Sound WoodHow much decay is too much in a tree?
After a sonic test, the question every client asks is some version of "so, is that bad?" They'd like a number: above this much decay, take it down; below it, keep it. It would make everyone's life easier. Unfortunately, trees don't work that way, and it's worth being able to explain why.
Why one number doesn't work
Two trees with the same decay percentage can be in very different situations. What matters is not just how much wood is gone, but:
- Where the decay is. Decay in the center of a trunk, enclosed by a continuous sound shell, costs far less strength than the same amount of decay open to the bark on one side. A hollow tube is surprisingly strong; a tube with a slot cut down one side is not.
- Which way the load comes from. Decay on the side under tension or compression in the prevailing wind matters more than decay on the side that carries little load.
- The size of the crown and the lean. A big, heavy, leaning crown asks more of the trunk than a small, upright one.
- The species and type of rot. Some decay makes wood brittle, so failure gives less warning. Some species compartmentalise decay better than others.
- The rest of the tree. Root decay, cracks, weak unions and dieback all add up.
- The target. The same tree is a very different concern over a playground than over an empty field.
The shell-thickness rules of thumb
You'll often see rules based on the thickness of the sound wood left around a hollow compared with the trunk's radius. One widely quoted version, often written as a t/R ratio, suggests that strength loss rises sharply once the sound shell gets thinner than roughly a third of the radius.
These rules are useful as a starting point, and they capture something real: a thin shell is much weaker than a thick one. But they have important limits:
- They assume the decay is centered and the shell is continuous all round. Many real trees, like one with a basal cavity, don't look like that.
- They don't account for the crown, the lean, the wind, the species or the target.
- They've been debated and refined in the research literature, and no single ratio is accepted as a universal failure threshold.
Use them as one input to your thinking, not as a pass/fail test.
What the sonic test gives you
A sonic map gives you some of the most important pieces:
- How much: the decay range, from the confirmed minimum to the possible maximum.
- Where: which side, and whether the decay is enclosed or reaches the bark.
- How sure: a tight range means good coverage; a wide range means more paths would help.
Sound Wood reports the result as a range for exactly this reason, and draws decay reaching the bark when the readings show it, because the shape matters as much as the size.
What the test doesn't give you
It doesn't give you the loads, the wind, the target or the tree's history. It shows one height. And it can't tell decay from a crack. All of that is the assessment around the test, and it's where your experience does most of the work.
Rule of thumb for the report: say how much decay there is, where it is, and what that means for this tree in this place. The percentage alone is never the conclusion.
Explaining it to a client
When a client asks "how much is too much?", an honest answer sounds something like this:
"There isn't one number that applies to every tree. What matters is how much sound wood is left and where it is, how much load the tree is carrying, and what's underneath it. In your tree, the decay is [enclosed in the center / open on the house side], and here's what I recommend because of that."
Clients usually respect that more than a number, because it shows you've looked at their tree, not a chart.
Where that leaves the decision
The decision belongs to the arborist, using everything: the visual assessment, the decay map and range, the type of decay, the loads and the target. A sonic test makes that decision much better informed. It doesn't make it for you, and neither does any rule of thumb.
When the call is close, there are usually options between "keep as is" and "remove": reduce the crown to lower the load, move the target, add support, or monitor with a retest. The map helps you choose between them and explain your choice.