |
The outdoor data shows significant seasonal swings. In Fargo, ND, outdoor EMC reaches a low of 11.8% in May and peaks at 15.3% in December. Sioux Falls, SD, ranges from a low of 12.6% in April and May to a high of 14.9% in December. In Omaha, NE, the reading drops to 12.1% in April before rising to 14.5% in December. Minneapolis–St. Paul, MN, records an annual low of 11.7% in May and climbs to 14.9% in December. Des Moines, IA, drops to 12.5% in May and hits 14.9% in December. Further south, Wichita, KS, shows its low of 11.7% in July and a high of 13.8% in December.
Best and Worst Months for Outdoor Air Drying
Air drying progresses most rapidly when outdoor EMC is low and ambient temperatures are warm. Across the Northern Plains, the most favorable drying months occur in late spring and mid-summer. Fargo, Sioux Falls, Omaha, Des Moines, and Minneapolis–St. Paul experience their lowest EMC readings during April and May, while Wichita reaches its lowest point in July at 11.7%.
Conversely, the worst months for drying occur in late autumn and winter. December marks the highest outdoor EMC across every single city listed, reaching between 13.8% and 15.3%. During winter, cold temperatures and high relative humidity stall effective moisture loss, and prolonged exposure during unfavorable seasons can encourage fungal and chemical stains in green wood.
Why Outdoor Drying Cannot Meet Interior Furniture Standards
A common pitfall in river table fabrication is assuming that thoroughly air-dried slabs are ready for epoxy resin. Table 13-2 of the Wood Handbook outlines recommended moisture content values for interior wood products such as furniture, woodwork, and flooring:
- Most areas of the United States: 8% average (individual pieces 6% to 10%)
- Dry southwestern area: 6% average (individual pieces 4% to 9%)
- Damp, warm coastal area: 11% average (individual pieces 8% to 13%)
Outdoor air drying alone in the Northern Plains cannot bring wood anywhere near the 8% target for interior furniture. The lowest outdoor EMC recorded in the region is 11.7%, while winter levels exceed 14% to 15%. If a slab is poured at outdoor air-dry levels, the wood will continue to lose moisture once installed inside a climate-controlled home. As moisture drops toward 8%, the wood shrinks. Because epoxy resin cures into a rigid plastic that does not shrink or swell with atmospheric changes, this wood movement generates severe shear stress along the bondline. Unrelieved shear force leads to edge separation, tabletop warping, or splits adjacent to the resin channel.
Air-Drying Timelines and the Move Indoors
The Wood Handbook explains that air-drying time to 20% to 25% moisture content varies widely based on species, thickness, yard location, and time of year. For 25-mm (1-in.) thick boards, low-density species such as pine, spruce, red alder, and soft maple may reach 20% to 25% moisture in 15 to 30 days when stacked in favorable locations during favorable times of the year. In contrast, slow-drying species such as sinker hemlock, pine, birch, and oak stacked at unfavorable times in northern locations require 200 to 300 days just to reach that same 20% to 25% range.
Thick live-edge slabs require even longer. Because air drying effectively stops far above interior targets, slabs must complete their drying cycle in a dehumidification kiln, heat kiln, or climate-controlled shop space. Bringing the lumber down to the recommended 8% average (or 7% to 9% for furniture grade) ensures dimensional stability. Before mixing or pouring, always consult the resin manufacturer’s technical data sheet to ensure the substrate meets all bonding criteria.
Sources
Published October 28, 2026. Sources are linked in the text.
Header image: original graphic by BigEpoxy.com.
Staff note: When checking lumber readiness, follow the sampling guidance in Table 13-2: test at least 10% of individual pieces in a batch (for example, taking at least six tests across 60 items) with a calibrated moisture meter to verify that the core matches the surface reading. (Source: USDA Forest Products Laboratory, Wood Handbook (FPL-GTR-282, 2021).)