How timber-frame sizing differs from dimensional lumber

Heavy timbers behave like deep, stocky beams — bending strength is rarely the limit. Three things usually govern instead:

Deflection. A 6×10 oak beam on a 16-foot floor span passes bending easily but feels bouncy. L/360 (1/360 of the span) is the floor target; L/240 is fine for roofs.
Horizontal shear at the supports. Timbers with tenons cut into them shear-fail before they bend-fail. A 1″-deep housing into the side of a 10″ beam removes 10% of the cross-section at the most-stressed point.
Bearing. Where the beam lands on a post or sill, you can crush the wood across the grain. Oak handles ~800 psi perpendicular-to-grain; white pine only ~350.

Why we derate the joint, not the beam

A clear 6×10 white-oak beam is monstrous in pure bending. Cut a 1½″ × 4″ × 5″-deep mortise into the end of it for a tenon, and you've removed roughly a third of the bearing area and added a stress riser. Connection derates here (15-40%) approximate what a timber-frame engineer would apply to the published reference values — they're meant to keep you out of trouble, not replace stamped calcs.

Species cheat-sheet for frames

White oak. Hard, dense, rot-resistant, very stiff. Traditional barn timber. Heavy to lift, dulls saw chains, but the strongest commonly-used domestic option.
Douglas Fir-Larch. The Pacific-Northwest workhorse. Excellent strength-to-weight, machines cleanly, dimensionally stable when dry. Most modern timber-frame shops ship in DF.
Eastern white pine. Soft, light, easy to hand-hew, but only ~⅔ the bending strength of oak. Common in New England barns — you'll see bigger sections to compensate.
Hemlock. Cheap and locally available across the Northeast. Brittle, splinters badly, prone to shake. Workable for non-critical members.
Southern yellow pine. Strong, resinous, great for the South. Heavy and tough on tools.

FAQ

What's the span limit for a 8x10 oak floor beam? +

With 10' tributary width and 55 psf residential floor load (L/360), about 14 to 15 feet between supports — deflection governs. Drop to L/240 and you'll get another 2-3 feet, but the floor will feel bouncy.

How big should a porch / pavilion beam be? +

Open structures carry much less load (25 psf is typical), and people tolerate more deflection outdoors (L/180). A 6×10 Doug Fir beam runs 18-22 feet pavilion-side. Snow load is what controls — a 60-psf ground snow zone changes the math fast.

Why are my numbers smaller than NDS span tables? +

Most published tables assume bearing-only end connections — a beam sitting on top of a post with no joinery. The moment you cut a mortise or housing, capacity drops. This calc bakes in a connection derate so the result reflects how the frame actually goes together.

Can I use this for green / unseasoned timber? +

Green timber loses ~10% strength compared to seasoned. The reference values here assume "wet service" already for heavy timbers — but if you're using freshly-sawn material on a critical member, knock another 15% off the result and oversize.

What about loaded knee braces? +

Knee braces shorten the effective span of the beam by transferring load to the post. A 16' beam with knee braces at 3' from each post acts more like a 10' beam in the center. Use the clear span between braces in this calc, not the full beam length.

Tributary width — what does that mean? +

Imagine your beam runs east-west. Half the joists frame in from the north, half from the south. Tributary width is the joist span on the north side ÷ 2 plus the joist span on the south side ÷ 2. For a beam down the middle of a 20-foot building, that's 10 feet.

Does this work for cantilevers? +

No. Cantilevers (overhangs past a support) have totally different mechanics — moment at the support, deflection at the tip. Don't use simple-span results for cantilevered beams.