Choosing Between Chain Derailment or Slack
Cutting above the standard 4.5-foot mark alters hinge mechanics, risk patterns, and wood quality. Learn the arborist-level physics and practical consequences.
- Boring above DBH reduces hinge diameter 8-15 % and wood strength 8-12 %. Leverage increases 4 % per 30 cm, raising barber-chair risk sharply on slopes. High cuts drop lumber grade from FAS to Grade 2, costing up to 25 % value. Shrink notch angle to 55° and increase hinge to 12-15 % of actual diameter. When obstacles force high bore, compare crane cost against damage/value loss.
You’re standing beside a leaning pine that must fall away from the shed. You place the bore cut at shoulder height—well above the typical 4.5-foot diameter-at-breast-height line—because the trunk is straight there and free of knots. As a certified arborist, I’m asked weekly whether that extra three feet matters. The short answer: yes, and the consequences ripple through hinge strength, fiber tear-out, barber-chair risk, and even the dollar value of the butt log. By the end of this article you will know exactly how high-bore geometry changes the physics of tree felling and what professional adjustments keep the job safe and profitable.
Why DBH Became the Reference Plane
Diameter at breast height—1.37 m (4 ft 6 in) above ground on the uphill side—originated in 19th-century European forestry as a quick scaling point for timber volume. The height conveniently clears most understory brush yet remains low enough to capture taper before the butt swell disappears. In modern arboriculture, DBH is more than a tally line; it anchors our hinge formulas and chainsaw handling ergonomics. Standard notch depths, hinge thickness percentages, and even the ANSI Z133.1 work-positioning guidelines assume the plunge cut begins within ±30 cm of that plane.
When the bore cut moves upward, every assumption changes. Lever arms lengthen, compression wood percentage drops, and the weight distribution across the hinge shifts. A 60 cm diameter Douglas-fir at DBH may taper to 52 cm only 1 m higher, thinning the hinge by 13 percent before the sawyer touches wood. That single fact explains why two seemingly identical trees can react very differently to the same notch geometry.
Butt swell: the natural flare at trunk base where diameter can be 15–25 % greater than at DBH, providing extra hinge mass when cuts are made low.
Mechanics of a High Bore: Leverage and Fiber Stress
Placing the hinge one meter higher increases the lever arm between the hinge and the center of gravity of the crown. In practical numbers, a 25 m tall red oak with the center of gravity at 60 % of height sees a 4 % increase in overturning moment for every 30 cm the hinge rises. That sounds small, but it moves the failure threshold closer to the green strength of the remaining fibers.
At the same time, the wood itself changes. Juvenile wood content—lighter, weaker, higher in lignin—dominates the upper trunk. Compression wood strips, which resist bending stress, diminish rapidly above DBH in conifers. Laboratory data from the USDA Forest Service show modulus of rupture dropping 8–12 % between DBH and 1.5 m in second-growth loblolly pine. In other words, the hinge you rely on to steer the tree is both thinner and weaker when you bore high.
Pro Tip: On any trunk over 45 cm DBH, measure diameter at the proposed hinge height with a diameter tape before you cut. If the loss exceeds 10 % of DBH, shrink the notch depth 5 % to keep hinge thickness in the 8–10 % range and reduce barber-chair odds.
Barber-Chair and Splitting Risk Above DBH
The dreaded barber-chair—a vertical split that races up the stem—happens when tension on the back side of the hinge exceeds the shear strength of the fibers before the hinge can bend. Raising the hinge places the fulcrum higher, increasing tension at the point where the stem is still rigid. Field observations show that on steep slopes (>25°) the risk curve steepens sharply once the bore moves more than 50 cm above DBH.
Hardwoods with spiral grain—sycamore, sweetgum, some red oaks—amplify this effect. The spiral grain acts like a pre-scored line, giving the split a path to follow. In extreme cases I’ve seen the split shoot 8 m up the trunk, ruining the entire butt log. Professional response is twofold: shorten the notch depth to create a thicker hinge and drive a wedge immediately after the back cut is started, before the tree commits to the lay.
Impact on Lumber Grade and Value
Sawyers working timber lots often bore high to avoid metal embedded lower in the trunk. Unfortunately, the most valuable clear wood lies between stump height and the first 8 ft log. Boring above DBH can turn a grade-1 butt log into a grade-2 second log, costing $150–$300 per thousand board feet on current southern yellow pine markets. The drop is caused by both the smaller diameter and the increased likelihood of fiber separation during felling.
For urban removals where the wood is chipped or firewood, the financial penalty is smaller but not zero. Crane companies price picks by complexity; a barber-chair that leaves a 4 m split stem dangling complicates the lift and may trigger overtime rates. When bidding jobs, I add a 15 % contingency to any tree where high boring is unavoidable due to obstacles or decay.
| Cut Location | Butt Log Grade | Barber-Chair Risk (Low, Mod, High) | Typical Value Loss (%) |
|---|---|---|---|
| Standard DBH | FAS/Grade 1 | Low | 0 |
| 30 cm above DBH | Grade 1–2 | Moderate | 10–15 |
| 60 cm above DBH | Grade 2 | High | 20–25 |
Adjusting Notch Geometry and Back-Cut Sequence
When high boring is the only option, experienced fellers shrink the notch from the standard 70–80° opening to 55–60°. A shallower notch leaves more wood on the compression side of the hinge, compensating for the smaller diameter and weaker juvenile wood. The back cut is then started 2–3 cm higher than the bottom of the notch—effectively creating a step that helps prevent the kerf from closing too early.
Next, hinge thickness is recalculated. Instead of the textbook 10 % of DBH, use 12–15 % of the diameter at the actual hinge height. For a 55 cm trunk at breast height that narrows to 48 cm at the bore point, target 5.8–7.2 cm of hinge instead of the 5.5 cm you would have left at DBH. The extra thickness buys time for the hinge to bend and tear, reducing the likelihood of catastrophic shear failure.
Crane-Assisted or Bucket-Assisted Alternatives
If the obstacle forcing the high bore is critical infrastructure—power lines, a greenhouse, a historic wall—consider bypassing hinge physics altogether. A 35-ton crane can pluck the top 12 m out of the crown before the sawyer ever touches the trunk. Cost runs $1,200–$2,000 per day in most U.S. markets, but that often equals the combined risk of property damage and value loss from a botched high-bore fell.
Another option is a tracked lift bucket. A 75-ft boom places the operator at 18 m, allowing conventional notch and bore cuts at DBH while the operator stands safely below the hazard zone. Bucket work trades crane rental for hourly lift fees—typically $250–$300 per hour—making it economical for single-tree removals within 2–3 hours.
What to Do Next
Before you rev the saw, measure trunk diameter at both DBH and the proposed hinge height. If the loss exceeds 8 % or the tree shows spiral grain, flag the job for professional review. Document the obstacle—photo, distance, elevation angle—and share it with a certified arborist who carries Tree Risk Assessment Qualification (TRAQ). The small consultation fee ($150–$300) is cheaper than a crushed shed or a fractured femur. Finally, if you proceed on your own, recalibrate your notch geometry and have wedges and a felling lever within arm’s reach before the first tooth enters the wood. Watch for tensioner warning signs while you work.
