Green Timber, Species, and Grading

Why green

Most timber frames are cut from green timbers, still full of sap. Big timbers take years to dry, so drying first is impractical, and green wood cuts far more easily with chisels and slicks. The engineering reflects this: the Wood Handbook notes that for lumber thicker than 4 inches, design properties are typically assigned on the basis of green wood, with no adjustment for moisture ¹.

Green framing has consequences you design for:

• Shrinkage. Timbers shrink across the grain as they dry, tangentially about twice as much as radially, and almost nothing in length ². A 10-inch-deep oak beam can lose more than half an inch in depth.
• Checks. Seasoning checks open on the faces of boxed-heart timbers. They’re normal.
• Movement at joints. Joinery must tighten, not loosen, as wood shrinks; drawbored pegs and well-placed housings do that.

Species

What North House uses and why: the majority of its projects use eastern white pine, milled locally, because it’s easy to work and “excellent to use green as it remains fairly stable as it dries” ³. Its low shrinkage (6.1 percent tangential, 2.1 radial from green to dry) is why ².

Common framing species in the Upper Midwest:

Pegs are traditionally oak or another strong, straight-grained hardwood, riven or turned dry so they don’t shrink loose.

Boxed heart vs. free of heart

• Boxed-heart timbers contain the pith near the center. Every one will develop seasoning checks, typically one big check on a face, but they’re stable and stay straight.
• Free-of-heart-center (FOHC) timbers are sawn from bigger logs with the pith outside the piece. They check much less, but large logs are needed and they cost more.

Grading

Structural timbers are stress graded, mostly by visual grading: an inspector judges growth characteristics that reduce strength. The Wood Handbook lists the main criteria: knots, slope of grain, checks and splits, shake, density, decay, ring count and latewood, pitch pockets, and wane ¹.

What matters most:

• Knots interrupt the grain. In a beam, knots on the bottom (tension) edge hurt bending strength most; knots near the centerline matter little.
• Slope of grain: grain running diagonally across a piece weakens it.
• Shake: separation between growth rings, which reduces shear strength in beams.
• Checks and splits: rated by the size of the actual opening.

Grades include Select Structural, No. 1, and No. 2, each with design values in the National Design Specification. If your building inspector requires it, North House notes that grade-stamped timbers and site-specific engineered plans can be provided ³. For a shed you may not need stamped timber; for a house you almost certainly will.

Seasoning checks

Checks alarm newcomers, but in a boxed-heart timber they’re expected. The Timber Frame Engineering Council has published a technical bulletin specifically on seasoning checks in timbers ⁴. Practical rules: orient a boxed-heart beam so the expected check is on a side face rather than the top or bottom edges, keep checks away from peg holes and joints where possible, and don’t fill them. Checks through joinery or along the bottom of a beam near midspan deserve a closer look.

Ordering

Order timbers sawn to full dimension with extra length, from a mill that saws timbers; band-sawn timbers have a smooth, even surface. Ask for straight logs, boxed heart or FOHC as you prefer, and species and grade. Stack them stickered and shaded if they’ll wait more than a few days, and coat the ends to slow end checking.

What to take from this

Frames are cut green and designed for green timber: timbers over 4 inches carry green design values. Eastern white pine is the easy, stable folk-school choice; oak is strong but hard and shrinks more. Boxed-heart timbers check but stay straight. Grades depend mostly on knots, slope of grain, and shake; checks are expected and should be oriented to the side faces.