Moisture content (MC) is arguably the single most critical property of wood for anyone building, renovating, or crafting with it. Getting moisture content right can mean the difference between a project that lasts decades with grace and one that fails within months through warping, cracking, cupping, or joint failure. Yet MC is frequently overlooked, especially by homeowners and less experienced builders. This guide covers everything you need to know.
Wood is hygroscopic — it constantly absorbs and releases moisture from the surrounding air until it reaches equilibrium with its environment. This equilibrium moisture content (EMC) varies by climate and season. In Southern California interiors, EMC is typically 6-8% year-round thanks to our dry climate and air conditioning. Coastal areas may run slightly higher (8-10%). Covered outdoor spaces in LA settle around 10-14%. Fully exposed exterior wood fluctuates between 8% in dry summer months and 15-18% during winter rains.
Why does moisture content matter so much? Because wood changes dimension as its MC changes. As wood dries, it shrinks. As it absorbs moisture, it swells. The magnitude of this movement depends on the species and the grain orientation. Tangential movement (along the growth rings) is roughly twice as large as radial movement (across the rings). A flat-sawn red oak board 12 inches wide can shrink or swell by 1/4 inch or more across a 10% MC change. That may not sound like much, but multiply it across a floor, a tabletop, or a set of cabinets, and the cumulative effect is devastating.
Measuring moisture content requires a dedicated moisture meter. Pin-type meters work by driving two small pins into the wood and measuring electrical resistance between them — drier wood has higher resistance. They provide accurate readings at the depth of the pins and are excellent for checking MC at specific points and depths. Pinless (dielectric) meters scan a larger area using electromagnetic waves and do not damage the surface, but they average the reading across a depth zone and can be less accurate on rough, wet, or irregular surfaces.
For the most accurate results with a pin-type meter, follow these practices: push the pins in parallel to the grain, not across it (cross-grain readings can be 1-2% low). Take readings at multiple locations on each board — MC can vary significantly from end to end and from surface to heartwood. For thick stock, use long pins or drill test holes to check MC at the center of the board, not just the surface. The surface may read 8% while the core is still at 15% — a recipe for cupping and checking as the board slowly reaches equilibrium.
For interior applications like flooring, furniture, cabinetry, and trim, you should aim for 6-8% MC in the Los Angeles area. Wood installed too wet will shrink as it dries to match indoor EMC, creating gaps between floorboards, cracks at joints, and loose-fitting doors and drawers. For hardwood flooring specifically, the National Wood Flooring Association recommends that wood MC should be within 2% of the subfloor MC at the time of installation.
For exterior applications, the target is less precise since outdoor wood is expected to move seasonally. Generally, 12-15% is acceptable for decking, fencing, pergolas, and exterior trim. The key is not to install bone-dry interior-grade wood outdoors where it will absorb moisture and swell, potentially buckling or popping fasteners. Conversely, do not install rain-soaked wood that will shrink dramatically as it dries.
Reclaimed lumber presents special moisture challenges because its history is unknown. A board salvaged from a dry barn interior may already be at 6-8% — perfect for immediate interior use. A board pulled from an outdoor deck may be at 20%+ and needs months of drying. Wood that looks and feels dry on the surface can harbor high MC deep inside if it was recently exposed to rain or stored on wet ground. Never trust appearance alone: always meter test reclaimed wood at multiple locations and depths.
If your reclaimed lumber is too wet for its intended use, the solution is drying. Air drying is the simplest approach: stack the lumber in a covered, well-ventilated area with stickers (thin wood spacers, typically 3/4 inch thick) between each layer to allow air circulation. Stickers should be placed every 16-24 inches and aligned vertically in the stack. Weight the top of the stack to minimize warping. Air drying in the LA climate is quite effective: expect MC to drop roughly 2-4% per month for 1-inch-thick boards, though thicker stock dries proportionally slower.
Kiln drying is faster and more controlled. A kiln uses heated air and controlled humidity to remove moisture in 1-4 weeks (depending on species and thickness) while minimizing drying defects. Kiln drying also kills all insects, insect eggs, and fungal spores — an important benefit for reclaimed wood. We offer kiln drying services for customers who need reclaimed lumber brought to interior-grade MC on a specific timeline.
One final critical concept: acclimation. Before installing any wood in its final location, allow it to sit in that environment for at least 5-7 days (2 weeks for flooring) so it can reach equilibrium with the space. Stack it with air circulation, not in a sealed plastic wrap. Monitor MC with your meter during this period. When MC has stabilized within 1-2% of your target, the wood is ready to install. This simple step prevents more wood movement problems than any other single practice.