Moisture Content
Moisture content (MC) is the percentage of water weight relative to the oven-dry weight of wood — it controls dimensional stability, fastener performance, and susceptibility to decay, making it one of the most important properties in lumber selection and installation.
In Detail
Understanding Moisture Content
Moisture content is calculated as: MC% = ((wet weight − oven-dry weight) / oven-dry weight) × 100. Living trees may have MC above 100% (more water weight than wood fiber weight). Kiln-dried lumber is typically 12–19% MC, and in-service interior wood stabilizes around 6–10% MC.
Wood shrinks as it dries and swells as it absorbs moisture. This dimensional change is far greater across the grain (width/thickness) than along it (length), which is why floor boards gap in winter and bind in summer.
Key Thresholds
- Below 19% MC: lumber is stamped "KD" (kiln-dried) or "S-DRY" (surfaced dry).
- Above 19% MC: lumber is stamped "S-GRN" (surfaced green) and will shrink further.
- The fiber saturation point (~28-30% MC) is where decay fungi become active.
- Below 20% MC: wood is generally safe from decay.
Related Terms
Delamination
Delamination is the separation of bonded layers in a laminated material — in plywood, it means the adhesive bond between veneer plies has failed, causing the plies to separate and compromising the panel's structural integrity.
Edge Swell
Edge swell is the permanent expansion of panel edges — particularly OSB edges — that occurs when the panel absorbs moisture, creating raised ridges at panel joints that telegraph through roofing, flooring, and other finish materials.
Exposure 1
Exposure 1 is a plywood adhesive classification indicating the panel uses waterproof glue that will not fail from moisture, but the panel is designed for temporary construction moisture exposure — not permanent outdoor use.
Related Materials
CDX Plywood
This is a test description for CDX plywood as we work on the technical backend of the website.
OSB (Oriented Strand Board)
Oriented strand board (OSB) is an engineered wood panel made from compressed wood strands bonded with waterproof resin. It is the most widely used structural sheathing panel in North American residential construction, offering uniform strength properties and lower cost than plywood.